One Huge Gap in Surround

Problem with the 7.1 analog inputs on my Yamaha HTR-5790 is that there is no way to make them do any processing, they only go straight through.  Therefore, I am stuck with whatever functions my Oppo BDP-95 has, and those are limited to volume, delay, and mixdown.  The Oppo offers no functions to convert 5.1 to 7.1 using Dolby Pro Logic II x, for example.  With an AC3 connection, I should be able to select whatever processing I like, but then I have a compressed signal, and therefore am losing the full resolution of DVD-Audio and SACD recordings.  I can see why the receiver is like this, it must only have 2 ADC's inside, and it would need 6 or 8 to handle processing of analog multichannel inputs.  Are there any receivers that DO handle such processing???

This makes one of the fundamental issues in Surround, the inherent incompatibility of 5.1 and 7.1, much worse.  To explain, requires a history lesson.

I was becoming an audiophile just as the original Quadraphonic wars were running.  Despite their differences in implementation, all Quadraphonic systems simply assumed a 4 channel speaker layout, 2 speakers in front, and 2 in back as mirror image.

It turns out that this 4 speaker configuration is fairly worthless.  Humans do not have very good front-to-back auditory sensitivity.  Things in back are somewhat muted, but it's hard to have any idea of where they are unless you turn your head.

When this was all re-thought for Home Theater, it was fundamentally changed.  Instead of Quadraphonic (because more is better or something) we got Surround, in which an ambient soundfield is created arournd the listener, starting with SIDE speakers.  This actually has some perceptual utility, maybe (though I think that often multichannel listeners don't appreciate the forward depth of stereo systems, which is often superior to multichannel in practice for various reasons, one of them being the confounding effects of the center channel, which IMO should be eliminated).

But somehow, the sense that "back" was needed still  seemed important to some people.  So, the 5.1 specification fudges the side speaker position so that they are also somewhat behind the listener.   Often the specification is something like 90 degrees to 110 degrees, but most people, especially the relatively uniformed, put their side surrounds much further back that 110 degrees.

When you get to 7.1, finally it is done correctly, with both sides and back speakers.  But now, the sides are supposed to be at 90 degrees, as close as possible, right on the sides of the listener.

So if you set up your system for 7.1, as I have finally done in my kitchen system (sans center channel, which is being automatically added back into the front stereo pair), it will not play 5.1 recordings, which are the vast majority, in the way that most 5.1 systems will play them.  There is no back signal at all.

Effectively you need some kind of processing like Dolby Pro Logic II x (which, btw, my Yamaha receiver has, it says so right on the front, but it is ONLY applied to digital inputs).  Although Dolby Pro Logic is mostly known for generating fake surround from stereo, it also features a matrix for converting 5.1 to 7.1.  Although you could simply duplicate the side signals in the backs, and some schemes actually do that, it's best handled through a matrix.  What I've heard described in a matrix where the sameness between the sides is subtracted from the backs.  It might also benefit from some kind of steering which leaves things alone if they appear to be concentrated in one speaker.

Now that we have endless named proprietary schemes for doing matrixing we don't have any plain vanilla matrix boxes to fool around with as in the good old days.  Stuck with the fact I can't actually use the Pro Logic II x on my Yamaha, or find a simple 5.1 to 7.1 outboard converter I could stick in between my Oppo and Yamaha, I now dream of building such matrix boxes myself.

Update:  I may already have the answer to this problem in an unexpected place.

Two years ago I started buying Integra Research RDC-7 surround preamps simply as stereo DACs because they have dual differential 1704's, something I was pursuing at the time.  For various reasons, I decided not to use this unit, and the low 1.0V voltage for I/O would be problematic in my high end living room stereo.  I had reached the point where I was thinking of recycling at least one of these beasts (the beat up looking one) and possibly both because of no perceived need.

Sadly, the manual for the RDC-7 is not actually available, what one finds online is the RDC-7.1, a different beast.

Reading the manual for the RDC-7.1 it is clear that Dolby Pro Logic II x can be applied to the multichannel input (it lists the various options for multichannel inputs, and that is one of them).

I suspect either this or something similar is available on my RDC-7, especially because it has an "Upgraded" tag on the front.  It appears the THX Ultra 2 has a "Cinema" mode which also expands 5.1 to 7.1 surround.  In any case, this $4500 surround controller is not limited to "straight through" or "pure" for the multichannel inputs as the Yamaha HTR-5790 is.  Perhaps that's the deal, to do the cool stuff you need to get the actual preamp/controller.
  
Since the time this was made and now, generally all Home Theater Receivers and Surround Processors have been eliminating analog multichannel inputs, which  I think is a bad idea.  In addition the the compressed AC3 "Dolby Digital" signal, newer products transfer multichannel through HDMI, or at least HDMI connectors.  This may have the advantage that a digital signal is being transferred, and a layer of DAC and ADC may therefore be avoided.  I think that issue is extremely overrated--surprisingly digital coversion, especially to 24/96 and above, is essentially lossless and about as good as a pure analog interface--the kind high end audio has been built upon from the beginning.  Plus, analog signals do not have complex clocking issues or cause RFI from high speed pulses.  I am also not sure you will always get a full resolution signal via HDMI from all kinds of discs.  I have heard HDMI clocking may be problematic (but Oppo fixed this in the BDP-205) also. 

However, if it turns out I can't use the Integra Research to do my 5.1 to 7.1 expansion, any HTR with HDMI multichannel inputs and Dolby Pro Logic II x (or perhaps THX Ultra 2) will do.

I will first see if my RDC-7 *can* convert 5.1 to 7.1 through the multichannel inputs, and if so, begin working on incorporating it into my kitchen system.   My Yamaha HTR would then do nicely as a multichannel amplifier, possibly supplemented by a high end stereo amp for the front channels.

Addendum: Chesky put forward an alternative use for 6 channels in which the back surrounds are removed (they aren't THAT big a deal anyway) and replaced with front speakers at the widest possible positioning.  (Stereo can't use the widest positioning because that would destroy the phantom center image.)  Yahama and others used the extra two channels as "Presence" speakers which are above the stereo speakers.  I didn't mean to necessarily disparage these ideas, I have just found it more convenient as well as more typical and supported to have surround back speakers.

Dolby Pro Logic II x is not the only process to convert 5.1 to 7.1.  I already made that clear, but in the minds of many, all that older stuff has now been replaced by Dolby Atmos.  I think thats way over the top for home theater, and particularly for 5.1 to 7.1 conversion.  I would stay away from any process that has mandatory automatic room correction, though I'm not sure Atomos actually mandates that.

By all acounts (though none might be informed) the 5.1 to 7.1 conversion in Dolby Pro Logic II x is a matrix applied to the surrounds ONLY.  It does not reference the front or center speakers.  So, in principle, you could do this or a similar conversion with two inputs and three outputs.  This is a relatively simple thing, and for a 7.1 system, perhaps just what is needed.  As I said, Atomos looks way over the top.

In addition to "surround" formats, there are also some special formats like Amiphonics and Ambisonics.  These may require stereo back speakers to work.  These formats are somewhat controversial and I still wonder how well they actually work.  However, I purchased a few recordings of this kind, and that is another use for back speakers.  In this case, replugging connections may be the best way of handling it, plugging the 5.1 surround outputs into the back.

Measuring the FPB 300 damping factor [Updated Jan 13 2019]

With sadness but hope, I've decided to mothball the Krell FPB 300.  Though it has been very entertaining to try to work out the bugs, after 3 perfect repairs, and 3 years of experiments to get it to work properly, I've decided to hang it up until I have the spare time to disassemble and repair, rebuild, or re-engineer myself.  Which may never happen, but at least I'll have time someday to take it apart which would be worth something to me.  I can't bring myself to sell it after all this time and money invested.  As they say, as long as there's money, there's hope.  I also have hope that eventually suitable replacement transistors will be available.  I remember the way this went with tubes, which have never been more available than today.

Meanwhile, I have measured the distortion performance in both channels at 4 and 8 ohms, and the numbers are excellent in the good channel, at all power levels, basically at spect at 8 ohms and slightly worse than 8 ohm spec at 4 (which is not specified).  The numbers are not quite as good in the "bad" channel, but 0.04% THD at 8 ohms taint that bad.  I actually measured 0.09% THD at full power at 10kHz after a full warmup in that "bad" channel, beating the spec IIRC.  (Now I can't seem to warmup anymore...)

The Hafler might measure slightly better than the Krell good channel into 8 ohms, I hadn't measured 4, up to significantly above rated power.  It seemed to have peak output of about 60V, which surprised me.

The Aragon hasn't measured quite as well, however, I haven't done a full set of measurments only a quick check while tuning the bias, and distortion at final bias was never checked, and actually the final bias may not be the final final bias either.  Still, I got the Aragon down to 0.07% at 8 ohms at modest levels at 1khz, in bias fiddling a couple of years ago.

Tonight I was able to measure the Krell at 220 Hz at 8, 4, and 2 ohms, and unloaded.  (This is the "bad" right channel.)  I put the load on one set of outputs and a 6 foot cable to DVM on the other output.

1 watt nominal level into 8 ohms, about 2.88V

220 Hz

8 ohms  2.797V (warmer)

4 ohms  2.785V (coldest)

2 ohms  2.762V (cold)

noload   2.812-2.810(warmer)

DF(220hz,8ohms,1w) = Eunl / (Eunl-Eload) = 2.810 / (2.810-2.797) = 216
DF(220hz,4ohms,1w) = 2.812/(2.812 - 2.785) = 104
DF(220hz,2ohms,1w) = 2.811/(2.811 - 2.762) = 57.4

(Note: the DF should scale as half for each halving of load.  8 ohm damping factor is the one normally specified.)

The amp did not run long enough to actually get very warm, but even during the 10 minutes or so of operation there was improvement from 28.12 to 28.10 in the unloaded output voltage.  I could not go back and remeasure 2 and 4 ohms because the amp shut down twice when doing these measurements, and I didn't want to push it any more than that.

I was also able to measure 10kHz at 8 ohms:

10kHz

8 ohms 2.201

noload  2.213

DF(10kHz,8ohms,1w) = 2.213/(2.213/2.201) = 184

Very little change in damping factor from 220Hz to 10khz.

The apparent HF loss of -4.5dB compared with 220 Hz is probably mostly being caused by my crossover and EQ....more than I expected but it achieves the desired result here (flat response).  Also a dB or so HF loss in the Fluke 8060 meter I am using at 10khz.  I know from previous measurements, specs, and reports, that the Krell is virtually flat well beyond 10kHz, but that is not the question here, so I'm not going to turn off the crossover, bring out a better meter, etc., to measure this better. Turning off the crossover requires a reboot of the DEQ's and has messed me up in the past (when I forgot to turn it back on, etc).

****

In the "good" channel, that runs cooler with no suspicious hotspot and seemingly does not trigger shutdowns, I measured slightly better low frequency damping factor, but more than just slight improvement in the change in damping factor at 10kHz.

DF 8 ohms, 1w, 220 Hz, 222 (equivalent to 0.036 ohms)
DF 8 ohms, 1w, 880 Hz, 207 (equivalent to 0.039 ohms)
DF 8 ohms, 1w, 10kHz, 204 (equivalent to 0.039 ohms)

This time I normalized all measurements to 1 watt by adjusting the volume to 2.88v in all cases, overridding what are mainly affects from my crossover on the frequency response.

The improvement is consistent  with a theory that that feedback and servo are keeping the bad channel in line, but also reducing the ability to cope with changes in load at high frequencies, if only by a small degree, since it's "resources" are already being used to fight the faulty transistor(s).

Though perhaps there was some improvement due to change in measurement strategy.

And you might imagine if there really were a transistor in very bad shape, the result would be much worse.  Differences like this might occur from differential aging as well as manufacturing and repair histories and usage histories.

I was hoping to see a bigger difference, so that the good channel might have damping factor like 500 or something, and there being something "obviously" wrong with the bad channel, more than just a slight variation, even if that variation is consistent with the theory that the bad channel is slightly faulty.  The distortion measurements showed the same thing, with the good channel meeting spec and the bad channel being slightly off spec.

The damping factor for the FPB 300 is not specified, actually.

Unobtanium Transistors

The principal problem with maintaining amplifiers like my Krell FPB 300 is that the power output transistors, MJ1302 and MJ3281, have long been unavailable.

Motorola discontinued these transistors while Krell was still making amplifiers with them, so Krell bought the entire remaining inventory in the mid 1990's.  They used them in many different amplifier designs, and according to legend, Krell freely installed new ones during amp servicing.  That was wonderful while it lasted, until about 2009 or so.

Now, to keep the amplifiers running with authentic parts, it is necessary to scavenge transistors from parts unit amplifiers, similar amplifiers that might be even more valuable than the one you are trying to fix.  There is no public place where such scavenged transistors can be safely purchased, maybe you know someone you trust (and, do you really trust them).

MJ1302A and MJ3281A are sold on Ebay and through Asian distributors are almost certainly fake parts.  I bought some of these in the hopes they may be OK now that Chinese fabs have been making them for well over a decade.

Similar silicon is still available in the TO-247 case in the MJW1302AG by ON Semiconductor, the Motorola spin-off which still manufactures basic transistors for linear applications.

The problem is that the FPB amplifier heatsinks were designed for TO-3 transistors.  To use TO-247 would require complete re-engineering of the chassis and heatsinks.  One might as well buy a D'Agostino amp.

Meanwhile, ON Semiconductor does still make TO-3 power transistors, including the MJ15024G, which others now use in power amplifiers.

What defines Audiophile?

Struggle.  It's not High End audio if it doesn't take some work.  Contrary to what audio store mavens would tell you, you can't simply buy musical enjoyment.  It's not "all about the music."  It's about getting the sound that makes it sing, understanding how to get there, and traveling at least some small part on your own feet.  As Euclid said about Geometry, there is no royal road to being an Audiophile.  Not only is it not possible to get their without your own work (which could be just...actually serious listening...if you have totally obsequious dealer you trust and tireless elves to help daily) it's not the same thing.  To be an audiophile is to be endlessly deciding how to do audio, but hopefully not obsessing too much over these decisions, but hopefully making better ones as time goes on, that lead to more rather than less enjoyment overall.

To be an audiophile is to at least nearly always be on the edge of Audiophilia Nervosa.  That is the dark side, which we must stay as far away from as possible, I believe.  Some guidelines for avoiding that:

1) There is no one right way to do things, starting from the fact there is no one right set of goals to be aspiring to, including the nature and extent of one's unpaid personal labor and buying in audio, and when and what to listen to and how.

1a) The enjoyment people seem to be getting from their audio systems often seems inversely proportional to how good they are by various objective standards.  Why can't we who take audio seriously have it both ways?  Have high standards when we're tightening connections, and then, just enjoy?

1b) I believe 1a also includes that...it doesn't take much audio capability to make audio enjoyable.  The main thing is in fact...the frame of mind about it all.  To be able to accept things as they are now.

1c) Once one has banished unquestionably horrid sound, most audio systems are capable of communicating sufficient information for a fully expansive experience each time.  In fact, this doesn't take much.  The truly great systems have even more information to choose from, but both are sufficient.

So, avoiding obsession includes accepting the fact that the present will never meet the highest standard, or even sometimes ones that were recently being met, but still the envelope of expending experiences must continue.

Antenna Pointers

Ground Loop from Outside Antenna feeds (which themselves have proper outside bonded grounds and suppressors)

Despite ground bonding, I was getting significant ground loop hum from the tuner connected to my outside antenna.  I tried various ways to fix this.  Two things "worked":

1) Running antenna lead to F connector surge protector on my Monster power bar, then to tuner.  This reduced ground loop hum a lot, but not entirely.  I think having suppressor outside and inside as well on antenna feeds is good practice for safety reasons.  I did not notice signal deterioration.

2) Running antenna feed through "Magic" isolator.  I got Magic isolators for TV use almost 20 years ago.  I was worried they might not work for FM, but the case says nothing about having an FM trap, so I went ahead, and noticed no signal reduction.  In combination with #1, this eliminated ground loop hum entirely.  The 60 hz hum at the input to my Behringer DEQ 2496 connected to the Sub Out line signal from my Yahama THX receiver dropped to -140dB.   Before doing #1, the level was worse than -90dB.  As I did this after #1, I did not try it without #1.

Ground loop reduction became the order of the day when I started running my subwoofer signal through the Behringer DEQ.  Isolating the signal between the receiver and the DEQ with a Jensen Isomax transformer actually made matters worse.  The Behringer has 3 wire AC input whereas the Yamaha is only 2 wire.  The ground of the Yamaha is therefore determined by what it plugged into it, or not.

However, until I discovered antenna grounding and isolating, I was forced to use a Jensen Isomax transformer on the audio output of the tuner.  Listening to radio without that, hum was terrible, like -50dB, and other sources not much better than -60dB.  Once I applied the antenna grounding and isolating steps, the isolation transformer was not only no longer needed...it was introducing a little hum by itself on the radio signal and was better removed.

NB: Ground loops did not appear until I started using the DEQ because nothing else in my kitchen audio system is grounded.

Before doing any of this, I tried measuring the ground voltage and current running through the coax.  It was extremely tiny.  But that's all it takes to induce -60dB hum.  I decided in itself it was "safe," not representing a ground failure, simply a ground loop.

Folded Dipole Antennas made from Twin Lead must have Soldered Ends

My biggest discovery of the year, the lack of soldered ends explained recurrent issues I had been having since 2002 when I made a special antenna tuned for TV Channel 9 which I am still using.  In the end, the folded dipole with unsoldered ends became a Channel 9 blocker for other antennas, so I couldn't get Channel 9 on anything.  Soldering the ends fixed everything, and I get perfect reception now.  Bare copper is prone to getting semi-conducting oxidation which does very weird things.

UHF Antenna location is extremely important, down to millimeters.

As I hoped, I was able to find a location on the south side of the kitchen for a UHF TV antenna that picks up all local stations perfectly.

The problematic multipath-prone TV signals were coming from the south.   Previously I had both UHF and VHF antennas on the north wall, but the UHF signals for Channel 4 (which actually broadcasts on Channel 48) and Channel 5 (which uses another UHF channel) had multipath issues with recurrent pixelization multiple times per minute.

I first figured the south wall would be better because I could position the antenna above the bathroom mirror on the other side of the wall which was probably not helpful.  It would also then have a clear path to a southern originating UHF transmission, instead of being blocked by HVAC and other metal objects around kitchen.

But even on the south wall, the precise position proved to be extremely important.  Too far one way and I would get dropouts on channel 29, which comes from the north!  Too far the other way, and I was still getting multipath issues on channels 4 and 5.  There was a happy spot right in the middle so I stuck my Clearstream Eclipse antenna with "permanent" adhesive strip right there and enjoyed perfect UHF TV all night on local channels.

Overnight the Eclipse had unstuck from the wall and slipped down just over an inch.  The performance was now abysmal again.  I now stapled the antenna back into place.  But making some error, I stapled it about 2mm lower than before (fortunately, I had marked the original position).

This was not as good, I determined that afternoon.  I was not getting as many dropouts, but I was still getting them.

Back on the wall, I found that positions above the original position I had found were even better.  In fact, the signal strength and freedom from multipath kept increasing as I raised the antenna all the way to the ceiling, so I stapled it there.

Now reception is perfect on all local UHF channels, with no dropouts.

New Time Alignment attempt

I tried the new Hantek scope, and tried to do an alignment using my calibrated microphone, held by a vibration isolator and a music stand (it might or might not have been better to have the correct adapter to mount the mike directly to the stand).

I could do a fairly good job aligning the sub and the panel on each side, but problematically I could not set the delay differently for each channel on either the subs or the panels.  I do not understand why, it looks like this adjustment should be possible somehow but it isn't, even on the subs, for which the DEQ is in dual mono mode.  So I had to pick a compromise, which still shows pretty good blending of the panel signal into the bass signal.  This is not entirely unambiguous on the scope at the finest resolution of 0.01ms.

With the super tweeters, I could not get a clear signal off axis or more than 2 feet from the super tweeters.  (On my iPhone using Analyzer from Onix3, I see the supertweeter output clearly from 4kHz to 20kHz...the former greatly attenuated fortunately.)  So I could not do the alignment other than by guesswork.  I could hear some differences using the single positive pulse (dirac, I think it's called) test signal, but I didn't know which was more like correct in the critical range of 6.26ms to 8.5ms relative to 10ms added delay for the panels.  Sometimes I preferred the former but recently I've switched to something like the latter, which I think is more correct. )

I think this was partly because the Hantek scope I has has a fan, which was creating enough low level noise to possibly mask the supertweeter pink noise output.

Previously, my old fanless Tektronix didn't seem stable enough anymore.  But as it turned out, even with the Hantek, I needed to tap off the preamp signal to the trigger input.  I could posslby get the Tektronix to work for this work, or get another fanless scope.

Italian Tuneup Fixed the Krell !!!

[update: sadly, it appears the Italian Tuneups I've been performing don't have long lasting effects.  My current plan is to workout a warmup strategy, that each time gets over an apparent thermal trouble spot that seems to be the crux of the problem, but once the amp gets into a funk it seems non-trivial to keep it going long enough to warm up without shutting down first.  Now I've come close to what originally worked with the first Italian Tuneup, and it doesn't seem to work into the 8 ohm load I figured I could keep connected in parallel with the switchbox, making it a trivial procedure to do the warmup.  So do I try different power levels, different frequencies, different waveforms, ramping up or down, or a different load?  What gives the best "kick" to get started?

So, the glow has faded, but the fact that it worked has indeed opened up a window of understanding and a window of opportunity I'm currently still trying (and tiring) to figure out, so this is still a work in progress on December 12, 2018.]

This may be my discovery of the year, or maybe the decade.  I've been struggling with Krell FPB idiosyncrasies for that long too (though, it has been worth it, see a future article).

It had gotten to rock bottom: the amplifier wouldn't even idle, with shorted inputs, and nothing connected to the outputs (which, with no signal, is fine for this amplifier).   It would just shut down for no apparent reason, in as little as 5 minutes after having been turned on.

The problem had started just one month after it had been serviced for the 3rd time in two years to fix this problem, twice by the Krell factory, which had finally refused to work on it anymore, and then The Service Department, recommended to me by Krell, after which it had worked more perfectly than ever before.  For one month.

All the capacitors have been replaced in these last two years.  Hundreds of capacitors.  And many other parts, many other repairs, and a few upgrades.  I approved every repair and all but one suggested upgrade.  (BTW, it might not be a good idea to perform Italian Tuneup if the amplifier needs other service.)

After I confirmed it was working perfectly after a month of simply listening to music, I brought on the ABX amplifier switcher, to allow me some ad hoc comparisons with the Hafler 9300, designed by the creator of the Acoustats.  I did a few casual nonblind AB comparisons, the Krell seemed fine, and the winner by a tiny margin (if I were actually hearing a difference at all).  But then a week later, when I wasn't doing any AB comparisons, I played a high resolution digital recording, the HRx sampler, at maximum "standard" level (defined in an earlier post).  And curiously it shut down during a softer passage just after a louder one.

Over the next month I performed a few dozen elimination tests, to try to determine if the problem was in the amp or something else, and then which channel of the amp, and what conditions caused the shutdown.  Several times before, it seemed like I had cured the problem, I even played the very same recording that made it shut down the first time since the last service, and it sailed through just fine.  But then it shut down later playing something else.

Now, with the amp only able to idle for a few minutes before shutting down, I thought certain it would need professional repair again.  I'd never be able to sell it for a decent price like this, and I didn't really want to sell it, but I might wait until next year so as not to be putting all my money into this amplifier, just my "surplus."

Or maybe, I'd try to fix it myself.  I was beginning to think about that again.

When I was in my teens (many decades ago), my best friend had a Pontiac GTO.  He loved it for a few years.  When I was riding with him on the freeway one day, we were taking an ad hoc trip from Los Angeles to San Francisco, he floored it for a few seconds.  "Blasting out the carbon," he said, "keeps the engine running smoothly."

I'm not sure if that was the proper technique, but there is something just like that called an "Italian Tune Up" which consists of something like full power operation for some period of seconds or minutes.  And done correctly, this does help remove carbonized deposits from valves and other things.  It's a widely recognized technique, especially applicable to cars that are only driven gingerly on a regular basis.

I decided last week I would try something like this on the Krell.  I waited for the weekend to get it all set up properly.

Into the living room I moved my Sound Technology 1700B, a digital Rigol oscilloscope, and a never-before-used "2000W" 4 ohm audio brake load from China.

The connections from the Krell merely needed to be moved around from the ABX box over to the Sound Technology analyzer, and merged with connections to the load which was newly terminated with Furtech locking bananas.  So it was two sets of locking bananas plugged it to each of the two balanced signal inputs of the analyzer, externally loaded with 4 ohms.

I first ran the apparently (and now proven!) dysfunctional left channel (overheating, anyway, I did not previously know this channel was wrong, but it was clearly running hot, and depending on the experiment the shutdowns seemed to predictably occur at a particular warm up temperature...around 135F).

I ran the channel with a full power signal, just below where visible change to the sine wave seemed to be occuring, which turned out to be around 705 watts into 4 ohms.

I measured distortion as 0.18%, a bit higher that I'd hoped.  Everything seemed fine for 4 minutes, and the left channel was barely heating up more than the right.  Then the shutdown occurred.  The temperature had risen a mere 14F from 78F to 92F.  Like nothing.

Now that test, I had presumed, would exercise the output function maximally, the emitter say (I'm not really sure...in fact I'm not sure this makes any difference, the current through the transistor always occurs in certain proportions, regardless of whether any flows into the load.  This is something I don't know.

But I decided to try a different approach.  After the amp had apparently cooled back down to ambient (I completely shut down the amp at the breaker), which required only a hour or so, I had planned to wait another day, but I was so excited by the prospect, and seeing the amp was now back to ambient, I plunged ahead.

I figured the "worst case" for heating the transistors is the famous 1/3 power output.  Into 4 ohms, the maximum power (before true clipping) is 900+ watts.  So 1/3 power is 300 watts into 4 ohms.

I miscalculated this as being sufficiently close to 30V to just go ahead with 30V, which is full scale on the 30V range of the analyzer.

Anway, that's what I did, I ran at 30V.  And I saw the left channel heat up faster than I've ever seen it heat up before.  More than I imagined it could do.  I was afraid it might shut down at any moment.  But it kept going and going and reached 175F (from ambient 78F) in 15 minutes.  I chose that moment to turn the amplifier off.  I did not want this test to potentially cause problems (though, actually, I've seen it go to 207F without apparently changing anything, and I believe it has a "kickdown" program at 180F which reduces the bias automatically to reduce the temperature).

Well, this was clearly different.  It had clearly blown past the old 135F where shutdowns were occurring.

I let the amp cool down to 160F and decided to just let it run at a maximum sustainable level for hours, partly as a test and partly because this would be another kind of exercise which might help.

I chose the level of 10V or 25W.  At that output level, the temp actually rose a bit from 160F to 170F or so, but I just let it keep running, and it ran for 3 hours without issue.

At both 30V (225W) and 10V (125W) the THD measured 0.07%, which appears to be what other people have measured into 4 ohm loads.  The specification is 0.03%, which appears to be for 8 ohm loads and using the balanced inputs (which I was not doing since my ST has only unbalanced signal output).

I then let the amp idle and cool down idling overnight.  It was still running ok.

The next day I played music at loud and medium levels for 2 hours, and again in the morning for 2 hours, and it still ran OK.

It is, apparently fixed.  And just as important, if the problem comes back, I know what sorts of things I may do to get it going again.

I DO have some ideas as to how to prevent the problem from coming back, as well as what ultimately causes it.  Before this "repair" I wasn't even exactly sure what channel was causing the shutdown.  I suspected the left channel because it runs hotter mostly, and shutdowns seemed to occur at certain temperatures in that channel.  Now, since I only treated the left amplifier channel to make the problem go away, it appears quite conclusively that it was/is the left channel of the amplifier at fault, and nothing else.  And somehow, certain kinds of operation cause it to change in some way that makes it start failing.

If all I do is Italian Tuneups, I don't really need to know the underlying cause.  But my most intuitively satisfying theory is that a certain power transistor on the left side is prone to something like carbonization.  It may have some cracks or grooves which get carbonized under some circumstances, causing an internal short which makes it misbehave.  This carbonization begins to vaporize or disperse when the heatsink temperature is around 135F.  If the amplifier isn't being played hard enough, the computer inside decides this looks like a serious problem and shuts the amplifier down.  WHEN the amplifier is being played hard enough, it quickly blasts through this carbonization before it has time to look like a serious problem to the computer, setting the stage for perfect operation until the carbon builds up again.

I suspect the computer program is being too paranoid, and be more tolerant of slight deviations from ideal transistor behavior to allow the normal decarbonization to occur.  But this suspicion is irrelevant for the forseeable future, because re-programming the computer is nothing I am going to have the time or expertise to do for a long time into the future, if ever, and it is fraught with risks as well.  It is quite likely the "paranoid" computer has saved the amplifier from certain destruction many times, and would be unsafe to operate with much less paranoia.  So meanwhile I simply have to live with the paranoid computer program, perform Italian Tuneups as needed, and try to figure out how to slow down the carbonization (or whatever) build up process so I don't have to do an Italian Tuneup often.  Though, in principle, I could do it ever day before serious listening.

I have many anecdotal experiences that suggest that playing very high frequencies, such as from high resolution recordings, or from FM tuner outputs digitized at 24/96, is an issue.  I noticed this long ago (because of shutdowns occurring while playing FM tuners at modest levels, and even the recent series of shutdowns which started when I played a high res recording), and decided that since I use supertweeters highpassed at 18 or 20kHz, I might as well lowpass the Krell at something like those frequencies.  And in one set of experiments, years ago, I decided the system sounded better this way (with the Krell lowpassed) and even measured better.  Now I never have much confidence in the reliability of my subjective "tests", and later measurements showed no measurable advantage (the Acoustats roll off steeply at 18kHz anyway, so it "doesn't need" to be lowpassed electronically to blend with supertweeters starting at 18kHz), so I had not been doing this recently.   But there is also little point to trying to drive the Acoustats higher than 18kHz because they have a strongly capacitive load which reduces impedance down to less than one ohm at the highest frequencies.  This is problematic for an amplifier which tries to run Class A for everything, and has a bandwidth of 300 kHz.

Another line of resoning with fewer supporting anecdotes is that operating with the ABX tester, and running the Krell with no load while still getting an input signal (the way I have been doing it) so the other amplifier can play the speakers, is not a good thing.  I was told (by Steven at The Service Department) that you don't have to connect speakers to the Krell.  But I did not specifically ask about continuing to run an input signal into it.  With many Class AB and true Class A amplifiers, this would not be an issue.  But it appears the Krell computer (the so called "anticipator") sets the bias level in large part based on the input signal, not just (or at all) how much actual current the amplifier is delivering.  I have seen some evidence that it runs hotter with no load attached than with the speakers attached when there is signal input, as any engineer would expect of a Class A amplifier because if energy isn't being dissipated in the speakers it has to dissipate within the amplifier itself.  Now with a real Class A amplifier this is not a problem, as the amplifier normally idles at the highest bias and has to be engineered for that.  But with a plateau bias amplifier, it depends on the programming of the computer--was it programmed to allow sufficient thermal margin in case the speakers are disconnected?  Now, I've actually done this many times, and the Krell did not seem immediately to be adversely affected.  It might even be, in a sort of Italian Tuneup kind of way, beneficial.  But because it is a highly suspect unknown, with a few supporting anecdotes as well,  I've decided that before doing more ABX or similar blind testing, I'm going to have to modify the tester to switch in a dummy load when the amplifier is not playing the speakers.  I believe this is dooable, there is already a 1k or similar load switched across the unused amplifier by the switcher.  I just have to rewire that 1k load as a connection to a external dummy load.

I'm going to fly with those two "preventative" ideas for now, and try to remember relevant playing details when the next shutdown occurs so I can more and more reduce the causes of the carbonization or whatever is occurring.

UHOH

All was fine on Friday evening, I started off playing music with gusto, quickly cranking up the right side to 145F.

Then, I wanted to do two things.  I wanted to take the historic photo showing the Italian Tuneup setup.  I'd cheat by actually measuring the right channel.

Now, just before, the amp had been heated up.  But I wasn't paying attention to the cooldown.

I got the right channel going, and was just about to measure, when, the Krell shut down.

I was thinking, maybe the right channel needs the Italian tuneup too???  (At the end of the day, there is no clear evidence for this.)  But as it turned out, the now idling left channel was right at that troublesome, for idling, temperature of 135F, where I'd seen the lions share of shutdowns and most of them while idling.

So I switched to measuring to the left channel, pounding it with 225 watts for a 175F warmup, then a slight cool down, then did a lot of interesting measurements.  The distortion remains at 0.07% at most levels at 1kHz, until very high levels.  At 20kHz, 1 watt or less do measure 0.07% as well (all into four ohms btw).  But cranking up to 100w the 20kHz distortion begins to rise.  At 600w, 20kHz distortion has risen to 0.4%.  (Recall, the Pass Labs XA200.8 has 1% distortion into 8 ohms at 20kHz at lower voltages.  I'm testing the Krell with higher power and tougher load.)

So now the left channel had been tuned again, so I switched back to the right channel (a process requiring turning off the amp) and now, to keep things going, I also hooked up the left channel for music playback.

Still, I got to shutdown before I could get a good run at 1/3 power.  Once again the left channel was at the suspicous 135F.

I'm thinking to do this right I'm going to need to run both channels simultaneously.  I have more dummy loads on order.

And maybe the thing is also, no power cycling when hot, and messing around.  Just blast to high temperatures by playing robustly, then keep playing--never pause.  Pausing to mess around messes whatever up.

And so, the waste of time continues...

Back to the Hafler 9300, sounding better and better.  I kick it off at exactly the same level I had the Krell at (with my 0.1dB level matching) and it sounds...wonderful in every way.

But later, once the Krell had cooled down to 85F, I decided to try exercising and measuring the right channel of the Krell.  I could pop off a quick 1/3 power burn and then measurment before the left channel gets back up to 135F, I thought optimisically.

So I ran the right channel at 225w for 15 minutes.  The back right heatsink was 163F while the power regulator heatsink in front was 173F.  Strange.  I proceded to do distortion measurements at different powers and frequencies.

The right channel seemed a bit lower in distortion, 0.06% at 1/3 power at 1khz, and basically 0.06% at other powers.  When I tried 20k, I got half the distortion in the other channel, 0.2%, at high powers.  At lower powers, 1W, distortion at 20kHz was only 0.065%, and at 10W it was barely higher.  So, slightly better than the left channel (despite running cooler) and especially so at 20kHz.

I kept testing for awhile, and strangely the left channel was not pressing upwards to the tricky 135F.  I decided to let the amp idle overnight.

Overnight, something amazing happened.  The problematic left channel is idling identically to the right, at 115F instead of rising to 140F as I had been seeing recently (when it didn't shut down at 135F).  I had only seen that after the last repair by The Service Department when the amp first returned.  Somehow, it seems to have fixed itself through the latest exercise, and even more strange, it seems to have been the Italian Tuneup on the right channel that has fixed the left channel, despite the channels having their own separate computers.

This is still a work in progress, I hope, and not a clean slate as might be hoped, but it continues to seem like the Italian Tuneup is effective medicine that does gets the Krell working better.  For now, I've decided to let the perfect 115F idle in both channels run for 24 hours or more so that it gets remembered.  It's remarkably free of clinking also.

Sunday Nov 18

The Krell has been idling without fail since Friday at 11pm after my last set of full heat stressing and measurments starting from cold (because otherwise I was hitting shutdowns, seemingly still left channel related because the suspicious 135F temperature there, but maybe not).

After that, curiously now, the idling has returned to the wonderful performance first achieved a few months ago when I got the amp back from The Service Department.  That was the first time ever (after 3 previous servicings by Krell factory) that the left channel idled normally at at temp below 150F.  Going way back, even after the first Krell service, the left channel shot up to high temperature quickly, hitting some stop it seemed at 180F, and then bouncing back down, then back up slowly, and so on.  I figured there was a bit of noise or something that kept the bias going up until the kickdown, presumably at 180F as with the earlier KSA "S" models like the KSA-200S, after which plateau 2 or one was enforced, until the temperature cooled enough to re-enable all bias plateaus, then off it went, and so on.  That's the way it was from day one in 2008, predictably making clinks on each kickdown.

Actually, though, Krell in their second servicing fixed the endless rising, but the heat was still too high at idle, nearly 160F.

But isn't this strange...exercising the right channel seems to have fixed the bias level holding in the right.  After 38 hours of idling, the left channel temperature is 121F in the "backmost" (frontmost in my current cooling optimized backwards setup) heatsink panel, 123F in the middle panel (where airflow is slightly reduced due to the sack of digital components upstream), and 117F in the front "power regulator" heatsink.  When the middle panel is warmer in the current setup, things are working good.  When the "back" panel is way hottest...that's what the weirdness used to look like, the back panel weirdly hot and causing the weird thermal cycling.

So it looks like both channels need the Italian tune up, even if it looks like only one is bad, they seem to have some weird connection, or perhaps did this time.  Actually, this narrative ignores that I did do some more testing on the right channel, just not the full warmup thing.  Maybe that's what cleared up the carbon.  But, anyway, I think two channel tuneup and testing is best and plan to do it that way in future.

The right channel panels, turned to have the lesser airflow, are nonetheless all 116F (uniformly for both back panels and regulator panel).  Note that the cooler regulator panel would be cooler too if on the other side.  So this channel is drawing less bias somehow.

My best theory that the funky transistor which sometimes goes weird around 135F otherwise has a slighly different characteristic that the DC feedback of the other balanced half fights against through the servo.  Given such a thing is happening, it's a wonder that distortion still measures about the same, 0.07% instead of the 0.06% of the right channel, and a bigger difference at 20kHz, 0.4% vs 0.2%, all into 4 ohms.

After playing some loud music for 60 minutes, I let the Krell idle for two hours.  It did so without issue, with the left heatsinks returning to 141F.  This suggests there is a bit of problem left, but if generally the Krell is started with some robust playing, to heat to 140F or so, it will run OK after that, and if not, it needs another Italian Tune Up.

Why Keeping the FPB Running is Worth It

For what it's worth, no amplifier I've ever tried in my Acoustat system has sounded better to me, in sighted tests, than my Krell FPB 300.  In blind tests, my results so far align with the literature of blind testing of amplifiers: I have not been able to provably hear the difference between the Krell and my other very fine transistor amplifiers like the Hafler 9300 and the Aragon 8008BB.  Nevertheless, I do not take these blind tests as conclusive either, and I accept that I like the Krell sound best, just not provably.  My audio hobby is emphatically not only about listening to music, it's about learning about many things, including audio amplifiers and how they sound, and even if they can be distinguished sonically at all.  That last bit is actually Audio Science, something I confess I do very little of.  But I like having the equipment for doing so.  And one place to start is with the comparison amplifiers of similar quality, but very different in design and construction, so my top shelf collection, of which my Krell is the crown jewel, the most expensive and exotic--and with lots of stuff to show for that.  It's nice having at least one of those--and I believe mine is still up among the best.

I do want the best sounding amplifier for my system to listen to music anyway, even if it's only that I think it might be the best sounding amplifier, without any proof it actually is.  Then with this "best sounding amplifier" I can compare with other amplifiers and those comparisons become meaningful and relevant.

It has these objective design attributes:

1) Class A operation (through Plateau Biasing anyway), so no crossover notch distortion and less distortion generally.

2) Very low distortion, including into 4 ohms or less, and up to the highest frequencies.  The spec is 0.03% at 8 ohms.  I myself have measured 0.07% at high power levels into 4 ohms, which is excellent and far superior to most power amplifiers.

3) Regulated power supply for the output stage--VERY rare among even high end transistor amplifiers.  Ensuring low dynamic modulation, another issue never measured but easy to understand.

4) Wide bandwidth: 0.5 Hz to 300kHz.

5) DC coupled, so no capacitor sound.

6) High power which doesn't sag, 450 measured watts into 8 ohms, 900 into 4 ohms, even more into 2 ohms.  My electrostatic speakers are very inefficient and have impedance that runs to less than 2 ohms at the highest frequencies.  They are also very revealing of distortion.

7)  It's generally a nice amplifier to use.  It has a nice soft pushbutton to start and the slow start causes no noise or light dimming.  The connections are very nice.  It looks very nice as well as Impressive.

I've often thought, if I had the money and space, it would be nice to have a newer and more highly regarded Pass Labs amplifier, such as an XA 200.8.  Now these monoblocks require more space than I have, and also consume 680W continuously each, for a total of 1340W.  That's about double my average power consumption with the Krell, which is already getting to be a bit much.  So they wouldn't be very practical for me, let alone the $44,000 pricetag.  But what if I had the money, the space, etc, wouldn't it be wonderful?

But it turns out, you can find on many blogs, many people do NOT find the Pass Labs amplifiers to be superior to their favorites, which may be old Krell, Levinson, and Threshold amplifiers.

And there may be objective reasons the Pass Labs amplifiers and other current favorites (including mine, in the lustworthy category) like Threshold and D'Agostino and Levinson might not be as good sounding as my FPB 300, as well:

1) They generally do not have the regulated power supply.  The XS 300 does.  The very top Levinson usually does.  Those may cost way double what even the XA 200.8 costs and consume even more power, just for this pretty obvious feature.

2) They do not have as wide a bandwidth, or as low distortion, especially at higher frequencies.  The XA 200.8 reaches 1% distortion at 20kHz and 8 ohms and conservatively rated power.  The Krell only gets up to 0.1% or so.

Those same considerations apply to most amplifiers ever made by Threshold, Levinson, and even Krell.

3) Output impedance.  SoundStage measured the FPB 300 output impedance as 0.07 ohms.  That compares very favorably with the 0.14 ohm output impedance of a Pass Labs XA 200.8, more favorably with the D'Agostino Momentums at 0.2 ohms, and the D'Agostino Progressions at 0.44 ohms.  These newer amplifier just aren't doing the damping factor like some older ones.

4) Power into 4 ohms, and 2 ohms.  Neither the XA 200.8's nor the D'Agostino amps produce more than 600 watts into 4 ohms.  My FPB easily cranks out 700 watts into 4 ohms still with still very low distortion as I measured, but measurements by others show over 900 watts into 4 ohms with less than 1% distortion.  It was measured by Martin Collums as having over 1500W into 2 ohms (which isn't specified).

Scanning the measurements of amplifiers, only a few stand out as having possibly better measurements for me (with a very inefficient 2 ohm minimum speaker).

1) The Threshold SA12e.  This is the pinnacle of Threshold designs by Nelson Pass and the most desirable (if you can handle the power consumption heat), which has far lower distortion and output impedance than other Threshold models.  It bests my FPB by having output impedance of 0.02 ohms, and possibly even lower distortion.  Nevertheless, I've seen reviews in which FPB 300 and SA12e are subjectively compared with the FPB being declared the winner.

2) The Levinson 33H is right in there also.  Newer Levinsons with hybrid "digital" output stage produce more noise at lower levels like 1 watt.  Lesser Levinsons don't have the 4 ohm power, etc.

3) The Soulution has lower distortion, as does the Benchmark, and the top Halcro, but not enough 4 or 2 ohm power.

4) No doubt many others I've never heard of, but generally less known.

Now, another amplifier which might be still better than the FPB is the Krell KRS-200, said to be a D'Agostino personal favorite.  And it ought to be, given the history, build, original price, and power consumption.  It's a true 200W Class A amplifier with no fiddling around.  There's also the KAS 1 and 2, which are plateau bias amps with regulated power supplies, much fancier and pricier and overbuilt than the FPB amps.  Actually, there may be a multitude of other amplifiers I don't know of, that would have the esseitnal features of regulated power supply, low distortion beyond 500W in 4 and 2 ohms, even at 20kHz and above.  But, of all the ones I read and hear most about most of the time, there aren't many.

All these amplifiers are incredibly more expensive than what I've paid for the FPB 300, even including all my servicing adventures.   I've read negative reviews on Levinson service which no doubt is very costly, Pass Labs gets the highest recommendations for servicing, which they hardly ever seem to need anyway.

Now I don't know, I haven't had those amplifiers in house, and possibly never will.  Even if I did, I still probably couldn't have a firm (and proven) subjective opinion.  But it appears, I have "a contender" and that's what I want.  I want an amp about as good as it gets, but not so much my finances are collapsing.  And I want to be able to compare nearly as good amps of radically different designs, and find if I ultimately can provably hear the difference.  My Hafler is an example of a radically different approach: a very simple design, using MOSFETS and almost all FET circuitry.  My Aragon is a simpler amplifier than the Krell also, but bipolar outputs like the Krell, with massive conventional power supply.  These amplifiers are all objectively excellent, which is a good starting point.

Despite limited power, the Hafler does wonders with high peak power and current, and indefatiguability.  The 9500 has considerably worse measurments, and the 9505 has way more complexity.  I feel the 9300 is the goldilocks of Strickland TransNova designs.  It has the lowest distortion, even lower than the Krell.  But it lacks the regulated power supply, and higher continuous power of the Krell.