The Carver Amp Challenge and the 21st Century and it's Failure
Some of you may be old enough to remember this article from Stereophile. Bob Carver claimed he could make an amplifier audibly indistinguishable from some of the best from Conrad Johnson. A high efficiency (not class D), solid state linear amp vs. a linear tube amplifier.
Carver's approach was to feed a speaker via both amps at the same time using opposite terminals. The speaker itself was the measure of accuracy. Any difference in output between the two amplifiers would cause audible output.
What's super important here is Carver invented a new way to measure the relative difference of amplifiers with a real load.
That's kind of revolutionary from the standpoint of commonly published measurements of amplifiers before. Steady state, frequency sweeps, THD, IM and S/N all failed (to my ears) to express human experience and preference. I remember a reviewer for Audio, I think Julian Hiirsch, who claimed that these primitive measures were enough to tell you what an amplifier sounds like. The man had no ear at all, in my mind. More here:
And here was Carver in 1985 cleverly showing that two amplifiers which measured reasonably well, sounded differently. We should also be in awe of Carver's ability to shape the transfer function on the fly. That's pretty remarkable too but not the scope of this post.
My point is, really, Carver showed us a revolutionary way to examine differences between gear in 1985 and yet ... it did not become widespread. << insert endless screaming here >>
As far as I know (and that is very little) no manufacturer of any bit of kit or cable took this technique up. We are still stuck in 1985 for specifications, measurements and lack of understanding of what measures cause what effects and end up cycling through cables and amps based on a great deal of uncertainty.
My points, in summary:
Most of what we consider state-of-the-art measurements are stuck in the 1970s.
There are a number of ways to improve upon them
No one has.
We should be a little more humble when asserting if it can't be measured it isn't audible because our measurements are not nearly comprehensive
I look forward to manufacturers or hobbyists taking modern equipment to pursue new measurement and new insights into our hobby.
Yeah, that was part of the story. It started with a bold claim from Carver stating his amplifier was indistinguishable from a tube unit. I believe Peter Aczel picked up the story in his review magazine, The Audio Critic. When it wasn't the same, Carver had to modify his amplifier in significant ways to make it sound the same, one of which was increasing its current capacity significantly. Those mods never made it into production since they were much too expensive.
Tweaking time domain energy here and there, that has all been done before, although I think Bob was the first to apply it to audio work, ignoring tone controls and equalizers, of course. Aczel's magazine went defunct before the review was published
but Bob still reprinted that review with permission to sell his amplifiers.
I don't agree that most of our measurements are stuck in 1970 technology. The test equipment we have today is like comparing a Lotus to a Model T. I wish I had today's stuff back in my audio design days! Back then, we used to laugh and say "the test equipment we have today is like comparing a Lotus to a Model T" when we talked about 1970 technology compared to the 1950 technology. However, I find myself saying the same thing today about current test equipment. I can do things in my lab today with 1995-2005 era used equipment that I could never do in the 70's. And, the best part, it is so much less expensive today than back in the 1970's when comparing performance to the dollar.
Likewise, I don't agree that no one has made improvements in measurements. Folks have, I just don't see them dumping their IP into public domain for their competitors to use. See, the thing is, audiophiles want a single measurement that is the cornucopia and tells all. That is unlikely to happen, the fact is our equipment is fine, the key is knowing how to apply the equipment, how to interpret what you are measuring, and then solving the problem.
It started with a bold claim from Carver stating his amplifier was indistinguishable from a tube unit.
His first claim was he could make it indistinguishable. Not that it started there. After that he produced amps such as the M500t. He may have produced others on this same idea, and it's arguable how well he succeeded.
His amp may ONLY have been an accurate reproduction of the CJ amp with 1 particular speaker. In any event, I don't claim his success, but I do claim his revolutionary way to evaluate equipment has not carried us forward, and that's disappointing.
one of which was increasing its current capacity significantly
Not my understanding. Among other things, Carver increased output impedance, going the opposite way.
don't agree that most of our measurements are stuck in 1970
technology. The test equipment we have today is like comparing a Lotus
to a Model T
Which has not actually revolutionized our understanding. We, the buying public, read reviews and measurements taken with test gear which can add more zeroes to the distortion measurements and more dB to the S/N measurements compared to that in the 1970s but we are not presented with revolutionary ways of understanding the performance of electronics and their audibility.
Likewise, I don't agree that no one has made improvements in
measurements. Folks have, I just don't see them dumping their IP into
public domain for their competitors to use.
It's quite plausible this is true, and that leaves us, again the buyers, in exactly the same place as if it didn't exist. We as consumers are stuck with 1970s definitions of electronics measurements, despite vastly superior measurement possibilities, disk storage space, CPU power to now investigate the dynamic performance of gear in entirely new ways.
PS, you may be a little confused since I think Carver may have used the term "current source" to describe the difference. That doesn't mean more current. It's a different operating principle than a voltage source.
To clarify some historical facts about the "Carver Challenge":
Aczel’s preprint of the amp matching effort Carver performed under his auspices was written sometime prior to January, 1983, while the Stereophile article appeared in October, 1985. So Carver’s effort with Aczel was performed well before the one with Stereophile, although the people at Stereophile were apparently unaware of it when their article was written.
Also, "The Audio Critic" didn’t quite go defunct; it resumed publication in late 1987, after a lapse of about six years during which time Aczel served as president of the Fourier Loudspeaker company (which did go defunct during that time, for a variety of reasons Aczel later described in TAC as including inadequate capitalization, too few dealerships, and a warehouse fire).
The target amp that was the subject of the Stereophile article, as stated in footnote 3 on the first page of the article Erik linked to, was a Conrad-Johnson Premier Four tube amp. The target amp that was the subject of Aczel’s article was a Mark Levinson ML-2, a very highly regarded and very expensive solid state pure Class A monoblock amplifier of the time, rated at about 25 watts into 8 ohms but capable of providing huge amounts of current and much greater power into low impedances.
The null test, as described by Aczel, was performed by connecting Carver’s amp and the target amp in the normal manner to speakers that were at inaudible locations, and connecting a monitor speaker and/or a meter at an audible location between the + terminals of the two amps, both amps being provided with identical inputs.
Although Carver was able to tweak his amps to obtain a remarkable null of 70 db or so in those efforts, as I recall he pretty much admitted in an interview with "The Absolute Sound" about 7 or 8 years ago that he couldn’t come close to maintaining that deep a null in production. And as Erik pointed out, the null was achieved with one specific speaker load.
I owned both the original M400, the cube-shaped amplifier in which Carver introduced his magnetic field power supply technology but which was not "transfer function modified" to emulate a different amp, and the subsequent M400t version which was designed to emulate the ML-2.
IME the original M400 sounded very poor, with a strange glare often being present. The M400t sounded surprisingly good, and was certainly an excellent value in relation to its price and its 200 wpc power capability. The most significant weakness in my experience with the M400t, using it with fairly easy-to-drive 7 ohm 90 db speakers, was that the size of the image it projected seemed somewhat constricted. But based on my experience with the two amps it would certainly seem that Carver’s efforts to emulate the sonics of the ML-2, and the technique he used in that process, provided considerable benefit.
Thanks for the clarifications, @almarg I was not aware of the earlier testing with the ML, so my thinking about the current improvements was probably not accurate.
I appreciate your detailed and informative corrections!
What this keeps tickling me of is the Technics digitally controlled Class D amplifier. They use DSP to pre-correct issues the amplifier has with the load. Amplitude and phase issues Class D amps tend to have in the top octaves.
I wonder if today we couldn't make better models, or get a better idea of what amps are doing, in such a way that would allow us to pick a sound.
Most of what we consider state-of-the-art measurements are stuck in the 1970s.
There are a number of ways to improve upon them
No one has.
We should be a little more humble when asserting if it can't be measured it isn't audible because our measurements are not nearly comprehensive
I look forward to manufacturers or hobbyists taking modern equipment to pursue new measurement and new insights into our hobby.
Same here.
Who knows how many more audiophiles there might be if they had not had their minds poisoned by Julian Hirsch? For years I was under his spell, so bad it took many trips and listening sessions to finally realize what a crock of bull it is thinking we can measure music in any meaningful way. Thanks to him and a whole bunch of others misled into his camp we had to go through a whole generation of high powered amps that measured great but sounded bad. We had to go through the CD! Gad!
Suppose it is too late to ask Stereo Review for my money back?
Seriously though, the damage is incalculable. Cable construction. Frequency response. Timing. Double-blind testing. On and on. All kinds of seemingly unrelated things that when you dig into them all are based on the false assumption that measurements matter.
And granted its a little complicated because sometimes measurements do matter. Where the damage is done is at the margins where instead of asking if this is an area where measurements matter people simply assume that they do. Which is wrong. Forced to choose, no measurements at all build everything entirely by ear, and the opposite, I'll take by ear every time.
I'm just thankful there are so many talented designers like Ted Denney and Keith Herron and Duke Lejeune who understand when and how to use measurements, and when to trust their ears.
Erik - Douglas Schroeder just reviewed a amplifier that does what you
are describing . Think it was a Goldnote amp . Something about changing
the dampening factor by a switch .
Yep, similar idea about Carver's challenge with the CJ. Nelson Pass has also written a little about using current source amps with certain types of woofers, with some interesting conclusions.
There is a lot going on... the how with reliability ( never a Carver strong suite imo and experience running a service department in a college town ) may not be shared much because it is valuable IP and patents are expensive and not always worth it...
the amp I am listening to has five parts in the signal path, no emitter resistors, floating ground outputs, tube front end, liquid cooling so bias is a set point, ten regulated power supplies, built in HRS isolation.... etc, etc.....
the null test works well in an anechoic chamber to match pairs of speakers.... but so few do it....the reason why ? Well first ya need a chamber, and when they don’t null out, ya got to know what to do, and want to do something about it vs just passing it along....
the null test works well in an anechoic chamber to match pairs of speakers.... but so few do it....the reason why ? Well first ya need a chamber, and when they don’t null out, ya got to know what to do, and want to do something about it vs just passing it along....
We could just as easily today measure the difference via the speaker terminals, and listen to it with headphones, or save it for spectral analysis. Carver was surely using a scope to figure out exactly what was going on and zeroing in with help.
I also don’t want to get hung up on the null speaker test, because I think there are a number of ways to get more data and be more accurate.
I wanted to point out this test came out in 1985, and nothing in popular reading has come out since its equal or better. That’s what I call stagnation. What the Carver amp challenge shows is the space available for innovation and research. To stop now is like stopping astronomy when we see the moons of Saturn for the first time. "Hey, that's all there is!"
I'm not saying we should do so verbatim. I'm saying that this was a ground breaking test that brought together several aspects of amplifier performance in ways the Stereophile suite does not.
I'm saying it illustrates how limited we actually are in understanding how amps differ.
I don't agree that most of our measurements are stuck in 1970 technology.
Can you name a new measurement since 1985 which has become part of the vernacular?
I wish I had today's stuff back in my audio design days!
Same. But adding precision is not a new measurement. It's just more precise.
Likewise, I don't agree that no one has made improvements in
measurements. Folks have, I just don't see them dumping their IP into
public domain for their competitors to use.
If true (not doubting it) that's at the core of our problem as end users.
To give an example of what I'm talking about as really revolutionary, consider the system Meyer's sound labs uses to tune pro audio systems in real time as the audience arrives.
THAT is, relatively, revolutionary, and takes huge advantage of DSP and computer processing power to coordinate hundreds of speakers at a time.
Being able to push the noise floor from -70 dB to -100 dB is not. It's the same measurement, but with better gear. And as @elizabeth has pointed out, you can buy for a couple of hundred dollars what would have cost tens of thousands in 1985.
Elizabeth, you have the right take, in my view. Carver is a genius and I would be keen to hear feedback from owners of his new Crimson 275 tube amp. I have casted about for this feedback since Sept 2018.
I don't think that manufacturers want a revealing measurement that could be used by consumers to understand real world performance, might hurt sales for some high dollar equipment.
I have had two Carver TFM-42 amps driving a pair of Caver Amazing Silvers for around 30 years working flawlessly. I just upgraded speakers to the Legacy Whisper XDS (delivered Saturday), maybe after some time I will take a look at the Carver 350s.
I own a M400T and still use it in my pool table room. I also owned and sold dual M500T’s. I consider Bob to be a true maverick with his innovative creations. Alas, Bob is getting up in years and is mostly on the retired genius path. So, future products may be limited. But, he did tell me that he was working on a new tube preamp.
After many years of using various amplifiers, I purchased the Bob Carver Raven 350’s and enjoying them as I write this. There was a problem with the XLR output in my unit that was easily fixed, so anyone buying one intending to use the balanced out should contact Bob Carver Corporation or myself for the correction.
@Elizabeth, @Lowrider - I think I can shed some light on Bob Carver "haters", although I think the term "hater" is really too harsh, at least for me.
I met Bob at a CES show many years ago. We were both a lot younger then and back then he had a personality that just didn't work with me. I am not sure what it was. Perhaps, I didn't like his bragging about the sound in his booth as a number of his competitors I visited had surprisingly better sound.
That was one of my jobs at CES, as a design engineer I had to check out the competition, check out the sound, and meet the designers, and get a feel for their philosophy. (Analog circuit design is about 1/3 science, 1/3 philosophy, and 1/3 art form.) Yet there he was Bob telling me his booth had the best sound at the show, and how the sound did this, and did that; yet my ears told me it didn't do any if the things he was claiming. I guess he expected me to believe what he was saying rather than what I was hearing.
The reason I walked over to his booth is one of our dealers came by our booth and mentioned how bad that sound was. So bad he decided right off not to pick up his line. In any event, I can see how Bob's personality back then could rub people the wrong way. I still wouldn't use the term hater though. Not too surprisingly, I never purchased any of his equipment since meeting him that first time.
BTW, wasn't he the lead designer at Phase Linear? I seem to think he was.
There is no reason to hate Bob Carver and no reason to be infatuated with him either. Every company he has ever owned has gone out of business except for his current company and that is only because he has not had time yet to let it run out of business many of his products, designs, and technologies were only for show such as his silly "magnetic" amplifier and auto-correlator circuits and then of course the famous "Flame Linear" designs that weren't properly conceived, specified, or manufactured he is a promoter, a showman, a "carnival barker" as Americans say!
@spatialking , interesting story. I knew Bob’s ego was beyond the pale back in the day, but that was very ballsy behavior.
Clearthink, I’ve read the stories where Bob was considered a showman or a barker. I can see how those in the industry resented that. Also, his history of company ownership raises concern about the future of his current highly regarded tube amps. Will there be any authorised service when he hangs it up?
I’ve owned two of his amps. A high powered Carver, which at that time, power was king for a young headbanger. Next was a Sunfire 300, which I consider a first step into a serious home audio system.
lowrider57" I’ve read the stories where Bob was considered a showman or a barker. I can see how those in the industry resented that. Also, his history of company ownership raises concern about the future of his current highly regarded tube amps. Will there be any authorised service when he hangs it up?"
It is not likely that is why all of his companys have gone "belly up" as Americans say.
The issue behind us hard nosed business types is that designing a reliable product, the real engineering work.. which is viewed as boring to many IS key to long term survival....
Bob and his now rather extensive lineage of products and companies in the wake, do not have a reputation for longevity or reliability.
as an aside, we were a very early PS Audio dealer - they offered quirky products, oversized power supplies, frequently great sound for $$$ and I remember only one amp and zero preamps in the service department.
i could set my watch for Tuesday morning Frat boys dropping off the Phase 400 for a weekly fix....
about a 2 week break even payback to switch to a Hafler DH-200 and 4 weeks for a DH-500
Seriously the speaker null part of the test is essential IF you really want to move the ball, just don’t pick a lame easy load... back EMF, phase angle, reactive and reflective impedance.... none of that shows up in the “ Julian Hirsch “ testing.....
tomic6012 , glad to see another Audionics fan hear. I've had a BT2 preamp and CC2 power amp off and on at three different times starting in 1980. I still have my last BT2 that I use as a backup to my Audio Research SP6. They have amazingly similar voicing.
I've used a pair of CC2's in mono mode with the power-hungry Thiel 3.5's ..... a match nobody would think to recommend but one which worked beautifully. I don't think I've ever had another amp that reached the "back of the stage" as effectivly as that pair.
Bruno Putzeys claims that he can make his n-core amplifiers sound in any desirable way. The OEM line is bufferless, manufacturers can bring their own sound with the buffer construction. I tested some of them (including those based on the expensive SonicImagery Labs opamps ) and for me the winner is the basic Hypex “tester” buffer costing a fraction. I understand that neutrality is not everybody’s cup of tea though...
@harrylavo : I bought my first Audionics CC2 back in '78. Later added a second one! Now, all these years later, still have one (number 3) in my collection! A superb sounding amp!
roberjerman2 - They do elicit loyalty, do they not. I was doing some work for The Abso!ute Sound in the years before they came out, and still had access to Harry Pearon's listening rooms. When I heard them, I was smitten.
Excellent article and thread. Thank you. The best thread I have found in a long time. In regards to the "glare" reference of Al's, I have found that with speaker placement ( I have Klipsch Klipschorns, KLH 30's, and CF 2's (you can tell I am a fan...lol)) and a little tweaking of the signal path the glare disappears.
I agree with that Bob is a genius. The simple fact that his 400 (in all of its iterations) is an original masterpiece of engineering that changed an entire paradigm, and is still running with the big guys almost 40 years later, is testament to that. That applies to just about all his products including his M500t, of which I am blessed to have four. As to any criticism of the "sound" of carver amps, music reproduction, like art, is in the "ear" of the beholder. The simple fact that Bob Carver opened up "upper class" sound to us middle class and poor folk, did indeed build a better "mousetrap", that is still sought out and used in the 21st century, is testament to his creative genius.
Of course it is understood that Bob Carver used a measurement device to tune his amplifier to match the two amplifiers, right? He didn’t sit down with an oscilloscope, he used the output of the loudspeaker to identify the differences. He did use his ears, not his eyes, and he did use a loudspeaker to do this.
Like for example a waveform that has low distortion however isn’t a match to the original despite not having more or less distortion isn’t going to sound the same. And yes a computer can analyze this difference. This difference can be shown graphically or like the speaker did, be represented audibly.
I am agreeing that despite the sound reproduced by a device having less noise, than another with more noise where the wave form is closer to the original can and often will sound better than the former in resolving as an approximate to the original. This is measurable.
Is it possible that science hasn’t discovered all of the parameters that we can perceive with our hearing? Maybe not, maybe we've been sold on the wrong metrics. Perhaps there’s bigger fish to fry.
One vast, gaping hole in Bob's idea way back then is the output impedance of the two amps involved in the test. If not identical, the one with the lower output impedance will dominate the results.
And if its not obvious, another serious issue is simply that neither amp will be operating correctly if connected to another amp in a null test! This is the sort of thing you don't do at home, lest ye 'let the smoke out'.
So, I kind of let this thread go in preference to newer threads talking about the same idea, that we’re stuck, but I wanted to take this up:
One vast, gaping hole in Bob’s idea way back then is the output impedance of the two amps involved in the test. If not identical, the one with the lower output impedance will dominate the results.
@atmasphere
Au contraire, mon ami. The clever EE trickery Bob does forced him to alter the output impedance of his amp to match the other. Using the complex impedance of the speaker in between two amps is how it happened.
Amp1 (+) =====> (+) Speaker (-) =====> (+) Amp2
The goal being to null the voltage at the speaker terminals with music, which, as you allude to, could only happen if the impedance of the amps matched. Now, thinking of this, Carver claimed he manipulated quite a bit, but it may have been all he did was change his amp’s impedance curve. Indeed in future amps, that was the only difference, whether you wanted low or hi Z outputs. I think the story if not the legend is he was able to change the distortion profiles. What if the latter part was bunk. His claim was to match the "transfer function" of the amps. That means voltage into a given load. I've never seen a transfer function that incorporated distortion.
So, arguably, and with some induction (i.e. guessing) Carver proved distortion didn't matter. If your distortion is low enough what matters is the output impedance. Well, I've taught myself something, thank you guys. :)
Ralph ( @atmasphere ), I believe Erik is correct on this, in the first part of his post just above (but see below re the second part of his post). Note the description of the null test I provided in an earlier post (dated 3-3-2019):
The null test, as described by Aczel, was performed by connecting Carver’s amp and the target amp in the normal manner to speakers that were at inaudible locations, and connecting a monitor speaker and/or a meter at an audible location between the + terminals of the two amps, both amps being provided with identical inputs.
Also, note that the two amps were not connected directly together, but rather through a monitor speaker or a meter, presumably some sort of voltmeter having a wide bandwidth.
As I mentioned in that earlier post, though, two major flaws in Carver’s approach were inability to maintain a deep null in production (which he pretty much admitted many years later, in the TAS interview I referred to), and the fact that the deep null he obtained with his modified prototype was obtained with just one particular speaker load.
I’ve never seen a transfer function that incorporated distortion. So, arguably, and with some induction (i.e. guessing) Carver proved distortion didn’t matter.
No, I don’t think that is correct, Erik. The transfer function of an amp is the relation between output and input, not just in terms of voltage gain or any other single parameter, but in terms of everything. If the transfer functions of two amps are precisely matched, or in the case of Bob’s prototypes matched to a null of something like 70 db with a musical test signal, then everything about what the amps are doing (gain, distortion of all kinds, noise, etc.) is presumably matched very closely. But as we’ve both indicated, just under the particular test conditions, including the characteristics of the particular speaker load.
No, I don’t think that is correct, Erik. The transfer function of an amp is the relation between output and input
Right, but have you ever seen a transfer function equation that included distortion?
If the transfer functions of two amps are precisely matched, or in the case of Bob’s prototypes matched to a null of something like 70 db with a musical test signal,
Right, and -70 dB puts it in the range of distortion.
What I’m getting at is that if the two amps matched exclusively on gain vs. frequency and output impedance, then you’d get to this point. What’s left? Distortion and noise.
Using the complex impedance of the speaker in between two amps is how it happened.
Amp1 (+) =====> (+) Speaker (-) =====> (+) Amp2
The
goal being to null the voltage at the speaker terminals with music,
which, as you allude to, could only happen if the impedance of the amps
matched. Now, thinking of this, Carver claimed he manipulated quite a
bit, but it may have been all he did was change his amp’s impedance
curve. Indeed in future amps, that was the only difference, whether you
wanted low or hi Z outputs.
This still seems really problematic, as Al pointed out- the speaker used as a load and how that interacts with the amplifier is a pretty big deal!
Now if this is indeed all about output impedance, way back in the 1950s Fisher, EV and a few others made amplifiers that employed current feedback as well as voltage feedback. Voltage feedback in an amplifier reduces its output impedance; current feedback increases it. The feedback was controlled by a knob marked 'damping factor', 'damping control' or the like. With an amp set up like that, theoretically there's your 'transfer function'...
And since distortion is one of the main things that we audiophiles hear as differences between amps, it would seem that the Carver approach is moot.
Not moot, contradictory, and kind of goes with some of the subtle things I've heard with cables, that the sound of an amp is much more affected by the impedance changes in a cable than we have thought up until now.
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