Also kind of interesting to point out that the other way, making the Conrad Johnson amp sound like the Carver, in an afternoon with a bucket of parts, would have been nearly impossible.
Maybe he could have taken feedback off the output? I'm not sure but lowering the output impedance of a tube amp sounds a lot harder than raising the impedance of a SS amp.
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Not explicitly, but the notion of a transfer function is a complicated subject. @almarg Yep, and noise and distortion can be a part of it. I’m just trying to parse out what Carver did during this testing. We think of him as a god, faithfully recreating the output of one amp with a complex load in a way no other EE could possibly do. I still think the test design was still ingenious, but if we limit our thinking to Carver changing just the frequency response and output impedance, then this seems like something any competent amp engineer could do in a day, which then explains what happened, which then informs us of what was important. Of course, it’s 3 stages of supposition, but it’s fun to think about. Best, E |
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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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. |
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. :) |
i will add your list ignored the great contribution of Dr. Matti Otala...TIM
Sorry, I meant to type that but instead typed IM. But ... 1970. |
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.
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What good would it do to recreate this test?
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. Best, Erik |
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!"
Best,
E
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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. |
@maplegrovemusic
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. Best, E |
@spatialking Apologies, I seem to have been very much in error. You were right. I was wrong.
Best,
E
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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.
Best,
E
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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.
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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. Best, E |
