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.

Emotiva headroom and distortion measurements (new unit)

Emotiva lists 1.0V as the standard output for the XSP-1, however, the distortion is actually spec'd at 2.0V.  No other voltages are listed.  I was reassured by Emotiva that "6 Volts" is the maximum, but I wasn't entirely clear on whether this was input or output, and I'm concerned about the 3.8V or so from the singled ended output of my DVD-9000 playing HDCD's.

I've measured the maximum voltages now, somewhat approximately because I'm only using "instrumentation loads" like my 10M DVM and 1M oscilloscope.  This is a factor that 10 years ago and before I simply ignored.

Anyway, for single ended I/O the maximum input is very impressive at 9.6V RMS (measured with the gain at -20dB).  Maximum SE output is around 6.8V (gain at +12.0) at onset of obvious clipping, a tad bit of obtuse rounding can be seen above 5.5V (looking on my newest digital oscilloscope).

Driving the output into very hard clipping close to 10V can cause square wave doubling.  This is an undesireable per se, but in most circumstances your home amplifier will already have exploded at 10V sine wave input.  This is not an instability, what happens is that after the top of the sine wave flattens, the center begins to grow negatively proportional to further drive, until this center downward spike reaches the bottom, making the clipping wave, now pretty much square, into a doubled square wave at twice the original frequency.  It shows essentially that multiple internal elements are clipping at not exactly the same time...therefore something like a push pull output.

True pro equipment can handle 10V as a standard level.  That's probably why Emotive lists the standard level as 1V, to make it clear it's not 10V pro gear--which is compromised in S/N at lower consumer levels.

These I will not be pushing the Emotiva limits in any way, though a recent adjustment to the Lavry was to set max level (-8dB) to -17dBU which is 5.4V, which would be a touch too high for single ended output on the Emotiva, but probably gives around 6dB headroom for balanced output (not measured yet, that presents some challenges).

Certainly the input will handle anything I can throw at it input wise.

On my Juli@ card, using RMAA, with balanced I/O last weekend I measured about 0.0005% or 0.0006%, with my residual at 0.0003%, meaning the actual distortion is around 0.0003%, as shown in website graphics (showing better than the claimed spec of 0.0005).  These are measured at the 4V input level of the card, minus about 0.9dB.  I measure this either to 96kHz sampling or 44.1kHz sampling with the same result, showing no super high harmonics, very clean top end.  Basically my residual and the Emotive line up perfectly, save a little more 3rd harmonic (the push-pull output cancelling any 2nd order distortion) and down below -120dB a tiny bit of 7th (generally, stuff below -110dB can be ignored, the Denon DVD-9000 is full of high frequency hash at -115dB but is one of the best sounding CD players I've ever heard).  It was this cleanness that sold me on the Emotiva, among other things, and having the best phono circuit in my collection clinched it as my living room analog preamp, requiring me to get a second for the bedroom I had originally purchased my first unit for several years ago.  The THD levels compare favorably with the BEST current Mark Levinson at $40k, and the Emotiva has the essential-for-me digital volume control if not as nicely implemented as the Levinson.

On my Sound Technology, I get measurements showing that single ended output should be kept below 4V.  (I actually don't use the single ended output for anything except driving the Sonos box at 2V in the bedroom.  The balanced outputs go to the Masterlink in the bedroom, and the Lavry in the Living Room, my two most critical things.)

6V output...4% THD
5V output...0.38% THD
4V output...0.0063% THD
3V output...0.0046% THD (this is about my residual on this analyzer when it's working great, once in a blue moon it can hit 0.0044%, but right after taking these measurements it decided to go beyond 3% resdual, a sign of internal overwarming or something, so I shut it down).

In balanced output, this would probably translate to a maximum low distortion output of 8V, well more than what I need for the Masterlink (4V) or the Lavry (5.4V).  Right now, with cranky Sound Technology analyzer, I can't do the measurement I had hoped to do this afternoon to confirm that.

Bless the Sound Technology, after 3 hours rest it came back on and I was able to record a better residual, which is I think not untypical actually (until after a few hours, it will go up, I adjusted my ST that way so I could do decent measurements without 3 hours warmup, I'm usually just doing a few measurements, though this way, in 3 hours you can't use it optimally).  That is:

Sound Technology Residual: 0.0033% THD

Keep those in mind for the above measurments, the 3V Emotiva output is not exactly the residual, it's 0.0013% higher, but that might depend on the number of minutes of warm up also not that much.  Actually I know from other measurements 2V is at 0.0003%, so the 0.0013% probably says more about the ST and it's state of warmup (remember, it failed just a few minutes later, and I had to let it cool down for 3 hours).

Anyway, with some problematic adapters they may have been contributing, I measured output distortion balanced, once again using the ST as primary source.

At 8V balanced, distortion was a hair above the residual at 0.0038%.

At 10V, distortion had risen to 0.03%.

At 9V, distorition was also around 0.03%

So my prediction about 8V being the highest low distortion output level seems correct, the peak optimal RMS levels are:

4V single ended
8V Balanced

At these output levels, THD has not increased since the lowest output levels, and may even have reached its lowest level.

It goes higher, but distortion starts rising, fwiw, still way below speakers and such a few dB higher.  I'd keep it below 6V single ended and 12V balanced in all cases with great prejudice, though it doesn't really break up till even higher.

I could even crank up the input level on the Lavry, now set to 5.4V, a few dB higher.

The Kitchen HT system sub now re-enabled for stereo

I mainly use my kitchen HT system to play stereo, but I have the two surrounds and sub to add in for actual 5 channel material.  I'm opposed to center channel speakers on various grounds, and I always use the appropriate setup which folds the center as a mono signal into the fromt left and right.

For stereo, I'd always used the "Direct Stereo" output of my Yahama HT-5790.  I measured this, and it clearly bypasses the digital conversions that the various HT modes require.  An alternate is "2 channel stereo" which does do digital processing for 2 channels.  The digital processing is clean except, of course, it introduces noticeable pre and post ringing, the kind that J Peter Moncrieff seems to be telling us is all fine and good.  (And all "objectivist" engineers also, I might add.)

So, using Direct Stereo, I have no "subwoofer" output, because the crossover is implemented in the DSP.

Recently I've been playing FM on the Kitchen Tuner because it attached to my roof level whip antenna which clearly surpasses all my other antennas (merely as a result of height, long I tested inside and it was worse than my indoor antennas).

After I moved on from the McIntosh MR78 with Modafferi Mod, which had a kind of steely, though clearly very high information, sound that was fascinating sometimes but not appealing and actually a bit tiring, I brought out the Sony 730ES (because it was most accessible in my conditioned storage building) and the sound is so much "nicer" somehow, combined with the much better antenna, it's somewhat intoxicating.

But what's always been noticeable was the kind of "miniaturization" done by the Kitchen setup.  This may be partly the almost nearfield location of the speakers, and similar factors.  The speakers themselves are the once Stereophile Class A Revel M20's, which go to 30 something Hz and cross over nicely at 60Hz, as I have always done in the bedroom.

I've always noticed the "heft" of the living room system playing FM, so preferentially even I play the Living Room stereo when I'm in the kitchen, even at the expense of an actual stereo image.  Appropriate somewhat to FM, it was the "concert is in the next room" experience, but like a real concert, instead of the table-top miniature in the kitchen.  Having the subwoofer for regular stereo, now adjusted pretty good, gives things the full size and heft, and even seems to be removing some of the steelyness from the tone--which might have been resulting from mismatch between extended high end and rolled off low end.  It may have been that lack of steelyness which I was confusing for the lack of definition in 2 channel (dsp) mode, much more than the digital resampling and DSP.

At various times I've played the subs, and though this speaker (an SVS 10" from the 2000's which was my very first "real" subwoofer and was a revolutionary upgrade to my bedroom system, replacing the McIntosh M22's I had been using as subs, but that was quickly upgraded to the SVS 39 inch 16 Hz tube). but in the kitchen it had sounded terrible, quickly invoking the "turn it off" reflex, so I guess long ago I had decided Direct Stereo was better and that was that.

But, I wondered, what if I just turned down the subs a little.  I had never really measured the system, or adjusted by measurement, or maybe I'd tried the auto adjust once and hated it, preferring my own manual "by ear" adjustments, notably using the built in level adjust test signals in the receiver, and balancing the different speakers and the subs by ear listening to the test signal.  This was set up before I had an always ready 1/6 octave RTA in my phone.

So I tried adjusting the sub level to get the flattest response using my standard noise source, Stereophile Test Disc 2.

And this required an astonishingly low setting for the subwoofer level.  After several days of readjustment and measurement and listening I've gotten to setting the subs at the low end of the second lowest mark on the level screen, with a 60 Hz crossover and "bass" set to "both".  As I was previously attempting to audibly match the level of the 1kHz centered pink noise signal with the signal in the bass on the Yamaha, I was setting to within the third highest mark.

If I set the "bass" to "swfr" it seems that the low bass disappears in "2 channel mode".  Im suspecting this is a bug in the Yamaha.  That's why I'm using "both," which I assumed would effectively use the bass as fill-in, and since I've set the back speakers to "small" they shouldn't get any bass from the "both".

What I really plan to do is use a behringer DSP to do the fill-in crossover and shaping for the bass, and I currently have a spare DEQ 2496 which I'm clearing the space for now.  Previously, way back in 2009 or so in fact, I bought an extra Behringer of the earlier vintage, DEQ 8020 or something, only 20 bit, and it was much more painful to use having few buttons and unintuitive to me displays, so the "equalizing the kitchen sub" project never got off the ground because I hated the equalizer.  Now I have a spare 2496 which I like (and will have to get another spare if I decide to continue this, or perhaps another miniDSP).

Now what I plan to do run the main speakers straight through, perhaps in Direct Stereo, and capture the extra LR line outputs in the back for the DEQ, which will then perform crossover, eq, level, and perhaps other functions.  However, I'll have to have a switched connection to the sub outs for multichannel inputs, which simply go straight through the receiver