Carver TFM amplifiers

I have my last chance to measure the Carver TFM-224 amplifier that I have been borrowing.

Just how does it sound like a tube amplifier?  (In my sighted testing, it sounded more like the Krell than a Parasound HCA-1500A.  In DBX testing, I couldn't reliably tell the difference.  Anyway, I believe Bob Carver was doing some trick here that I might be able to reliably tell in DBT eventually.)

Well, for one thing it appears to have rather low damping factor.  I'm seeing close to 9 across the audio spectrum.  Tube amplifiers often have a damping factor of about 8, and sometimes less.

Meanwhile, I'm not seeing significant variation in output vs frequency (and, actually, my I didn't re-measure the input, it's a long story but the impedance of the input matters a lot, and I chose the "high z" output mode of my Keithley 2015 analyzer, since the amplifier input is fairly high z.  So, you can't necessarily even believe the measured rolloff  (around 0.1dB at 20kHz, about 0.3dB at 20Hz, relative to 1kHz) since those could be generator errors, to some degree, but the generator errors would not likely be making the response look better.

BTW, Damping Factor is surprisingly easy to calculate, measure output voltage under load and then unloaded, and then: Df = Vl / (Vu - Vl).

20kHz
8 ohms: 2.6858v
unl.: 2.9806v
df: 9.1

1kHz
8 ohms: 2.7307v
unl.: 3.0343v
df: 9.0

20Hz
8ohms: 2.6608v
unl.: 2.9530v
df: 9.1

The damping factor is remarkably low for a transistor amplifier, and it is remarkably constant across the audio spectrum for any amplifier.  Usually amplifiers have lower damping factor at 20kHz, not slightly more.  But the low and constant damping factor is not the real limits of the amplifying circuitry, it is the dialed in characteristic, "fake" damping factor if you will, so it can be as constant as anything so long as it doesn't go higher than the "actual" damping factor the amplifying circuitry is capable of.

Silver Cables, and Zobels

Here's a company that sells interesting silver wire, teflon jackets.

According to subjectivists, silver wire may sound bright, etc.  (or everything else dull, if you're a silver favoring subjectivists).

The objective truth is that silver is better (though, most objectivists would say not so much as to be important).  Copper is worse because copper oxide is a very nasty conductor (a semi-conductor actually) and it forms readily wherever there is exposure to air.  And even if this is just a thin film, that thin film is specially important because of skin effect.  But there is further oxidation even below the film.

However I wouldn't jump to weird geometries.  Well shielded coax is hard to beat for rejection of interference in interconnects.  It is way better than loose twisted pairs, even if those twisted pairs are shielded.  Given high RF nowadays, and notably in my own house, I would not use anything but the most well shielded coax.  And all my equipment is high bandwidth, making ultrasonic interference rejection especially important.

Tinned copper, silver plated copper, gold plated copper or vacuum annealed copper (what Cardas does, for example) are three ways of addressing the oxidation problem in wires.  While audiophiles may think they love bare copper, bare copper should never be used, including in wire assemblies.

My own practice doesn't approach this yet, I used to think bare copper was the way to go.

Back to the geometry, speaker cable can be twisted pair.  Actually it turns out, that RF rejection and RF impedance are issues, just going for the least inductance might not be the correct answer.  I'm beginning to think it isn't the answer for my Aragon 8008BB.  I'm planning to switch back to zip from star cross, I think the extra inductance may be good for this amp (which has no zobel at the output).

I've been reading about that too...some say zobel is always necessary and you can't hear the choke, while John Curl, and Naim Audio, say you can hear the choke and they don't use one.  An interesting take may be that if you don't use the zobel on the output, you are achieving the same effect by overcompensating the amplifier, which is worse.  Iverson seems to have always used the zobel, and I think it becomes extra critical in high feedback amplifiers.

Anyway, Naim assumed they didn't need the zobel because they were assuming people were using zip wire, as they universally were in the 70's, or wire with sufficient inductance to provide the zobel effect.  Later, when specialty cables didn't, Naim stepped in to make their own "audiophile grade" medium impedance wire.

Here "zobel" is actually being applied to other named type networks, such as Thiele (sp?).  But it's the same idea.

Likewise MIT cables put their zobel networks at the load end, which is suitable for correcting the cable itself.  Perhaps the zobel should be built-in to the speaker itself, or something plugged into the speaker.

Some people say the MIT networks aren't just zobels, they are effectively tone controls.

It's not clear how much tone control you can actually do...and that is very amplifier dependent too.

A month ago the John Curl debating thread ("Blowtorch Preamp" topic) was discussing current mode amplifiers.  Mainly this was driven by Richard Marsh, whose been deep into the topic.  John Curl and others were quite interested.  Some objectophiles say it's a nonsense idea, not real electrical engineering.  Amps, volts, and impedances are real things, but "current mode" is just a marketing term.  Such as used by Krell (not mentioned in the Blowtorch room).  But seeing Marsh and other pretty serious guys considering it seriously, I think it's more than just a marketing term, it has some kind of effects, for example high bandwidth becomes not a problem generally.

Amplifier Voicing

I've been beginning to think that there is indeed a basic subjective difference among amplifiers which was aptly described as Yin and Yang by Harry Pearson.

The Aria amplifiers designed by Michael Elliot (best known as the founder of Counterpoint who made many innovative tube audio products in the 1980's) are available in a upgraded XL version with custom voicing.  When you get the custom voicing option (which adds $1499 to the price) Michael Elliot comes to your home to do the voicing himself, by making small changes to 4 resistors which are initially matched to 0.1%.  He says:

By making subtle shifts in the operating points of devices in the amps, I can shift the "tonal center" of the sound upwards, to emphasize upper harmonics, speed, and transient impact, or downwards, to provide a rich, pure midrange with velvety soft highs. 

 I find this idea to be very interesting.  I do seem to be able to categorize my amplifiers as Yin (the dark, the receptive, the mother womb) and yang (the bright, the fire, the rising white light).

But while most (including Elliot) claim to make their default "neutral," I am thinking there may be no neutral, just a possibly smaller tilt one way or the other.  Neutral is a point so small you never land on it.

Another thing I agree with Michael, high input impedance is essential a line level (and good amplifiers).
On this page he says higher is always better, never accept a line input stage with impedance less than 50k.  He chooses 100k for his line stage, and a mind boggling 470k for the Whole Tone power amplifiers.  My intuition would be to go for 75k on both.  Power amp inputs are not in most cases higher than line preamp inputs.  In most cases, preamps are actually buffered attenuators, reducing level somewhat.  Thus the power amp should not have a higher impedance corresponding to higher input levels.  But his super high impedance on the power amp probably helps move the effective voicing from slightly dark (typical of tube amps) to neutral.

Here's a discussion of OTL amps, which features the designer of Atma Sphere amps describing many things.  Most of the alleged downsides of OTL are prejudice which followed just a few bad apples.  Just a few of the dozens of "totem pole" designs by Julius Futterman and others were unstable.  Most have been rock solid, and other manufacturers using the Circlotron circuit, like Atma Sphere, have no stability problems at all.  And with Atma Sphere there is no loop feedback at all, total "class a2" operation--with a tad of sliding bias. I'd be very interested in how well those work with my Acoustats.  The only downside for me is rather high price.  If I had the time, the optimal approach would be a direct drive tube amp like the one designed by a friend of mine.  I don't think the direct drive amp originally made by Acoustat was very good.

Marantz in the 1970's

Here's the finally updated AudioKarma thread on the giant Marantz factory in Chatsworth.  The background story is that after Superscope/Marantz opened this around 1976, they had spent so much money building it, and the market had fallen off since construction began, and costs in Japan had gone so far up, that they were broke and sold out to their foreign partners (a typical story in business, the Taj Mahal Building story).

Add to that my first personal story.  I had lusted after Marantz equipment ever since seeing a 20 in a store in 1970.  But I did ponder many alternatives before deciding what to get to replace my lousy and ill-functioning Dynaco SCA35.  I tried to have the SCA-35 repaired at a repair center on 30th Street in San Diego California in 1973.  The repair person did a bunch of things, but when I got back to my dorm room in Los Angeles, I had the same original problem--a frying noise.

I had talked to that erudite repair person about my choice in getting something new.  He wasn't inclined to see anything wrong with the SCA 35.  But anyway I proposed a Dynaco 120 plus a PAT-5, and a Marantz 2270.  His answer was that there "was no comparison."  I found this answer unilluminating and maddening.  Did he mean the Marantz was so much better?  Or that I shouldn't even be considering such choices?  He seemed to know more but be unwilling to say.  Maddening!

I though the Dynaco more reasonable for the money, but the Marantz seemed nicer somehow.  So when Pacific Stereo in Pomona dropped the price of 2270 to $399 in the summer of 1974, I couldn't resist any longer.  The frying-sound amp had one more chance at repair before becoming my sister's amplifier, but it failed that and was virtually donated when she got a nice reliable Kenwood amplifier that served her for 20 years and perhaps still.

At first, for a few weeks, I hated the 2270.  Among other things I found some of the controls a bit scratchy.  I took it back to pacific stereo for repair.  I got it back a week later identical.  Oh, well, I found after a few days the problem went away.  But also, all along, I was noticing a thin-ness compared to my SCA 35.  The SCA35 tube amplifier was giving me nice rounded out bass, whereas on the transistorized reciever the bass was all dried up.

By a couple months later, I had as much as forgotten that initial impression, and now it seemed, and decidedly so, the transistorized receiver had much tighter and more accurate bass.  Anyway, I was fine with the sound of the Marantz and the whole system I had (Dual 1209 turntable, large Advent speakers, Shure V15 III cartridge) for about 3 years before nagging doubts set in.  And for the first year, I felt like Big Man On Campus as people came and wanted to hear my system, I weirdly brought it all to the girlfriends house over christmas vacation, and so on.  By the second year, I was in the second tier, and so on.

Anyways, it was in the mid 1970's that I was hanging out with some friends of mine back in the San Fernando Valley, and one of them worked at G.A.S, which was located in Chatsworth (as was Electro Research, Quatre, and perhaps other small high end audio companies).  The first time we were driving past the Marantz building on Nordhoff (a street whose name is still familiar from my childhood in the Valley), it blew my mind there was such a big building which I had not remembered from my high school days.  I think I may have been told it was Marantz/Superscope, but that was so unbelievable to me then it didn't sink in.

A few years later when we drove by, I was told only that it was "nothing, anymore."  "What was it?"  "Nevermind."

I think only in 2009 or so I learned and it really stuck that the giant building we had been driving past was the Marantz/Superscope building from reading the beginning of this AudioKarma thread.  Although I knew Marantz had an "A line," whatever that was, and my 2270 was in the "B line", I just never imagined Marantz having such a large factory building in the USA.  Anyway, I was back in LA, and I wanted to drive past the building, but the photos on the AudioKarma thread had gone.  So I went driving around a bit, and saw nothing.  I did find the now mostly re-purposed Harman International campus.  It seemed only JBL had their original offices, other divisions had left and were replaced with marketing firms and the like, and available space.

Anyway, now the thread has been renewed, and the photographs restored later, and now we can see that the address is 20525 Nordhoff Avenue, which is also Nordhoff and Mason (another familiar sounding name), and the address is strangely close to my childhood street address in nearby Woodland Hills.  (Mason was the street at the western end of my childhood subdivision, I recall, the street that ran by the junior high I went to.)

Up until the Millenium, I had just not imagined Marantz being in my own back yard (or, actually, former back yard by then).  They were always in my imagination far away, and I still imagined Long Island City being central, though it hadn't been since 1964, the year Saul Marantz sold out to Superscope after the excess cost of developing and manufacturing the Model 10b tuner (and, also the newer manufacturing plant in Woodside?  I think there may have been a Taj Mahal factor rarely mentioned.).  Marantz headquarters and operations quickly moved out areas around Los Angeles, famously Sun Valley where Superscope was based, with the mass manufacturing moving to Japan, Standard Radio, the later owner of the Marantz name in Japan, until they changed their name to Marantz Japan in 1975.

OK, I did know enough by 1973 that Marantz was now Sun Valley but I figured that was just a distribution hub since manufacturing had moved to Japan.

Still it had seemed to me that Marantz was respectable right up to the time I bought one, by the summer of 1974, and by late 1975 they were possibly behind Sansui and Pioneer.  Real hifi was G.A.S. or Audio Research.  In fact, when I worked at a high end shop in San Diego in 1978-79, current production Marantz was lumped in with all the asian brands into something derogatorily called "mid fi."  No one mentioned (or even seemed to know about) the A line Marantz components made in USA (until the ending of Superscope, when the SM-1000 was actually built in Japan instead of back at the Superscope factory).

But I had sort of heard about the A line briefly, not enough to know anything about it really, and in a backhanded way, in the fall of 1975.

Hearing that I had bought a Marantz receiver, my then G.A.S. employed friend wanted to have an amplifier comparison.  This was in the fall of 1975.  So I brought my 2270 receiver, he brought his SAE clone, and there may have been something else, to somebody elses apartment.  I can't remember much about this except that I myself couldn't hear much, if any, differences between my 2270 and my friend's clone of the SAE Mk XXXI, designed by James Bongiorno.  But others didn't seem to find my receiver very acceptable.  Sonically I recall they criticized the bass, and the dynamics.  This was attributed to damping factor.  Then afterwards, the main thing I remember amazingly clearly (though I didn't understand most of it at the time) is a little bit of conversation:

Friend: "Do you mind if we take the cover off?"

Me: "No, go right ahead.  I did that the first day."

Friend takes the cover off of the Marantz, and some other guy takes a careful look.  (Was this other guy Andy Hefley?  That's likely, he was a good friend of my friend, though it could have been someone else from G.A.S.)

Friend: "Castor?"

Andy: "No, B Line."

Me: "What's a bee lion?"

Friend: "You wouldn't understand."

And so it went.  I didn't really understand the A and B lines until I read about it on The Vintage Knob or some other website decades later.  The A line was the Superscope-era Marantz line built in the USA, and the most famous designer of A line amplifiers during the 1970's was Mike Castor.  These were the likes of the 250 and 500 amplifiers.  The B line were the Japanese made products at lower prices, like the 22xx series receivers (at least most of them).

I didn't understand, but this was the beginning of the end of my honeymoon with my Marantz.  From then on, it wasn't the shining light, it was the slime I had mistakenly fallen for in my ignorance.  Audiophilia nervosa had started setting in.

Now I look on this whole situation a bit differently.  The then-infamous "B-Line" components were actually pretty good.  They were robustly built (as Marantz ads of the time liked to show) but not indestructable: I burned through one set of output transistors around 1979 and got them replaced.  But generally speaking these units were solidly built, with good transformers and decent circuits mostly.  I felt the phone preamp decent, the line preamp only adequate, and the tuner and power amplifier very good for an inexpensive receiver.  For quite awhile now the B Line components have enjoyed highly collectible status and often sell at prices higher than "high end" components from the same era.  People seem to feel better about them now than they did then.

Also I think what my friend and Andy and others were on to at the time was bogus.  There was this feeling that fully complementary circuitry as laid out by James Bongiorno was fundamentally way better.  Now many people have dumped the fully complimentary approach and have gone back to the quasi-complementary amplifier outputs used in the earliest B Line components.  Why?  Well with the wisdom of much time, it appears the quasi complementary outputs might actually work and sound better.  Or if not better, perhaps not enough different to worry about much.  And that was what I myself though I had heard in 1975.

But thinking back, something else is pretty clear too.  My high end audio friends were a bunch of snobs.  That may have rubbed off on me also, but I think I wasn't really much of a snob until I myself worked at a high end store in San Diego and learned the terms "mid fi" and so on.

(As far as quality construction, the very top Sony equipment starting with the F line like the 3200F amplifier and 5000F tuner, then the top ES models up until the last super CD players, such as the X779, are models of superb construction and, I believe mostly, longevity, etc, far surpassing my Marantz receiver.  The two earliest SACD players, the SCD-1 and SCD-777ES were overbuilt to the max, but I believe certain design problems led to them not having the same reliability as earlier units.  Since then, Sony and Pioneer (Sansui sadly no longer exists) aren't what they used to be.  While perhaps the SCD-5400ES player (is that still sold new?) was the best performing SACD player if not CD also, and CD about as good as Sony has ever done (I have a peculiar like of the 1 bit interpretation as opposed to the sigma delta, so I don't believe this, and I think the final X779 and 707ES may have been better for CD's), but they are not overbuilt in the same way as that now golden era of Sony products, perhaps 1966-1996.  As far as Woodside, the 20B tuner was a marvel, though not always appreciated, but a friend of mine had a Model 23 (?) receiver, the smallest one, and the FM tuner it had was terrible compared with my 2270 being perfect--then.  It was then that I no longer regretted, though I should have, getting one of those...they looked fancier and were made in USA, but Woodside models weren't available for long.  I'm not sure that any were any were even as good as their Japanese counterparts, except perhaps, the Model 19...oh oh boy did I lust over that for a year before finally deciding only to buy the 2270.)

Wizard on the Hill

John Iverson was the greatest story teller of all, but I've got my little story to tell about him.

A very geeky friend of mine was building DIY transistor amps in the early 1970's, I think he had his first one in 1973, while we were still in high school.  This was cutting edge stuff at the time.  His first was modeled after the least powerful SAE amplifier, designed by James Bongiorno.  He bragged about the leading design principles.  Bongiorno did become well known soon as one of the top audio designers.

By 1975 or so that friend was actually working for James Bongiorno, in the original G.A.S. factory in Van Nuys, CA, middle of the San Fernando Valley.  I was no longer living in The Valley, but had come by from Pomona College near San Bernardino.  My friend introduced me to James at that factory, who was busy wrapping up some piece of work.  The great James made some joke, I think he had already thought of the name Charlie the Tuner.


James: "How do you like your Tuner, Charlie?"

Me: "What?"

Friend: "Oh he's making a new tuner, called Charlie.  It's some kind of joke."

Me: "Oh, sounds cool.  I like good tuners.  I liked those ones with scopes.  But also that have good sensitivity."

Almost three decades later James would be arguing at me about tuners on the FMTuners group.

Anyways, that same friend left G.A.S. and lived with me during his first year at Cal Poly (and my second-to-last year at Pomona College).  And by that time he didn't think James was necessarily the very smartest designer, though he had done some good things.  The very smartest designer was John Iverson.  And that was not only his view before long.  Iverson quickly became famous for the A-75 amplifier, and rumors of the unique massless Force Field speaker which some people got to see.  Then, later, well known for the EK-1 strain gauge phono system, which got rave reviews but was not easily found or purchased.

*****

During my high school and college years when I was living in Los Angeles I sometimes dropped into Mel Schilling's Music and Sound store, which was just on the side of my neighborhood in Woodland Hills.  Little did I know Mel was or would be a parnter with Iverson (he certainly was by the last time I visted M&S in their larger store further down on Ventura Blvd).  It seemed to me all the interesting people and things were on the East Coast.  That's where I thought Marantz was (though they had moved to the Valley before I entered High School...and before I finished High School had moved to Chatsworth, just a few miles from where I lived).

So while I thought all hi fi production and cool people were on the East Coast, such as Marantz (I believed them to be in Long Island City or Woodside New York) in actuality, they had started moving to my home city of Los Angeles while I was till in elementary school.  While I was in college they opened one of the largest manufacturing buildings ever in Chatsworth California, not far from my childhood home of Woodland Hills.  Also in Chatsworth later were G.A.S. and Iverson's Electro Research (I had no clue where that was, once again I figured far away).

Iverson was the mythical wizard, my friend spoke of (sometime later telling me about the truly magical force field speakers, sometime later after they had already been dismantled, but I was scared to go near them anyway), and before long I was reading brochures about things like the amazing EK-1 and A75, they looked and read so cool, and reading praise of these things by J Peter Montcrieff in the International Audio Review.  Mentions may also have early occured in Stereophile if not The Absolute Sound.  Most of these review rags had said barely anything about Bongiorno, for example, who himself often claimed he virtually invented the modern transistor amplifier complimentary ("symmetry") circuit.  (BTW, a friend of mine and I and other seem to be thinking quasi complementary is still better anyway, for various reasons, even transistors today decades later aren't really complementary enough.)  But Iverson seemed to get lots of buzz.  But perhaps it was all because I visited Music and Sound and Mel Schilling was Iverson's partner.  I had no clue about any connection between Mel Shilling and Iverson.  I though Iverson lived on the east coast, where all good hi fi came from, I thought then.

Anyways, despite living near the Wizard for years, and haunting a few of his haunts, knowing some people he knew, I never met the Wizard while I lived in LA in the 1970's.

I met him in the beginning of the years of his decline, we all know now, his desperate national tour after after giving up the big factory on Empire Blvd. in Lake Havasu (the desperate tour did pan out fairly well, actually, and his boat was beginning to rise again before his biggest fall a couple years later).  All seemed fine to me (not paying close enough attention).  Iverson had come to speak at a meeting of the San Diego Audio Society, of which I had been "President" for a decade.  (I didn't actually do much more than print and mail the meeting announcement, after being told by a friend that Stereo Unlimited had scheduled a speaker.  No, we didn't use email or the internet in the 1980's.  I never had to round up speakers...they just appeared largely through the efforts of Bruce at Stereo Ulimited, or a friend of mine,...and they included many of the greats.)

It was one of the most memorable meetings.  I was almost...almost sold on getting an Eagle 2A right on the spot.  I can't remember what discouraged me.  Perhaps I thought an Eagle 7A would be even better.  I couldn't make up my mind because I felt I didn't need the power of the 7A but I would have loved the higher bias.  Of course I also liked the lower price of the Eagle 2, and people seemed to praise it more.  Anyways, I didn't buy anything, but I found Iverson to be the most charming speaker, and especially when he invited everyone in the end to an after-meeting dinner.  That may have been the only time that ever happened.  And it was the most fun part of all, while Iverson continued to tell one far out story after another, of which I sadly can't remember anything.

I liked Iverson a lot, I thought.  He wasn't the scold I had imagined (Bongiorno, for example, I would describe as prickly, for as much as I respected Bongiorno I could hardly avoid setting him into a rage of some kind by showing the tiniest questioning of his authority), instead Iverson was just a really cool guy you could say anything to and he'd have a great answer.  And that's probably part of why my friend thought he was the smartest also. But despite both my friends (my high school era friend and the one at the audio meeting in 1988) being Jewish, neither mentioned any trace of anti-Semitism.   Only now in 2016 I read about Iverson and hear him described as flaming bigot, anti-semite, and so on.  I never saw that myself, though I did hear the some of the libertarian right wing anarchist side in 1988 (it seemed to me then he wasn't nearly as far out as many), and it may have been that part which was central to his undoing (as one who strongly avoided paying taxes, for example, leading to increasing avoidance of and encroachment by authorities, I think that he would not contact the police when perhaps he actually needed police protection, as might well have been necessary in his final predicament).

Another friend of mine who was actually at this 1988 meeting wasn't so impressed.  He pointed out that he was apparently living in a motorhome with his wife and business manager and selling product out the back door, and consumed two bottles of whiskey during the meeting and dinner, and was clearly lying about a number of things and worst of all didn't understand my friend's own wisdom.

Following Iverson's stories, one might have imagined Iverson on top of the world, with his own private jet and so on.  I figured that was exaggeration, but I never imagined him on hard times.

Anyway, I'll concede to the frequent consensus he was a technical genius, sadly never one who reached his potential, and a life-work sadly ended at least partly due to a dark side of himself, and the cruel world itself, but I also think of him as a kind of maniac, who toiled with an extreme manic intensity to produce highly polished and perfected designs.  He was a child prodigy at transistor circuit design, and could get complex arrangements working just right.  I think he highly polished the designs he came up with, but not to say he found the best possible designs necessarily, it seems there is always an even better design because the design space keeps expanding.  But he did some things right that others have had a hard time duplicating, and it seems nobody has really duplicated or bettered the Iverson thing in an Audio poweramp.  It was a special and so far unique magic, that required a particularly manic design and construction, and required a somewhat maniacal designer.

Comparing his 7A to Ampzilla I realize that the use of servo loops, for example, isn't entirely optimal, though I consider it far better than the usual capacitor coupling (or worse, transformer coupling).

While servo loops isolate the capacitor a lot, it's still sloshing around and messing with the low and other frequencies, and it's no way to get to a "null" of amplifier vs source, for example, which I suspect Iverson was interested in, as many were up until the 90's.  Much better simply to have correctly and thermally balanced push pull, which Iverson always achieved by design, careful matching, hand assembly, and fine tuning.  Of course the best of all is intelligent designs, and some believe the most intelligent design makes adjustment unnecessary,  but I think there is usually a performance price to be paid for that, and if your goal is to reach the very best performance--a true null not just an approximation--you cannot rely on slowly approximating solutions.  Then if something needs to be right on, to reach the very peak, this may require adjustment.  (Now it seems recent designers like Nelson Pass have created circuits that are inherently balanced and don't need any adjustments.  I think that's part the progress of cleverness--in particuar Nelson has been very clever, and part the giving up on the pursuit of the nth degree of nulling, and part the luxury of huge available chassis dissipation made possible by ten to eighty times pricing, most often for far less available power.  Even with amplifiers weighing 10 times more Nelson claimed (until the recent .5 upgrade, followed by even better .8 upgrade) not to be interested in creating "arc welding amps."  So you might not be able to arc weld with a 300 pound amplifier, but the 30 pound amplifier from Iverson will arc weld all you want, and still not sound "transistory" with a smooth and wide open top and a warm middle.  A classic "transistory" Class AB hard, steely, cold, clinical soudning amplifier would never become the choice of J Gordon Holt ever.  So you get both the arc welding, and the refined sound, in the original $895 30 pound amp from Iverson.  In 1985.  A miracle!

I further speculate that Iverson may have been reaching for a particular possible design that gets you to null rather than just "good."  And that would be the upper end of the Baxandall optimal feedback.

Baxandall determined in 1960 that negative feedback doesn't actually reduce distortion overall unless applied in the correct amount.  Incorrect feedback actually increases distortion, though it often pushes distortion up to higher and higher harmonics, for example, so sometimes with limited bandwidth measurement miss this.

The correct feedback for the simplest transistor amplifier is either 4 db or less or very high, 30dB IIRC.  Most transistor amps err with feedback somewhere in between.  Some of the better ones use simple circuits and low feedback, but not necessarily low enough.  The 4 db would apply to a single transistor, with multiple transistors the optimal feedback would be even lower.  And on the high end, a more complicated amp might need even more feedback--if it could be applied.

Now you can go after the high end of optimal feedback.  But it's very hard because the open loop amplifier has to be very linear and very stable to handle such a high feedback level.  It takes very careful, even maniacal design.  And that's how I consider Iverson a kind of maniac over these details.  And stiff, probably regulated power supplies also.  And finally he obsessed over reducing propagation delay closer and closer to zero, which seems unnecessary to me, but certainly important in feedback amplifiers, and for achieving simple good nulling results.

The one thing I am sure about is that he obsessed over his design details.  And that's partly why his amplifiers are so widely regarded as best, to those who have spent some time with them.

A few other notable amplifiers are based on the high feedback method, including Halcro and Wolcott, very very expensive.  Even those wouldn't replace an Eagle 2 for many, since the Halcro lacked the low impedance drive (really doubling down to 2 ohms, and playing at even less) and the Wolcott was actually a tube amplifier, also certainly not a low impedance champ.

Iverson was creating his super high technical performance well regarded high end amplifiers, hand built in the USA, for peanuts, in the 1980's.  Earlier products under the Electro Research brand (originally started by Iverson and Schilling) were mind blowing expensive in their day, though by todays's standard extreme bargains as the high end icons they were and still are.

The Iverson Eagle 2 was highly praised in Stereophile mentions.  It was said to be much better sounding than nearly all transistor amplifiers, with excellent midrange, highs, and bass.  It had much cleaner and more enjoyable upper 3 octaves than the Eagle 7A.  It was the personal reference amplifier of the founder of Stereophile himself, the very respectable J Gordon Holt (curiously having the same middle name as Iverson)--a person who could have the best of transistors tubes or anything.

Enough!  I have finally bought an Eagle 2 this week, and will see how it compares.  If I like it, I will have it upgraded by Russel Sherwood.  My hope is that it doesn't have the usual Class AB transistor amplifier sound (though I've seen many offhand statements to that affect, always by non-owners, owners always describe it as far better than other transistor amps in midrange presence and high frequency smoothness).

One way or the other the wizard was lost, not long after I finally met him.  But the legend, and hopefully the amplifiers, live on.  (BTW, I wonder about the intended buyer for Iverson's special gun. If such a buyer was intending the gun for criminal activities anyway, why not one more?)

Class AB I believe has an inherent "bright" or "thin" sound once you get past the Class A portion.  Feedback, especially the low baxandall optimal feedback, can't entirely eliminate this.  But high feedback can, at least if you clean up the notch itself, and use reasonably high AB+ biasing, Class AB  with high feedback can duplicate Class A sound I believe.  But few designers push the feedback high enough.  I speculate that Iverson did, he pushed the open loop gain and feedback higher than anyone, and this gives his amp a peculiarly good sound.  He's not getting stuck in the middle feedback morass.

The Eagles also run pretty high bias.  The 7A has considerably more bias, but the 2A is better anyway I gather, and is simpler and more effective being non-bridged.  In fact I remember Iverson and company steering me to the Eagle 2, Iverson spent all his time demonstrating the 2, and then seemed to think that was the best for me.  I wanted a low power amp in the Eagle 7 case running higher bias, stubbornly, couldn't decide.

You can achieve very nice low distortion with low feedback with Class A, or at least Class AB+ in most cases, most of the time.

Here's interesting info on the EK-1.  I regret not snapping one up when I had the chance.  A friend wanted info on adjusting "phase compensator" controls.  According to this thread the Singapore factory did so with a square wave test record, according to one owner, who did the same and was happy with his.  This does not guarantee that this method was approved by Iverson.

What Iverson has designed in the EK-1 is clearly a pair of discrete op amps, with associated circuitry, to implement a strain gauge preamp.  Nothing all that magic, the question is how good are his discrete op amps?  One number I hear claimed was better than 125dB S/N with inputs shorted.

That's pretty good, and that was actually just a guestimate anyway, and I've heard the opposite claim that EK-1's are noisy, and that might be true in some circumstances.  It appears that the designers use of  a solid stainless case is not coincidental.  With the kind of power Iverson is running in those op amps, one might expect high RF succeptibility.  The stainless steel probably works in most cases, anyway, that's one explanation for claimed variability.  Another is states of misuse and disrepair.  And, of course there is always: people make stuff up.  So what to believe?  I'm pretty open to believing it works well for some, not so well for others.

Anyway, nowadays one wouldn't do this sort of thing.  One would simply use the best available chip op amp, which is still the OPA 211, which has a hybrid silicon germanium process.  It can't be beat with silicon transistors in terms of low noise, distortion, etc.  And then an ordinary machined aluminum case would do, and if executed well, it would work for anyone.

And one would apply the best available EQ, well understood, the reverse of the RIAA, flat from 500 Hz down and flat from 2122 Hz up.  Fine tuned to each cart, that's what Iverson did and it's still good.

So many shortchange things, like saying you don't need EQ with strain gauge cartridges, and get away with none, or a highly uncompensated response, bass heavy and all.

It becomes so common, it takes unusual strength of character to stand out sometimes and do things the right way.  Iverson had a lot more moxie to do things to the nth degree than most at the time.

I liked Iverson so much...he had that charisma that some people do...I wanted to join his circus kind of feeling...I could so easily get an Eagle 2 to replace my stupid frying-sound Citation II.

I remember the moment of feeling like Iverson and Munro were family, after having talked about the two Eagle amplifiers and which would be better for me.  Having easily given up on the very kindly sale due to my indecision, Munro asks if I'm bothered by Iverson after he walks out.  No, I say.  I feel afraid for him.  "Tell that to John," she says huskily, smiling, turning away with a cough.

But the feeling to buy an Eagle amplifier, which few Americans realized they had the chance to, passed.  I think shortly thereafter a friend pointed me to the same of an Amiga 2000 computer with all accessories.  I jumped on it, which led to endless friends and clubs still going on, through my "exotic computers" club.  So with all that going on, I forgot about the Iverson amplifier, and when I had money again to consider buying somthing, I didn't much believe in amplifier differences, and besides the HCA-1000A got good reviews, seemed to be designed well by John Curl, so I got one of those.

Now I wonder if it isn't at least fool around with the hypothetical idea that amplifiers do sound different, and what special magic Iverson achieved, which I even vaguely remember to this day.  The HCA-1000A is still good, the HCA-1500A better, but I think I like the Krell FPB 300 still better, what else is there?

*****

I also specifically remember feeling a bit worried about Iverson hearing him describe some of his exploits and anti-government feelings and the Audio Society meeting and afterwards.  I remember being very worried that this wasn't going to end well for him primarily, though sometimes he made me fear from the government also, that didn't worry me as much.  At this point I find it very hard to believe that Iverson lived much beyond the disappearance.  I think he desperately wanted to make a likely illegal sale in order to raise funds to free himself from debt, or to make a new expansion, in one way or another most likely to save his company and his career, just like DeLorean famously tried.  But Iverson's buyer was too unsavory, and prepared to just ambush him and rub him out.  The buyer wanted the gun for personal reasons...as a doomday style weapon at their own residence or venue, where they could let loose and kill lots of people, but not worry about all the lead in the next days.  Almost certainly a mobster type, American or not.  I think the real military could find someone else to make the gun if they wanted it.  Mobsters have to go with what they can find and trust.  And really really don't want anyone else to know or be able to buy or duplicate it.  Iverson was good, but his unique genius at such things has been often oversold, I believe.  He was just willing to go farther out on his own for audio amplifiers than most others.  And a few other things.

Iverson was a shining light, I don't think he could hide for so many years.  And I don't think he really was the James Bond he portrayed himself as.  He was son of a reclusive but curious TV repairman in LA, and a great story teller, and a brave man but possibly too brave in one instance.



*A friend speculates that Bongiorno and Iverson simply copied circuits from the semiconductor manuals.  That is precisely what Nelson Pass said was his training, at ESS he read the semiconductor book and thought about all the circuits they showed, something for every occasion.  Nowadays the deepest design is still done by semiconductor designers, the ones who design integrated circuits.  But this is indeed a brand new world of powerful design tools and so on.  Not a world of wiggling leads.  Iverson was the man for and of his time of discrete circuitry, for doing his kind of thing, making miracles.  As always it wasn't individual circuits but a particular combination of them set just right which made Iverson's amplifiers great, and most ultimately in the Eagle 2 series because it wasn't an overreach but the simplest Iverson.  Likewise Bongiorno, there may well have been some cleverness in his combinations, but I do find his audio work prior to the 21st century to be not quite as good.  (I suspect he did far better in his post-millenial products than before.  The original Ampzilla and others were clear and dynamic amplifiers but lacking the smoothness and midrange transparency of Iverson's amplifiers, starting from the jaw dropping A-75 if not before.  It was no random thing that Mel Shilling co-founded Electro Research with him...he was clearly the best.

Iverson was the golden boy, and widely recognized as such at the time, and he probably could have had the world if things had worked out better.

I remember the praise J Peter Montcrieff of The International Audio Review heaped on the Robertson 4010.  They invented a whole new kind of power, and found that the 4010, a relatively low powered amplifier, had 1,000 (or perhaps 10,000) watts of Moxie Power.  In contrast, some other 300W amplifier was found to only have 3W of Moxie Power.  This is related to the increase or decrease of power into lower impedances, I believe.

The Robertson audio products were made by the Singaporean firm named Robertson that had been signed up to build the EK-1.  At some point, Iverson and Shilling bailed, selling the firm to the Singaporeans.  (The TAS story lays much blame on Iverson's absolute insistence on a cast Stainless Steel chassis, and not being satisfied with the castings.  This dragged out the introduction of the product for a long spell and jacked up the cost.)

It is not clear to this day whether the Robertson 4010 was actually designed by Iverson and obtained legally in the sale of the company, ripped off from Iverson by Robertson (as some have suggested), or a similar amplifier created using only the ideas of Iverson, whatever they actually were, as told to or figured out by the Singaporeans in the course of their interactions with Iverson.  In one way or the other the 4010 is an Iverson-like amplifier, the Moxie Power being found in all the others, probably better in the real, fully credited Iverson amplifiers.  The Eagle amplifiers were made under Iverson's supervision in one of his USA factories, the largest and most famous one being the 10,000 sq foot facility at Empire Drive, the final one being just a small building near his house.

(I called Iverson the Wizard on the Hill because in the Lake Havasu subdivision where he lived, which wasn't necessarily over opulent, he nevertheless had a somewhat unusual lot that had a small hill that the house was perched atop.  Such goodness for an audio guy.  I have the similar fortune of having a large setback to my house, and no nearby house having facing windows.  This makes for being able to be playing music at reasonable levels without anyone nearby even noticing.  A hill would be even better, and Iverson would have needed that.  Back in The Valley days he may have lived on a hill as I did and as did many people.  And then also, it just sounded good.)

Aragon Preamp

Recently I've been using the Aragon preamp merely as a source selector, just using the unbuffered tape outputs, with no AC power being provided to the unit.  However, looking at the schematic, there are 2k output resistors to the tape outputs so it's not 100% transparent.  With my low cap cables that likely makes a negligible difference, but when convenient I'm going to bypass those resistors.

I prefer the Aragon preamp as source selector to the DB Systems selector simply because the Aragon is a big heavy box which doesn't move when you turn the knob.  It also looks very cool, is very heavy, and has wonderful chassis mounted jacks.  I wouldn't say that it sounds any better than the DB Systems selector though.  I thought them about the same, but the 2k resistors would darken it somewhat.

I'd always thought the Aragon 28k preamp to have a slightly "dark" sound through the main outputs also when I was using them--though they seemed to measure as perfectly as anything.  Well now I see that a darkened sound might compliment the Aragon 8008BB power amp, which I'm finding (now) to have a (somewhat excessive) brassy top end.  I'm going to be examining the Aragon and other amplifiers technically this weekend, following a week of A/B/X testing last week.  I did not reliably hear differences according to a simple ABX test, but I still strongly believe the differences I hear anyway.  The basic differences are that the Krell is the best sounding and has sweet and entirely unagressive highs, next the Carver TFM-24 which was designed to sound like a tube amp, next the Parasound HCA-1500A which was slightly brighter than the Krell but very transparent--perhaps the most transparent, and the Aragon which is sounding transparent but harsh at times.  Kudos to John Atkinson, still one of my favorite audio reviewers though I often criticize him, who admitted recently he hadn't heard a difference in an ABX test.

A great test disc is the Tchaikovsky Pathetique on RCA Living Stereo hybrid CD.  Just after 10 minutes into track one, there are some biting violin riffs that can be positively strident with some amplifiers.  It sounds more or less OK on Koss ESP-950 electrostatic headphones, which produce the most  transparent and accurate sound I have.

I'm not sure if I like the Aragon preamp circuit at all, but it's apparently the same as in Aurum preamps (in Aurum they use 2 per channel because it's a balanced preamp).  It looks like a miniature power amplifier all bipolar with feedback and adjusted bias.

Here's a discussion on modifying the Aragon Aurum preamp.  It includes a crucial section on how to do the bias adjustment on all Aragon preamps.

I've been thinking of modding mine in a few ways, adding balanced inputs and outputs and making the selector deal with that.  The balanced outputs would be pseudo-balanced with ground as the negative.

That would allow me to hook up balanced device(s) and run balanced to Lavry ADC, rather than doing an unbalanced conversion right at the Lavry input jacks with adapters.  Then I could insert the ABX test box and since it would have balanced inputs and outputs there would be minimal degradation.

It would also allow me to have a polarity reverse switch on the Aragon (or the ABX).

But it would have been better with the balanced already designed in.

I would use the line amplifier only when doing gain adjustments on devices for ABX comparison.  Otherwise I'd just use the unamplified tape selector.

But doing that kind of modification, I almost might just as well get a fresh chassis, and indeed I've been looking at the many interesting clone preamp chassis on ebay.

Hacking I'd like to do

Here's a start to hacking the Sonos API.

I'd be more interested in hacking my Tact RCS 2.0 digital preamps.  Most of the "hacking Tact" groups are really about audiophile mods and don't sound interesting to me as I simply use digital inputs and outputs.  What I'd like to do is bypass the system so that I can set parametric EQ's without having to select a room correction (I don't actually use the room correction feature).  And more conveniently change system polarity with one button press.  And so on.  Making it more useful to me bit by bit.  Maybe add tone controls, something more like Cello Audio Pallete.  Mark Levinson's latest company has a software Pallete but it requires a computer.

PCM 1704 still the best chip

A comparison of several top sigma delta converters and the venerable PCM 1704 puts the 1704 on top.

Nowadays, however, it may be possible to do even better but at even more cost building your own flash converter, from FPGA's or something like that, as MSB does, and I think one of the Swiss high end companies also.

As recently as 2013, Lynn Olson of Positive Feedback was still saying the PCM 1704 was the best--better than the then-champion ESS 9018, the TOTL sigma delta converter still used in many well known recommended components.  And he says specifically what I say, that sigma delta converters sound like something is missing (I would say "lifeless") while the grand flash converter is full of life.

Recently I've had to take my Krell FBP 300 offline for repair and put the Aragon 8008BB back online.  It has a far more agressive high end, and can easily sound strident playing SACD's such as the RCA Living Stereo hybrid disc with Tchaikovsky's Pathetique on my Sony DVP-9000ES.  When I played the CD layer on the Pioneer PD-75, it sounded about the same (another bitstream 1-bit sigma delta converter).  But on the Denon DVD-9000, it was several notches better, much cleaner.  The Aragon is what you'd call a "revealing" amplifier but what it may be revealing is its own high frequency instability being set off by high frequency noise from the 1-bit converters.

The Guyz Behind Mondial

I put my Aragon 8008BB back online on Sunday, after a week of A/B/X testing with a Parasound HCA-1500A and a Carver TFM-24.  I've got an RMA to return my Krell FPB 300 to Krell for repair.  I miss the sweet sounds of my Krell FPB 300 and want it back online soon.  But that's no excuse for downtime.  The good audiophile must make do as well as possible with the best that is on hand.  The show must go on.  I have no patience for audiophile nervosa.

Just last week I discovered an obituary for Paul Rosenberg, the Mondial co-founder who along with Tony Federici were responsible for the great Aragon and Acurus components of the 1990's. This was also my discovery of the audio journalist Ken Kessler.  Paul and Tony sound familiar to me I think I once met them.

Ken affirms by telling the story of the big Mondial amplifiers having been designed by Dan D'Agostino.  In an alternative accounting I find more believable, the circuit and circuit board design are credited to Mike Kusiak, whose initials are one the board.  Robbii Wessen did the industrial design.

Coming from the pen of Ken Kessler, who appears to have actually had some good times with Paul and Tony, who probably heard it from them, this may be the true story, or at least "true enough" in the sense that D'Agostino might not deny it--there may have been collaboration involved.  I still find it hard to believe he got deeply into the details, I would more imagine that Dan may have proposed the basic circuit (on a cocktail napkin or something like that) and Mike and others did the layout and fine tuning.

The Aragon is a nicely made amplifier.  However, from a cold start it sounds more than a little strident.  It appears to need about 60 minutes of warm up to sound best, and even then it may be strongly on the Yang side (like the Guyz themselves?), not unlike a lot of high end audio products.  My actual Krell, however, sounds sweeter.

I have a strange memory which may not be entirely true.  I recall someone like Tony demonstrating an Aragon amp at a meeting of the San Diego Audio Society, possibly following a Krell demonstration, sometime in the mid 1980's.

Tony heard my thoughts on the Krell and said to me, "I have an amp just like what you want."  He said.  "There's no fan, it's not as expensive, has lots of Class A power, and sounds warm like the Krell."  So he led me and a few others into a smaller room, where he played some music on the amp.

It wasn't bad but was sounding a bit too bright.  Tony agreed and wasn't happy.  Then he noticed the tiptoes the store owner had put under the amp.  "I told them not to do that," he said.  "I remember Brian bragging he could put anything on top of those."  "But not my amp," he said.  "My amp needs to get warm, and it only gets warm down on the low feet.  That gives it the warm sound," he added.  "Here, I'll just put this towel on top of the amp.  That'll help it warm up."  We listened to it later and agreed it sounded more warm and better.   But I was bothered afterwards about Tony just leaving the object on top of the amp.  "Oh, you can't hurt it," he said.  "It would shut off it it got too hot.  It's got that notch too", he grinned.  I would not recommend doing this, btw.  The temperature regulation circuit might fail to act in time, especially in an ancient unit.  Nor do I strongly remember any particular person saying exactly this, but it was something that still left me a bit worried, especially using tube amps whose misuse can be very dangerous.  Now I hear others recall Paul and Tony as care free, so it seems consistent that Tony might be pretty laid back about warming up his amp.  And it now does seem they're sensitive to airflow like this, due to relatively weak bias regulation, at least in my unit (which was an early Connecticut unit, and also features the very robust original transistors).

I wasn't that tempted.  I used tube amps for midrange and bass mostly through the 1980s, which I had obtained for very low prices, and the high cost of even value amps like the Aragon seemed a bit out of my league, though by that time I could have afforded one, even with my barely affordable house payments.  At the same time, my Citation II tube amp, which sounded wonderful, often had a frying sound, and I wanted desperately to get rid of it and get something else.  But I couldn't get myself to do that either.  Finally when I was out of a job and selling my house I sold the Citation II and basically have never much looked back at tube amps.  Maybe just because I got sick of them?  I confess now I have regained a bit of interest in tube and other amplifiers recently, coincident with running various different backup amps.  Even if I can't prove I even hear a difference, as in one session of ABX testing.

When I got finally got an Aragon in 2010 as a backup for the Krell, it had very low feet, not original, actually just 1/4 inch stick on rubberized felt pads.  I ran it on top of a flat board (NOT the carpet!).  Still, it ran much hotter, 180W at idle.  When I put the Aragon on the brass carpet spikes from Mapleshade, this elevated the amp about 1.5 inches above the carpet.  The ventilation was vastly improved, and the amp temperature went down from 135F to 115F.  But part of that change was from the bias dropping.  The idle was now 120W, which BTW is close to specs.

The bias circuit is a simple transistor, no IC.  I think it has a function that runs the amp hotter when it is cold, then slows down as the amp heats up, reaching an equilibrium of sorts.  But perhaps on the tall feet it is not getting hot enough to get going.

"I told them not to put the amp on tiptoes," Tony said in my extended memory.  "This amp, mine, is biased to sound just right on soft feet.  That makes it sound more relaxed, just like a real Krell."  It does turn out that my Aragon was early production, made in Connecticut.  This memory makes me wonder whether my copy is the same one I encountered before.

After listening to the Krell for months, and then testing the Parasound and Carver amps on my Acoustats, the Aragon sounded beyond just bright, it sounded strident.  I found one passage about 10 minutes into Pathetique on RCA Living Stereo Hybrid SACD that was screechy.

I was determined to find out why the amplifier sounded different, and badly so at that.  So I first measured the Carver amp which I will soon be returning to its owner, and then my Aragon.  The Carver looked pretty good.  The Aragon looked horrible in the right channel, precisely where I had heard the screechiness.   It was showing 0.18% THD at 1kHz and 0.84% IM.  The left channel looked ok at 0.012% THD.  These were measured, in fact I did all my Aragon measurements with the actual Acoustat speaker loads.  I did the Carver measurements earlier using an 8 ohm dummy load.  The Carver measured 0.0029% THD.  I used RMAA to do the measurements with a Juli@ card on an old PC, the measurements were very contaminated in the case of the Aragon by ground loops because it has a grounded plug, the Carver only has an attached two wire cord.  I spent hours unsuccessfully trying to fix the ground loop, first by re-routing the AC connections to a single power strip, and then by bypassing the Juli@ card itself with an external optical input DAC from Schiit.  Nothing worked, the very complicated setup using the DAC only made things worse because there is still a ground loop formed by the outputs of the amp to the inputs of the Juli@.

I was about to cut out some new felt feet and do that heat raising trick by lowering the bottom of the amplifier a fraction of an inch from a flat board to block ventilation somewhat but not completely.  But then I thought, why not tackle this bias thing head on, by adjusting the bias?  Better to run higher bias WITH better ventilation also, for the best of both.  In fact, the improved ventilation made possible by the taller feet means I can run even higher bias more safely than a lower bias with deliberately blocked ventilation.

I had avoided doing this before because measuring the bias on the left channel is all but impossible, though very easy for the right channel which is on the side of the chassis.  But since the bias problem appeared to be only in the right channel, this would be a piece of cake (well, as bias adjusting goes...it's a long multi-hour process).

So I went ahead.  It turns out it is very easy to adjust the bias on the Aragon 8008BB, even on the left channel.  There is only one potentiometer on the entire amplifier circuit board, right in the center.  It's a multiturn sealed pot which couldn't be nicer to work with.  It took several attempts to turn the screw enough even to make any difference, then I discovered I could turn it around several times to achieve the desired result.

You measure the voltage in mV across the bias resistor for every transistor.  Aragon specs put these at 12mV for the inner channel, and 8mV for the outer channel.  At first this bugged me a lot, but now I understand...you set the bias so both channels reach the same temperature.  That's actually very important when you have a stereo amplifier.  When the channels are thermally balanced they will help keep each other at the correct point.  When they are not thermally balanced there will be a thermal bias oscillation where one channel gets hotter then cools itself down, followed by the other doing the same thing.  The larger the thermal imbalance between the two channels, the greater the thermal oscillation.

Only after much turning of the bias pot on the right channel did I get around to hooking up the now quite complicated measurement apparatus, which also took very precise location of the spike-footed amplifier to reach all the necessary cables, especially the speaker load through the Canare Four Cross speaker wire.  I aimed to achieve a bias like I had before of about 0.26mV, with a heatsink temperature which would now be lower than the 135F the amp had idled at with the original low feet.

The measurements came back excellent!  The THD distortion dropped from 0.18% to 0.0048% and the IM dropped from 0.84% to 0.036%.  Once again I think the real numbers might be even better if I could remove the ground loop problem.

Now however I noticed that the left channel wasn't getting warm enough any more, and sure enough its distortion numbers had gone way up.  So I had to readjust the left channel up to get the distortion down again there, though it seemed that once I got the left channel THD down to 0.86% simply adding more bias wasnt help much.

So I removed the cover of the right side wondering how hard it would actually be to change the bias, if not measure it.  It turned out to be remarkably easy to change the bias by turning the sealed multiturn pot in the center of the board.  At this point, I didn't need to check bias, I could measure the distortion.

Finally I idled the unit for hours, and decided several times to back down when the temperature in one channel or the other exceeded 125F, or if they appeared to differ.  I tried to adjust both channels to the same long term operating temperature.

After a 3 hour idle test, temps ranged 120-130 in the right channel, and 120-127 in the left channel, with low temps in front and highest temps in the middle.

I decided that was close enough, perhaps hotter than desired...but I need the Class A for good sound.

Which is indeed enormously improved.  This is still a Yang sounding amplifier, but very clean now.  Not as warm as the Krell but a lot closer.

DSD in blind testing

Here is the abstract of the report on blind tests done on DSD in 2004.  DSD was apparently compared with PCM 176/24, and participants were unable to hear a difference.

Of course this cannot in principle satisfy anyone who takes the "listen for yourself" advice seriously.  That advice is an nothing more than an invitation to be conned by slight of ear.  If somebody bothering to make a "superior" sounding system cannot PROVE it sounds better to at least some people if not everyone, they ought to just quit.  And the only way it can be proven is through repeatable blind test results.

This is especially true if we are not just talking about one person's private sound reproduction system, but a scheme to be generally to be used in recording and distributing music.

Squeeze Me Together

I've been enjoying high rez music through my Logitech Transporter SE, recently purchased as NEW! on eBay.  Logitech released these SE units, lacking the famous selector knob with force-feedback, in 2011 at a somewhat reduced price from the original model from 2009.  Many complained that Logitech was selling off the spare parts that might be required to repair the original Transporter model, which had been ingloriously discontinued.  But few complained about missing the selector knob.  Most users used it a few times, but then settled in using either the remote control or the Squeezebox web interface or both.

Squeezebox had been an independent company, for some reason then bought out by Logitech who didn't seem to carry the concept very far.

Actually, IMO, the concept doesn't need to be carried very far.  I'd be happy (at least for a start) with a simple Ethernet to SPDIF device.  I imagine such a thing could be built to high standards and still sell for less than $100.

One thing, you do actually need to decide which files the SPDIF is going to be outputting.  So, essentially, you need some kind of "player" (which selects file, plays it, allows for pause or stop).  However, as I understand it, this functionality exists in the Squeezebox software itself, now Open Squeeze.

Now at least I've got the bill filled, plus lots of extras, with a real Transporter.  It turns out some of those extras are at least temporarily useful.  For some reason my Tact processor isn't accepting 88.2kHz inputs, even over coax or AES/EBU from the Transporter.  It does accept 96kHz inputs.  And, it turns out that 88.2kHz is quite a popular high rez format.  So as I do with all my various disc players, I take the (extremely high validated quality) analog output from the Transporter, and resample it to 96kHz with my Lavry AD10.  And, as that always does, contrary to audiophile purism, it sounds marvelous.

Sadly, at this time I don't run balanced all the way to the Lavry…none of my selector devices permit mixing single ended and balanced inputs.  I now intend a large rebuild of the Aragon 28k to turn it into a mostly-passive selector device which would take both balanced and unbalanced inputs, and send a single unbuffered balanced output to the Lavry, with polarity reverse and other features.

However, in that longer term, I should also fix the Tact, and/or exchange the bedroom and living room Tact devices (because I think 88.2kHz still works in the bedroom unit) so I can have direct digital (though I suspect it would be Very Hard to tell the difference).

Anyway, though I don't need it now, here's a discussion of how to replace the Transporter using a Raspberry PI.

You need:

1) Raspberry Pi 2, Model B
2) Hifiberry DAC+ or Hifiberry Digi+ (spdif output!)
3) 5v, 2.5A power supply micro-USB
4) Case
5) 1gb microSD card
6) Picoreplayer (software)

Apparently Hifiberry and Raspberry snap together.

In the week since I ordered the Transporter, apparently Google and Amazon have figured out what I want better than I could.  Now I'm seeing tons of "network players" that might work nearly as well or perhaps even better than the old Transporter.  I have some fondness for the actual Transporter however because (1) I read about it in Stereophile and was incredible impressed by top notch measurements in 2009, (2) that high performance has also been validated in measurements and usage by Archimago, and (3) it uses an allegedly open source Squeezebox software.  The two downsides are that resolution is limited to 96/24 (well that's the same as my system anyway) and that it doesn't support DSD or DSD128 (which would be handy to have to play-into my system through analog reconversion).  I had been thinking about getting an extra gadget, a Pono, to support DSD but it would require the new preamp to use the best-quality balanced outputs of the Pono.  Also the Pono would let me experience first-hand the moving-average type reconstruction filter Ayre has developed.  And that brings to mind a 4th limitation of the Transporter: it doesn't have optional digital filters.  (Now I can imagine a more "ultimate" network player as being one that supports HQPlayer.  However, most machines capable of supporting HQPlayer are going to be troublesome "real computers" that I'd rather not have to deal with.  HQPlayer is part of the emerging player standard of Computer+USBDAC which I am diametrically opposed to.  Somewhere I think the creator of HQPlayer might describe a DIY network player+DAC.)

Anyway, here are some of the alternative players:

Pioneer Elite N-30 (listed for $250!)
Teac NT-506 (does DSD128 and may have variable filters, high quality clocks and clock input!)
Arcam AirDAC

Great American Sound

One of my best friends worked at G.A.S. for awhile, he introduced me to James Bongiorno in 1975 (I also met him later a few times) and Andy Hefley (who became one of the principals after James left).  All very interesting experiences.  I never bought a G.A.S. product new, I was a student at the time and they seemed too expensive, but later I did buy a second hand Son which developed an RFI problem I never fixed.  Having seen my own GAS unit have troubles, and other troubled units, I often wonder whether reliability was the actual issue which brought them down, not one of many others which have been alleged.  But the split between the founder and the remainder certainly didn't help either.  At the time of my last visit to the GAS company in Chatsworth, only a few miles from where I had attended all my primary and secondary schools, James had already left the company and GAS had installed Wave Soldering machines all over (this led to their debt problem I suspect).  Wave soldering was a very new thing at the time, and can itself lead to unreliable construction if not done correctly--which may not be easy.  OTOH, I suspect they went to Wave Soldering after the regular soldering wasn't going very well either.  Andy was doing the final feedback loop adjustments on his prototype Godzilla, which that evening we played at his beach house...

Given the charismatic James, it's no wonder every G.A.S. product was iconic and worth learning about even if his associations or companies didn't last long.  The first amplifier, Ampzilla, has been said to be one of the most copied transistor amplifier designs.  James seemed equally proud of the Thedra preamplifier, which near the time of his death he said there had never been anything like it before or since and he'd never use anything else (he did also create a new generation Thedra, much different, during the 2000's).

Here's a copy of the Gassette, a flyer from GAS describing the Ampzilla and the Thaedra.

So what's special about this?  Many things have been widely copied, others not.

1) Separate MC and MM phono stages (no transformers!)

2) MC stage has servo loop (uniquely so as far as I know)

3) Line (tone) amp also has servo loop

4) DC coupling enabled by servo loops (no coupling caps!, except in phono)

5) Tone Controls with far-out hinge points (160Hz and 4kHz)

6) Tone and Volume controls use stepped attenuators using sealed metal film construction.

7) Low filter is Bessel type with 5 positions.

8) Very low impedance line outputs

Like many before his time, James insisted that preamps should have tone controls.  His final redesign of the Thaedra had the most flexible controls he had ever done (though still not quite what the Cello Pallete offered).

That's very interesting I think now.  Personally, no gear that I have owned has had easily accessible tone controls (I'm not counting fancy DSP) since 1979, about the time JB left GAS.  Before that time, I had been using a Marantz 2270 receiver which did have tone controls which I never used, and it especially irked me that the "stepped" controls were not stepped at all, they were using ordinary pots with a mechanism which made it seem like the controls were stepped.  Indeed when set to "flat" the Marantz 2270 controls were not exactly flat, as you could easily see on a square wave.  And there was no tone defeat switch.  I couldn't wait to ditch the Marantz once I discovered this.  Actually this kicked off a year of experimentation as I tried different IC op amps to replace the line amp in the Marantz.

If only I had bought a Thedra preamp, it might have been all different.

Amplifiers ARE different

Krell FPB 300 has been taken offline.  Amplifier a friend says may be the best I have, the Parasound HCA-1500A, essentially the brilliant Curl amplifier design that has lived on through upgrades in the 3500, JC-1, and beyond.  Curl knows what he is doing, we agree.

It doesn't take long for me to hear the difference.  It is brighter, more forward.  The image comes slightly in front of the plane of the speaker.  And deep images are just as deep, kind of beyond forever in the background.  The Krell has a more subtle depth, what I call "layered," which seems more real.

About the brightness too, one might consider it "more open," or even "more transparent."  But I feel the Krell is equally transparent, just more subtle.  The Parasound, to me now, seems artificially lightened, as if someone turned on the stage lighting.

I'm thinking this comes down to You Can't Fool Mother Nature.  A low bias Class AB amplifier is not going to sound like a Class A amplifier with enormous reserves.  The Class AB no matter how well designed is ultimately going to give away the cheat.  (And it could be that my 20 year old Parasound needs a bit of a tune-up, to reduce the crossover notch.)

And the regulated power helps too.

I need the higher power too.  By my meters and calculations I have already put the Parasound beyond its ratings and probably it's limits, discovering the huge musical crash that starts "Her Majesty" on Abbey Road.  The Parasound clips at a lower input voltage which my DAC can exceed, but it also has more gain so I may have been playing louder than usual too.  So without adding more gain to the Krell, the actual limits are about the same, but I can't push the Krell into clipping or near it, and I can with the Parasound, adding an extra barely noticable strain (you can't tell much in the Her Majesty crash).

But this could be all wrong.  I'm not setting up my ABX test rig to test Parasound among other smaller amps until I get the Krell back from repair.  If I feel safe about this, I may ABX the Krell then also.  Hooking the Krell up has always seemed too dangerous though it has always been in the hoped future.

This is threatening the universe of magic to collapse for sure.  But I know how to understand such things.  My suspicion already is that I won't be able to tell differences.  But that won't mean I'll concede nothing matters.  I'll just know I've found a real subjective/objective anomaly, a doorway to greater discovery.

My long term plan has always been to have ABX available at all times.  Preamps IMO should have ABX capabilities built-in, and that's what mine will when I get around to building it.  For starters all inputs should have adjustable gain using the lowest noise amplification if needed, OPA211's.

I'd been playing the marvelous Saint Saens on Reference Recordings on SACD on my 9000ES.  After a month offline, I hooked up the Denon DVD-9000 and stuck the disc into that machine to see if its CD layer was HDCD.  Indeed it was, and I played that.  Did not sound anywhere near as good as the SACD.  Grainier, lumpier.  I'd never heard that same grainyness on the Krell.

So the Parasound is making SACD sound relatively better, and R2R sound relatively worse.  I'm also suspicious of the CD layer on that SACD.

Vintage, and why Magazines cannot be Truthful about It

Well, Stereophile is "trying."  Art Dudley is a vintage equipment expert who talks about specific equipment a lot (like Quad ESL 56's) but for October 2016 writes about vintage in general.  And it's mostly a bunch of nonsense, as is what I'm going to say.

I guess you could say I'm a vintage guy also.  I have systems filled mostly with equipment from eBay and Audiogon.

This is NOT because I have any particular love of a era-specific vintage sound (though I am interested in certain things, such as the sound of 1-bit converters doing DSD which I think is "the real thing" not obscured by fancy sigma delta DAC operations, as described here:

I suspect many delta-sigma converters with a DSD input might not be switching all of the switches from 0000 to 1111 at the same time, which would emulate a Sony-type, late-Eighties single-bit converter. It might sound a little weird, but a simpler method would be to internally translate DSD into short word-length PCM patterns, taking advantage of the dynamic range of the 5 to 6 switches (partial analog, as it were).

No I buy pre-owned equipment for several obvious reasons:

1) Affordibility!  The Krell Amp I paid $2700 for originally (actually the tab is about $4000 so far, including the first repair in 2010) cost $9000 when new in 1998.  But forget that, the most obvious brand new replacement now costs $36,000 (the Momentum amp in Stereo)!  There's been enormous aspiration inflation in the super high end.

I'm now faced with needing the second Krell repair.  I'm partly hopeful that they didn't fix it completely the first time (I think they didn't bother much with the left channel last time but did great job on the right channel) and once they've rebuilt both it should be more reliable longer.  Often this happens...once something is finally repaired completely--often taking multiple attempts--it then goes for a long time without needing more.  But you have to be persistent and willing to pay.  Looking at the alternatives...I will need to pay.

Sure there are other $6000, say, brand new amps which would fill the bill also.  But they wouldn't have the ingredients of the FPB design in the slightest.  No Class A operation to hundreds of watts.  No zero feedback output (or so I thought until today...now it appears there *is* feedback around the output drivers...so this means that the Krell has nothing on Pass Labs or the Class A Levinson designs...which I will may someday consider).  No servo loop DC coupling.  No full power regulation (Pass's XS 300 has that...for $85,000).  I'm not even sure D'Agostino's Momentum offerings have all these features*, and maybe I could live without some of them, but I don't know, I also think these features are part of a very special magic I need to drive my Acoustat speakers.  (*I haven't seen that the Momentum amps use something like plateau biasing, but if he says they are Class A they must.  Pass, of course gives you more like old fashioned Class A with his XA amps...with the bulk to match the wattage!)

2) Selection.  A lot of great stuff isn't made any more.  Or if it is, it's just way out.  Such as, say, direct drive turntables.  Some believe this is a better approach.  (Actually, I merely think it is interesting and might be better in some ways.)  OK, I could buy a VPI direct at $30k, or a Continuum at $90k, or a Rockport at $150k.  Gone are the days you could get TOTL Pioneer for $2500, or a decent Sony for $500.  But many of these classics are still on eBay, sometimes working sometimes not, for about those same prices, and some of them can be fixed.

3) Buying experience.  I do know and understand the high end audio buying experience.  I've bought my first high end item, a pair of Infinity Electrostatic Headphones, at Mel Shilling's Music and Sound in 1975.  And that was after years of visiting audio salons in Woodland Hills, CA (where there were 3 major high end audio stores when I was in high school).  I've been to high end salons countless times, though less recently.  I even worked in a famous store, Audio Directions, for a year (as a technician, but of course everyone in a store is also a salesperson).

And...I hate it.  You have the limited selection.  You have the factory mega high prices.  You have to make a decision in a limited time, and you can't really tell anything, even when you have very patient and unsnobby salesperson (I will concede audio salespeople today are generally far nicer to me today than in 1974).

I will have to concede that in it's totality Im not totally fond of the vintage buying experience either.  First there's the enourmous fear (will it actually arrive?  will it be rusty junk?  will it smell like moldy tobacco?).  Then, sometimes some of those fears come true.  Then, even when they don't come true, you have only have a few carefree years before needing expensive or even unobtanium repairs.

But the high is looking at the stuff in endless pictures, reading about it in endless blogs, arguing about it, showing it off, taking your own pictures, and, sometimes, actually using it.

The real advantage to the vintage BUYING experience is that you can take your time, and nowadays you can do it anywhere--because you can do most of it online.  To be honest, I don't miss the brick-and-mortar salons that much.  Even with nice salespeople, they are still salespeople.  I'm sorry, but I prefer to think about things, and especially   make buying decisions...Alone!  And back to #1, the essentially unlimited choices in vintage.  BTW I've found a new aggregator HiFi Shark who looks in all the online sales forum, so now almost anything can be found somewhere.

(Now for some people, Vintage is friends, connections, local places.  Those are the lucky people who live in such places, or perhaps just the well connected and informed in ways that I am not.  But for them even more than me, Vintage is Everywhere!  Everything!  And not just in stores.)

I guess the bottom line is I'm going to be sticking with the way I do things.  I do sometimes buy new equipment when it seems to meet my needs best.  I promise to stretch a little to do so whenever reasonable.  It can be better to pay now rather than later.  But generally I will be making up my own mind, primarily with my own head, ears, and heart.

Meanwhile, the magazines are supported by advertising primarily by manufacturers but also dealers.  So of course they are going to paint a picture where every day, in every way, things get better and better.  In high end audio, this doesn't mean they really get better, it mainly means they get more expensive.  There is no scientific proof that audio amplifiers have gotten significantly better since Frank McIntosh sold his first amplifiers.  That was when amplifiers were finally made to be good enough, and good enough in 1946 is still good enough now.  There is no scientific evidence that in a blind test you'd be able to hear the improvements since 1946.  In a blind test you might well prefer the 1946 model.  This is not to say there isn't a kind of technical progress in many ways.  Things have gotten a lot better in a lot of ways.  But, by and large, these differences don't matter, except to those who believe they want them.

Here I follow the audio objectivists.  While subjectivist audiophiles can paint one of many narratives (which have changed and folded and recombined etc etc) following the progression through technologies of amplifiers, turntables, etc.  The worst subjectivist audiophiles can be bullies just as bad as the worst objectivists.  And the bully subjectivists are usually worse, IME.

The fact is that these narratives are so wrong they even go circular, as has now happened with turntables.  What was old and abandoned long ago, is now The Thing all over again.

This just shows all these narratives are mostly crap.  Audio is more like women's fashion than a march of progress forwards (though has happened also, mostly in objectivist ways).  Audio has not only been good enough to be greatly enjoyable for over 100 years, but in many way certain things cannot be objectively proven to be better than they were decades ago.  Amplifiers are in fact the key example: almost all differences which people think they hear in modern amplifiers can't be shown to be there in blind testing.  I don't follow the objectivists completely (nor do they themselves except in rhetorical fantasy) that this means we conclude there are NO differences.  I would take it as meaning there may be little-or-no differences.  Far less important differences, if any, than people imagine.

Now in transmission systems, we've gone in both directions, with MP3 and Cassette Tape being convenient but unmistakeably low fidelity.  With 16/44 uncompressed digital audio we get to stuff that even in 1983 is so transparent most people would have to strain to hear further improvement (in blind testing...not their imaginations) but some can do it...as has been shown by recent DBT with different digital filtering technologies (or at least there has been one demonstration of such...by Meridian selling MQA...perhaps we should see if there is replication...).  I believe it is true and proven that digital filters can make an audible difference in digital, though it is less than most who believe they hear differences generally imagine.  One of the cornerstones of HDCD, for example, is that the digital filter can be varied by the producer to take on one of several shapes of different utility.  And now MQA is clearly a similar kind of thing but far more advanced.  And the DSD's are a different kind of digital that remove the digital reconstruction filters, but introduce their own formula of noise shaping as a substitute, which categorically you could describe--as yet another kind of filter.  So that's the fine edges of the digital stuff.  Will there be fine edges on the fine edges?

When people like Cookie Marenco say that if you can't hear the difference between a copy of a CD in the original...I'd pay pretty good money to see someone do that in DBT.  I've run very careful DBT on subjectophiles and seen nothing that would surprise an objectivist.  If what Cookie says is not pure nonsense, it would be very very hard I believe.  Marenco runs a production company that makes recordings in DSD, and someone I'd put with the crazies in subjective audioland, though not perhaps as a successful audio recording entrepreneur--which is what counts as they say.  There are very smart DSD pushers like Hiro and Miska on Computer Audio and I'd admit I've learned from them, but also Archimago who I consider far more fair minded.  DSD64 is a waste it appears.  DSD128 looks like a decent performing high rez format, though is it really necessary compared with well done space conserving 24/96 done by the likes of Ayre QA-9?  And I will say flatly that I personally believe that it will always be possible to find a way to make 24/96 sound as good as the best of anything (in equal number of channels, etc), possibly with a few more tweaks we haven't thought of yet.  But if a producer chooses to use DSD128, well whatever, I'll analog convert it to 24/96 (when and if I ever get suitable equipment) which I believe can always be made sufficiently transparent for human audition.  There's probably a reason why Sony hasn't, for example, published DBT showing the superiority of DSD, 1-bit, or any of their digital technologies as far as I know.  It can't easily be proven, and basically hasn't (I reported on high end German DBT which failed to show difference between comparable DSD and PCM--hope I find more about that).  Demon Sony Digital seems anti-consumer in endless ways, such as the inability to do any downstream digital processing.  Are we to throw out our digital EQ (and for me, crossover) just for some fantasy that might be so inaudibly different as never to be established by DBT?  The whole thing is much like a Super Audio extension of DRM, DRM which doesn't allow you to modify anything, even to adjust the presentation size to fit your room.  If I'm wrong, then at best DSD64 was a premature broken format, with even DSD128 available before the penetration of suitably capable technologies which may one day give us DSD128-compatible DSP---and or a case of requiring endless upgrades to finally realize the alleged potential--not to mention costly equipment).

So, given all the lies, crap, goodness from the beginning, and background progress not particularly aided by the subjectivist occupation of Audio, there is absolutely no reason you cannot do your own thing, just like me.  Though of course I believe you might do a little better after reading me.

Handling the DSD noise

I was noticing that the Krell FPB 300 was clinking a lot, yet I was only playing at a fairly soft level, playing the RCA Living Stereo hybrid SACD of Wallenstein Symphony of the Air.  I think that these old analog recordings often contained HF noise that was ignored, and in DSD encoding there is no low pass filtering.  And I think this range of early manufactured SACD's and/or this specific one is notorious for HF noises as described in the pages of The Absolute Sound.  HF noises that can potentially be destructive to amplifiers, etc.

So I thought, what if I low pass the Krell at 20kHz?  This might not be a good idea in general, but since I am adding in the supertweeters at 20kHz, it might be the right thing to do anyway.

So I tried it, and even ignoring the great reduction in amplifier clinking I thought it sounded better too.  More relaxed sound.  BTW I noted in a wonderful performance of the San Antonio Symphony on Friday that live music has a particular relaxed sound that is often more like LP's than CD's.

I fine tuned the adjustment and the pink noise is flatter than ever, and notably dipped at 20kHz when I turn the supertweeters off (it was a bit like that before also).  So that the high pass and low pass match I now have them both set to 12dB/octave, which would increase the supertweeter because it used to be set to 24dB/octave (LR) with two LC filters.  But I didn't delete the second LC filter, I moved it down to 9kHz because the first filter seemed to run out of attenuation (maybe or maybe not, I'm not sure the meaning of the -15dB setting when using LC, does that limit each filter to -15dB attenuation?).  The supertweeter also has a notch 4-6k to remove any metallic sound.  AND it has it's own built-in filter which I've never really accounted for, I think it's around 10kHz.  But you see my filters have already attenuated it greatly by then (and perhaps my second filter is redundant).

On the panel DEQ, the HC (12dB/octave presumed for HC, this is a cut!) seemed to cut less highs (by the graph) than H6, which would be 6dB/octave.  That's true, the shallow filter would have to start
earlier and cut more prior to 20kHz than the 12dB octave.  One more reason to prefer HC to H6, though one might argue a 6dB octave cut would be less sonically harmful.  In a quick audition I couldn't hear the effect of either filter, though it was my impression from extended listening that applying HC/20kHz to the panels sounded more relaxed.  Actually it's hard to know the true acoustic cut because the speakers themselves roll off as steeply around 18-20kHz.  Meanwhile the amplifier is pushing harder and harder due to the capacitive load.  Certainly doesn't make sense when a supertweeter is covering the range with wider dispersion and response to 35kHz.

I had been planning to just play this disc in CD mode to keep my amplifier from blowing up, but it sounds so much better in SACD, and now I can play it at +3dB on the Krell (since the 9000ES output is a bit low) at it sounds fabulous.

I'm wondering how much time the amplifier will run before the next repair.  And how it will go when it goes next time.  It is clinking more.  That could just be loose panels, but when I sturdied the amplifier current draw, the clinking is generally after-the-fact but often does follow high current use.

The way the Krell works is that more bias current is demanded based on a comparator, so linearity is always maintained.  However, when the amplifier ages, more current is demanded more often until fixed limits apply.  So you get sharper bursts of high energy, which leads to more clinking.

I'm keeping the 18 year old Krell plugged straight into the wall with the factory power cord which is quite robust.  That way I'm operating by the required standard for safety listing.  No more monkeying around with switches and current testers.  However it goes now, it should be safe.  It could go for a another decade or a minute, nobody knows.