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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.


https://www.stereophile.com/content/carver-challenge


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:


https://www.soundandvision.com/content/reconsidering-julian-hirsch

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.


Best,
E


erik_squires
bradthediscoman
1 posts
 
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.
rixthetrick's avatar
rixthetrick
1,060 posts
 
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.
atmasphere's avatar
atmasphere
11,879 posts
 
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'.  
bigkidz
3,397 posts
 
@atmasphere - exactly plus who cares anyway the end sound is what is important
erik_squires OP
16,281 posts
 
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. :)
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