Have I finally fixed the Acoustats???

It's now been 6 days since I replaced the electrolytic capacitor in my right Acoustat with a new 50uF 100V capacitor from Madisound.  There have been no failures so far.  Alas, it appears both my subwoofers are now defunct, so I've been playing the Acoustats full range, which I concede has virtues, including being a more complete test of the speakers...  Sound has been marvelous, though I also realize now some of the excess bass reverberation in DJ voices in NOT because of my subwoofers.

Months of previous testing suggested the problem with that speaker was not in the Low Frequency circuit, which has the LF transformer, where failures are often said to "usually" occur.

I had been thinking, therefore, for quite some time that the failure might be in the HF transformer.  But Andy Szabo, the AcoustatAnswerMan says that is so rare he has never seen a failed HF transformer.

Then I had also been thinking the failure might be in the resistor.  But unlike the resistor in the left speaker, this one does not have a hairline crack in the visible wire contact.  So I had not suspected a fault in the right resistor.  (The "cracked" left resistor seems to be fine according to all tests done so far.)

But it did seem to me, even when my first interface failed in 2010, that a likely failure point might be the electrolytic capacitor.

Although it seems at first blush a failure in the 47uF capacitor wouldn't cause the amplifier to detect a "short" (in fact, the load would be considerably reduced) I reasoned earlier this year that any such intermittant failure might cause a kind of "flyback" phenomenon when connection is restored that temporarily stored energy is then reflected back at the amplifier in an instantaneous transient, causing the amplifier to "see" what looks like a short for the briefest of durations, such as 10 msec.  The Krell might be able to decide to shutdown in less time than that.  This was my reasoning why the cracked resistor might be causing a problem (although, it could cause OTHER problems if the crack was in the portion going to ground).  And the same reasoning would apply to the electrolytic capacitor.

That is now what I now think was happening with the right speaker.  A momentary failure in the capacitor was being seen as a short by the amplifier.

The left speaker was fixed two months ago by connecting the LF transformer to the correct winding for 1+1 operation, suggesting there might be a fault in 2+2 tap I had connected to.   I didn't really think that changing this would fix the problem, but so far it seems to have done so.

So, six months wondering in the deepest audio desert may have come to an end.  Now, among other things, I need to get the subwoofer plate amplifiers replaced.  A second time in left subwoofer...however that might be fixed by replacing the fuse which I haven't tried yet.

Now, many previous times, I thought I had fixed the Acoustats, not so much by fixing the acoustats but by changing something, like putting the subwoofers on a different circuit, only to find later that each such change didn't prevent another amplifier shutdown a week later.  I'm only 6 days into testing the speakers with the present level of changes, so there's still some possibility of a failure.  But because this has been a holiday vacation week for me, I've done enough testing now, the cure seems pretty well established.

Emotiva Preamp better than Breeze "6010D"

[See final update.  Distortion of the Breeze Preamp is comparable to the Emotiva so long as the Balanced Input on the Breeze is not used.  Using only the RCA inputs, the distortion is 0.0006% or less through the RCA outputs, through the balanced outputs setting a new record at 0.0003% or less.  The DC offset can be nearly zeroed in all conditions at the same gain level, but changes with different gain levels, though I've adjusted it to 3mV or less provided the volume control is not advanced beyond the 3PM point, which I should never need to do.  At max, it's 14mV, which may not be OK for all amplifiers.  This is acceptable for use in gain adjustment for blind testing so long as only the RCA inputs are used and volume kept below 3PM.]

Clearing off the audio bench, again, I finally figured out the R/L mismatch issue that's been preventing me from doing computer measurements for the last few months.  One of the TRS to XLR balanced cables has gone bad.  (The old one was a Belden from Blue Jeans, my usual cable supplier.  TRS is not their usual thing.  Perhaps not XLR so much either.  Their specialty is premium 75 ohm Canare RCA's on Belden cable, and I've never had one like that in hundreds fail.  I took a break right here to order a replacement set of Mogami Gold TRS to XLR cables from Sweetwater.  Will be going that kind of route for TRS cables from now on, and perhaps XLR's as well.  The Mogami XLR's look better strain relief'd than Blue Jeans does.  I've now decided I like Gotham Audio the best for AES XLR, but they don't make TRS to XLR cables.  Mogami has a different "CORE" line of "professional use" cables but they are not the optimal shielded cable, which I need for my instrumentation bench.  Mogami Gold ought to be durable enough, I'm not doing daily gigs in beer joints.)

So, to finally measure the Breeze Preamp using RMAA, I borrowed a TRS to XLR from my keyboard.

First I measured my Emotive XSP-1.  It is impeccable.  Excellent construction with excellent design, including full balanced operation made possible by discrete resistor based attenuation with 0.5dB steps.  This is the kind of technology you used to have to buy $6k (now $20k) Krell preamps for.

Actually I first measured the residual.  It's 0.0003% distortion.

When the Emotive is hooked in, the distortion rises to a mere 0.0005% distortion.  This suggests that the Emotiva actually has about the same THD as my residual, around 0.0003%.  That's what decent (not stellar) engineering can do nowadays.

Then I hooked in the Breeze in place of the Emotiva.  Distortion rises to 0.0068%.  Despite being based on the fabled German MBL 6010, which was measured as having less than 0.002% distortion by Stereophile at 2V, and using even better IC's, the even more fabled AD797's.

I was hoping to use the Breeze as my measurement preamp, but clearly it's not good enough for that, while the Emotiva is.  I'm not thrilled about using it in my living room system either.  The plan was to use it to equalize the gain for ABX purposes.  In fact, the original plan was to use it to convert the unbalanced output of the Denon DVD 5000 (which I was then using as my midrange DAC) to balanced for running the Krell FPB 300 on a different AC circuit, for which balanced operation is a really good idea.

Well, I ultimately decided the Denon DVD 5000, which measured 0.0038% distortion, though having much better S/N than the DVD 9000 for some not yet understood reason, wasn't clean enough.  It doesn't have the ultimate "good" IC's opamps that are available now, it used an earlier generation which aren't very well regarded anymore.  So I've switched to using all Emotive Stealth DC-1's, which have AES inputs, XLR outputs, and measured as good as good or better than I am able to measure, 0.0003% distortion.  Emotiva uses the good IC's, and uses them in textbook quality engineering.

So, if a DAC with 0.0038% distortion isn't clean enough, a preamp with 0.0068% distortion certainly isn't either.

I may just have to get Emotiva preamps for all gain stages.  My main quibble with the XSP-1 Mk 2 is it's rather large and has way more features than I need.  The Breeze seemed to be exactly what I needed but it doesn't seem good enough.  It has XLR and RCA inputs, selectable, and XLR and RCA outputs which are always running independently, and a volume control (based on a single high quality potentiometer, because the level adjustment is done in an unbalanced part of the circuit, with balanced to unbalanced converters preceding it, and unbalanced to balanced converters following it--but this is what you have to do if you are not using precision discrete attenuation like the Emotiva).

Also, I measured fairly high DC offsets from the Breeze.  Around 30mV DC.  Now, the Breeze actually looks fairly textbook also.  I wonder why the distortion measures as high as it does.  One particular point, the Breeze has pretty high 2nd harmonic, filling in the second harmonic that is virtually nil on my analyzer.

I didn't want to tinker with the DC offset trimpots which are clearly right there in the Breeze without measuring the distortion first, fearing that fixing the offset would make the distortion worse.

It's quite possible, I think now, that fixing the DC offset will also, at least to some degree, fix the THD as well, which looks like it may be generated by asymmetry at the summing point of the Breeze.

Update:

I have adjusted the offset(s) in the breeze.  It did not lower the distortion.

The two pots near the input seem to control the unbalanced input.  I adjusted those to reduce unbalanced-to-unbalanced offset to 0.01 mV or less.

The two pots near the volume control seem to control the balanced output.  I adjusted those to balanced-to-balanced offset of 0.01mV or less.  I went back and forth on these adjustments, after many preliminary attempts in which adjustments seemed to make no difference.

Then unbalanced-to-balanced has about 2mV offset, and balanced-to-unbalanced has about 4mV offset, not perfect, but not horrible.

None of this improved the distortion appreciably from 0.0065%.  At one intermediate point, I measured 0.0061%.  Adjusting the balanced output offsets seemed to make no visible difference on the distortion spectrum from the balanced outputs as I was watching it live.  All my distortion measurements are balanced-to-balanced.

Maybe I need 3 Emotiva preamps, or more.  I need at least one for the Living Room, and one for the Laboratory.

I wish I could buy something smaller and simpler, like the Breeze, with performance as good as the Emotiva.

Another Update:

The Breeze is absolutely fine for single ended inputs, except for an unfortunate tendency to change the offset as the volume control is turned.

The RCA output distortion is essentially at my residual at unity gain.  Actually the number is 0.0006%, run through the "processor loop" of the Emotiva (to convert the balanced I/O of my Juli@ card to unbalanced) measured at around 2-3V output--and I can't read the exact number because RMAA is pulsing).  At 4-6V output, distortion rises to 0.04%.  And 1-1.5V, distortion is barely changed from unity.  I would only need 3V or less with most amplifiers.

Adjusting the pots near the input seems to affect the RCA input balance.  Adjusting the pots near the volume control seems to mainly affect the balanced input balance.  Adjusting both, one can make both zero simultanously at unity gain, but both rising to 14mV at volume control maximum, and falling to just slightly below zero.  Below 3PM on the volume control, RCA output offset is below 3mV for both inputs.  For my ABX gain adjustment purposes, I should not need higher than 3PM on the volume control, which is more than a little over unity gain.

Neither set of controls seems to have any affect on distortion through the balanced input.

The balanced input measured even lower distortion.  Taking the RCA output through the Emotiva processor loop, but then taking the balanced output of the Breeze, yields distortion measuring 0.0003%, lower than I see through the Emotiva balanced outputs, in fact, 0.0003% is precisely my residual...the Breeze is not raising the distortion number...meaning it can't be known but is probably significantly lower than 0.0003%.

The source of unnecessary distortion seems to be: high outputs (above 4V for single ended, unable to do this test for balanced) and use of the balanced input.  The balanced output is fine.

It looks like I can use this for ABX gain adjustment after all, just always use the RCA inputs.

The Balanced Inputs aren't horrible, at 0.0065% THD, but it represents unnecessary distortion by today's easily achievable circuits (including LM 4562 op amps and AD 797 and OPA 211), which can achieve 0.0003% or much better.

The fact that the distortion occurs only for balanced input, and has majority 2nd harmonic, indicates that the issue is with the balancing of the input differential amplifier for the XLR inputs.  One would need to fine-tune the gain of one of the two IC's used, and there are no pots for that.  If MBL didn't use trimmers, they might have used more tightly matched resistors setting the input gains.

FM whips

I now have a Sark 110 Antenna Analyzer, and I checked out the Magnum Dynalab ST-2 and the GODAR FM DXR-1000 whip antennas sold for FM.  The Godar is actually sold for all VHF and UHF purposes up to 1000 Mhz.

Both have a VSWR of 10 or so through the middle of the FM band.  This may be about as good as you can do with a skinny whip around this size.  That represents a loss of a few dB compared with a dipole.  An old style rabbit ear dipole, or twinlead folded dipole, has considerably better performance, as I found inside my kitchen over 10 years ago.  I had mounted an ST-2 to the wall, and it was useless compared with the rabbit ears.

But, the advantage of the whip, is you can easily mount it high, higher than anything else because it isn't heavy and doesn't need much support.

And so, it turns out in my "ambitious" new antenna plan, I'm going to mount one of these two whips on the highest point of the fascia board around the roof of the house.  Finally, after a lifetime of wanting to have an outdoor antenna.  I have three other much better outdoor antennas in my collection, but they all require much more ambitious mounting, such as a tower--which is my current long term dream.

But, which one?  Well despite nearly identical performance through the 93-108 Mhz part of the band, the Magnum Dynalab has a notched optimum in the low band between 88-93 Hz.  There it actually has excellent performance, with VWSR dipping sharply to as low as 1.8 at 88 Mhz.  The GODAR has a similar notch, but it's uselessly lower, at 60 Mhz or so when the antenna is fully extended.  Despite what you might think, changing the length has little effect, simply raising the low end VSWR a little, slowly.  It's not like you can tune it in, at least as far as I can see up to 200 Mhz.  The low end notch in the Godar perhaps helps extend the VHF below the FM band, but doesn't help the FM band.

So, if the low FM band is important at all (and to me, it's the best part of all) the ST-2 is the better antenna.  And actually it might even be just a hair better through the rest of the FM band also, except perhaps near the top of the FM band.  (Funny, my hearsay belief had been precisely the opposite about the ST-2, which was why I ventured beyond the mainstream ST-2.)  Many observers believe the solid steel whip construction of the ST-2 looks more durable than the folding style DXR, though the DXR is much stouter and solider than the usual folding whip.  All the better to have the ST-2 at the crown of the roof where it carries the most wind load, and th DXR further down and minimally extended for the scanner 140 Mhz and up.

Now matter how long you extend it, the Godar has an ultimate optimum just below VSWR 10 around 140 Mhz, perfect for scanners, with basically flat response from 100Mhz to 200Mhz otherwise.  Making the Godar as short as as it can go barely affects this at all, except it slowly raises the VWSR below 140 Mhz, becoming significantly worse through the FM band.  It looks to me like the Godar is a nice antenna for my scanner 140 Mhz and above, extended simply to get the desired height, no extension at all might be the "optimum" for scanner purposes except for height.  The GODAR is claimed to be useable to 1000 Mhz, I might keep it fairly short for the best 1000 Mhz performance, which I can't easily measure now.

The antenna generally sold for scanners nowadays is the discone, but any of those is going to require a more serious mount than simply being screwed to the fascia board, as can be done with these skinny whips.

My first priority when I get a serious mount, will be to put up a yagi FM antenna, such as my AP9.

Questioning Beliefs

Testing to find all the Acoustat intermittent shorting faults continues.  This now 6 month process has often revealed my tendency to jump to solid beliefs without sufficient evidence, as one belief after another was demolished by further testing.

There is no evidence the Krell FPB 300 amplifier, fully serviced this year, is at fault, except for apparently detecting a highly intermittent "short" problem which simpler amplifiers, like the Aragon and the Transnova (my Hafler 9300) brush off as long as they don't persist too long.  It plays another low efficiency speaker set, the Revel M20's, at as high output as I can tolerate.  It just doesn't tolerate shorted outputs for more than microseconds, which is apparently what the Acoustats are/were doing.

The Krell now also seems to play the Left (modified) Acoustat speaker OK.  I tested that for 3 weeks including the FM tuner (which seems to cause more shutdowns than anything else) without shutdown, now that I have reset the LF transformer to the correct "Red" wiring which has the highest step-up ratio and matching for a 2 panel system.  That seems to have fixed or at least bypassed the fault, which also appears to have no fault in its HF circuit (much to my surprise, as I had long figured the problem to be intermittant contacts in the HF variable resistor...but those are now very well cleaned and treated with Deoxit).  The Left HF circuit by itself did not fail in weeks of testing.  Then, I disconnected the HF and reconnected the LF correctly to the Red wire.  That did not fail in 3 weeks of testing, so I then tested the speaker full range for a week, and it did not exhibit any problem.  However, as I describe below, there has been one failure which *might* have been caused by the Left speaker...there is still room for doubt...in fact any test of an intermittant problem in a limited time span could be wrong...there is simply no absolutely perfect test as long as the problem is intermittant, and the ones I've been looking at now have become the kind of thing which *might* happen once a month, making it very difficult to track down.

The Right Acoustat seems to have a problem in the HF circuit.  I'm unsure if there is a problem in it's LF circuit, which I might have also changed (since I didn't write down what it was before disconnection).  I've had the HF circuit disconnected for more than a month now just to determine if there is ALSO a problem with the LF circuit.

Setup like that, I have had exactly one shutdown when playing both the R and L speakers, but with only the bass transformer connected on the Right side.  So does that indicate a fault with the L or R ???  Two weeks of followup testing on the Right by itself have not revealed a further issue.  If none appears for another week or so, I'm going to write off that incident as a freak event, perhaps even power-related.

Meanwhile, I tried a super high power test, which yielded further ambiguous results, possibly caused damage.

I have a polarity test app which came with a wave file featuring a full level impulse sustained to a quite low frequency, possibly below 1 Hz.  And it's a positive only impulse.  This is a potentially destructive signal.  And it repeats many times per minute on the wav file which lasts for 30 minutes.

But I can play it continuously for 30 minutes or more on both Acoustats, at 0dB (DAC is set to output 2V, which means the 600W potential power at 2.35V is cut to 255W peak rms), with the 100Hz crossover.

The crossover is very important.  If I disable the 24dB/octave crossover at 100W, the LF transformers heat up greatly.  I never see that heating up anymore with the 100 Hz crossover.

I ran full power (255W peak impulses with extended LF) into the right Acoustat for over 30 minutes. I did not notice the heating up.  Then the fuse blew.  Then I discovered the interface box had heated up to 120F right above the LF transformer.

I have not repeated that test.  On the right speaker, with both transformers now connected, the 3A fuse blew immediately when switching in the signal.  So, technically, this signal is not in the safe range of operation.

I'm suspecting the fuse blew immediately on the right side because both transformers were connected, creating a much greater HF load.  However, it could also be because...the R channel transformer still has an operational fault, even using the correct wiring.  If so, I'm not even sure how important that operational fault is, considering it's out of the safe operating range anyway.

Subsequent to testing both speakers to the full range signal, now known to be out-of-safe-range, I tested both with the 100 Hz crossover re-engaged, for at least 30 minutes, without failure (and the interface box seems to have negligible thermal rise, 2 degrees above ambient or less).  This actually applies the same peak voltage, equivalent to 255W, into the speakers.  And I have played the speakers for awhile.  So it does not appear I actually damaged them with this test.  But it raises further doubt to the belief that when the recent amplifier shutdown playing both speakers occurred, it was not because of the right LF transformer but the left.

BTW I've also shown that the Behringer DEQ does not switch out LC filters when you turn them off in the PEQ menu.  You have to restart the DEQ to get this change (potentially unsafe...) to take effect.  However, for a quick and possibly destructive test, the PEQ's can be completely bypassed in the bypass menu, and that takes immediate effect).

Engaging LC filters, however, does seem to take immediate effect.

******

But setting aside all the endless mind-numbing testing with ambiguous results, always incomplete, and on to what it has also brought to attention.

The Acoustat interface system is ingenius, but unlike the Transnova amplifier I don't see why it need to be considered a perfect design.

Listing to just the LF transformer alone, correctly set to the Red winding, yields fabulous sound.  Perhaps better in some way than with both transformers connected.  It has a greater coherent wholeness, and the speaker sounds far less up-tilted.

As I write this, however, I haven't spent much time listening to the newly corrected Right speaker, which now has greater output from the LF transformer with the Red connection.  That was a huge and unquestionable improvement, perhaps the biggest upgrade in speaker performance since I got the acoustats.  Never mind all the amplifier changes, etc, running the speaker on the correct transformer winding is huge, and huger than huge when it's all part of a dual frequency band network, as it off balances the two parts.

In fact, I could say the entire "lost 6 months" so far, since a speaker damaging accident was rewarded by this one discovery, so important is it, that I had the speaker(s) on the wrong LF transformer winding.

But now that I've measured the fairly decent HF performance of the LF transformer by itself...essentially flat to 1Khz, it makes little sense to me why the HF transformer should be crossed in starting around 350Hz.  And I'm thinking the 1-transformer impulse looks nicer.

But these are also things I need to test better.

I've enjoyed thinking about the possibility of bi-amplifying the acoustat interface LF and HF circuits, but it would probably not be safe, as both amplifiers would share  the same ultimate load on the secondary side of the transformers.

Still, I like the idea of removing the HF network, or at least the capacitor part, from the box for better free wheeling experimentation.  I'd be interested in only rolling in the HF transformer above 7kHz, or finding some way to get phase perfect impulses.

But the issue might not even be in the dual transformer arrangement either, but in the very idea of a need for extended high frequency power response.  Perhaps rolled off highs have a way of just sounding better.  Lots of classic systems have that sound.  Maybe this is even especially true for the FM source I listen to nearly always in background, and maybe less true for more 'clean' sources.

So perhaps the "solution" could just as efficiently provided with digital EQ, applied to the sources that need it.

Anyway, the speakers own internal engineering is now exposed, it can be questioned with and tinkered with, and great impacts and questions arise.  Differences that hugely affect measurement and acoustic performance.  Much greater than the imaginary wiring and amplifier differences.

A friend thinks I'm beginning to question the virtue of 'flat frequency response' above other things, which is good, to his mind.

Distortion by the Numbers

John Sau has posted about how low THD+N should be to absolutely guarantee it is inaudible.

If the system reaches 100dB peaks, we can guarantee that the distortion will be inaudible if it is -100dB, because 0dB is the ultimate limit of hearing sensitivity.  Regardless of masking.  (Actually, in many cases, distortion is masked at much higher levels).

I like this.  I expect many of my numbers-phobic audiophile friends will not.

However, distortion meeting the astronomically small numbers recommended may be inarguably inaudible, however it is far less distortion than is required to achieve basic inaudibility virtually all of the time.

With devices which achieve 0.03% THD, and -110 to -120dB below peak noise (and peak...is an attainable level in my system), my system does not quite meet the recommendation actually, but I believe is adequate, as far as electronics are concerned.

The Acoustats are low distortion compared to typical box speakers, but perhaps not quite the lowest distortion in the lowest distortion speakers of all time.  They have more distortion in the bass than elsewhere, since I cross over at 100 Hz at 24dB/octave, that bass distortion has slight effect, and the rest is well below 1% if not 0.1%.  Still, in any fair assessment, the speakers generate the most distortion in my system, if not most systems, except for some tweako ones.

Amirm is back!

I was saddened to see that Amirm was bounced out of two other websites I have enjoyed.  He, and his purely objectivist interlocutor Ethan Winer, were two of the most interesting people at WhatsBestForum, for example, and now they've both been bounced.

I wouldn't say I would agree with him always, or even a lot, but he's interesting to read and one of the better ones at combining subjectivism with some kind of objectivism.  About as entertaining and probably at least as accurate as J Peter Montcrief--who doesn't seem to have written anything in a long time.

Well, now he has founded a new audio website himself, and it looks good enough to be added to my sidebar (actually, some of those you might think would have been taken off long ago, but whatever, I like to tease, just like these guys do).

https://audiosciencereview.com

Looks like it's filled with refugees from WBF and the like.

Another Amplifier ???

I saw a very nice looking Hafler 9300 just listed on eBay for $395.  I took a look at this review

http://www.hifi-classic.net/review/hafler-9300-120.html

They measured distortion of 0.0025% at 1 watt, the most important watt, where it was continuing to decrease.  And bandwidth over 200 kHz.  Rated distortion is 0.02% at 150 watts into 8 ohms, and 225 watts into 4 ohms.  They measured over 400 watts of dynamic power into 4 ohms.

OK, so I know most subjectivist audiophiles aren't impressed by such measurements, but they are very impressive to me.

It is of course the Jim Strickland Trans Nova design, with help from David Hafler.  Two of the greatest designers in the history of audio, and this (along with the Hafler 9500 and 9505) are the last Trans Nova amplifiers built by the Hafler company.  Hafler was a longtime friend of Strickland and he loved the Trans Nova design, a big part of the reason why he bought Acoustat.

Fosgate rebranded the Hafler 9505 as a Fosgate model after they bought Hafler.  Another endorsement.

All the Trans Nova amps will drive anything, and especially Acoustats of course which are about the hardest "4 ohm" speaker to drive ever made.  But these are the last Trans Nova amplifiers, and probably the best.

The 9500 and 9505 are the big versions, with about double the power.  They command significantly higher prices.  The 9500 got a rave review Stereophile, and had stellar measurements also.

Then I looked at the schematic, and knew instantly I had to have this amplifier.  It is simple, and perfect!  Check this out:

3 stages, can't get lower than that without compromises.

Dual diff amp JFETs in front.  The optimum circuit.

Bipolar drivers.  Also the optimum.  Substantial current gain is required to drive the capacitance of the output stage.

MOSFET outputs in the Trans Nova design that drives anything, with no protection circuitry needed.

My friend Tim was blown away as well.  He suggested it could well be my best sounding amplifier.  He noted the very low feedback (remember the rule, you either want very little or lots and lots...this is the very little side).  He noted the lack of emitter resistors on the outputs.  Normally those are required for thermal stability...but because they add additional feedback they actually create a bit more distortion.

Somehow this amp gets away without the emitter resistors, but still drives anything, with low distortion.

A miracle that hardly anyone noticed.  Except that if you search for reviews by people who have owned 9300 or 9505, they say it's the best they've ever heard.  People puts lots of effort into refurbing them (the low profile capacitors were custom made and can only be substituted with more smaller caps on a circuit board stuffed in) and then say afterwards, it was well worth it, the amp is so wonderful.

So now I have a simple amplifier to try.  Probably as good as any by Pass, or maybe better.

This is not your father's Hafler DH300, a good but fairly standard amplifier.

This looks like it may be the crown jewel of simple yet capable amplifier designs.

And, it must be great for driving Acoustats.  Some of the Pass designs are not, especially the simpler ones.


*** Update: Measurements

My unit is mint-like, came in factory box without any visible issues.  A chunky amp just slightly bigger than Parasound HCA-1000A, with nice rounded-edge heatsinks that look and work very well, and weighing quite a bit more, feels like a brick.

I had no scope hooked up, just relied on readings from my ST-1700B, which I discovered starts malfunctioning after about 3 hours.  I deliberately tuned it to work best before 3 hours, so I wouldn't have to wait that long.  Back then, a decade or so ago, it still worked OK after 3 hours.  Now it doesn't so much, and at 24 hours...it is hopeless, all distortion measurements are 100%.

Anyway, it seemed output could reach to about 60V into 8 ohms, that's 450W peak.  I measured at 30V, which is 112.5W, not quite rated power of 150W/8 ohms, and then did one measurement at 40V, which is 200W, which is above rated power.  At 40V, 1kHz, distortion is 0.045% L...better than many if not most amplifiiers within their ratings, but slightly worse still respectable at 0.16% R.

At 30V, distortion (THD+N, residual 0.0028%) was .005% L and 0.0055% R at 1kHz, .005% L and .0075% R at 20 Hz, .029%L and .05%R at 10kHz, and .067% L and 0.08% R at 20kHz.  The 20kHz distortion doesn't quite meet spec, but is nearly as impressive as everything else.  This is outstanding technical performance, the best I've ever measured in a power amplifier, and nearing my residual at 1kHz and 20 Hz (!).  There may be some aging effect as the amplifier is 25 years old, possibly explaining the out-of-speck 20kHz performance.

Stellar performance...AND it is a very simple design with low feedback.  An incredible "sleeper."  Great peak power, high power bandwidth (actually...I did not measure power bandwidth).

Offset a tad high, at 28mV L and 31mV R.  Perhaps some kind of aging has affected both the high frequency distortion, which is a bit out of spec, as well as the offset.  Still, few amplifiers have lower distortion at 20kHz, and the offset is tolerable for a power amplifier and low impedance speakers.

Idles to 114F to 120F on the heatsinks, less toward the back.  This is notably but not dangerously warm, and also shows the rather high bias operation, even with heatsinks which look to be larger than those on the equivalently powerful Parasound HCA 1500A.

This does look like it would be second to none on the Acoustats, except possibly in dynamics as it is unlikely it has power stiffness as the Krell, or quite as much output at 4 ohms.

But, like the Aragon, there is no limiting circuitry to cause the amp to shut down.  It just keeps on swinging.