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Bob Carver Amplifier Challenge

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Many years ago Bob Carver made one of his solid state amps sound like a very high cost tube amp. It convinced Stereophile they sounded the same. Anyone here know the details of what he did to his amp to achieve this? I here talk of matching transfer functions. What was done to do that? What parameters of the amp were altered?
Neither high output resistance, nor noise and hum make solid state amps to sound "toobey". What makes, first of all as less as possible of distortions specific to typical opamp topology, especially dynamic in nature distortions. Adding resistors in series with loudspeakers, yes, can make some of them lower.

Here is a lecture on BAF of Bob Cordell, who makes toobey-sounding solid state amps.


Here is a lecture of Bruno Putzeys who makes toobey-sounding class D amps:

The youtube videos are interesting. But I am most interested specially in what Mr. Carver did.

You need to go no further then right here on the forums: Pass Diy.

I have built both SS and tube preamps and amps. Pass DIY designs are a great place to learn about how SS can sound very tubey. Have you spent any time on the Pass forum?

For Pass, it’s really about positive and negative 2nd order harmonics (various levels) with a lesser amount of 3rd.

These distortion products are simple psychoacoustic parlor tricks to get a SS designs to sound like tubes. Works very well.


I haven't looked at the pass forum yet. But what I am seeing is that to make it sound like a tube you make the amp have more distortion and less damping factor. To me, that is not the role of an audio amp. That is the role of a tone control or an effects box.

I remember Popular Electronics had a construction project for a device, with a tube in it, that you could dial in the amount of "tube" that you wanted in your music. That seems like the proper way to do it.

Mr. ggetzoff, do you still that way about the 737 MAX?
"If it's not Boeing I'm not going."
But what I am seeing is that to make it sound like a tube you make the amp have more distortion and less damping factor.

Absolutely not. In my SE amps I can select 4 damping factors; low as pentode output, medium as triode output; high as usual push-pull with feedback, and active servo control, with negative output resistance. In all 4 cases it sounds tubey, transparent, the only difference is in the resulting flatness of frequency response. Also, distortions are diminishingly low, until it approaches soft saturation. Very toobey sound.

Here is the video:

Wavebourn - Integrated Edelweiss-3 with damping control switch | Facebook
Years ago . . . Stereo Review did a double blind listening test of amplifiers, several solid state, and one Futterman OTL tube amp.

There was NO statistical proof that those particular amps could be identified as a tube or solid state amp.

Take an amplifier, use a Totem Pole Output stage (either tube or transistor), and then wrap Lots and Lots of negative feedback around it. Congratulations, you just intrinsically made them all sound the same.
I agree that boosting up the GNF makes components sound the same and sound is converging. There are many subjective aspects of music, and GNF has not been the definitive answer to those. Anyone with a quality amplifier would could change the feedback levels knows that.

I believe that the answer to modern era tube amps is mostly into the OT design which is based on symmetry and double quad cores.
(edit: recent SS amplifiers of good subjective qualities are using a high value capacitor in the output, we are still on the fence with class D, they are not a panacea.)

What is the goal is to achieve that high GNF sound with keeping the dynamics, low sound details and intelligibility with realism.
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What I failed to say was:

1. The Stereo Review double blind listening test was decades ago, on some amplifiers of the day. These amps may have had some other things in common than just Totem Pole, and Lots of negative feedback.

2. Even if some of todays amplifiers that use Totem Pole outputs and Lots of negative feedback are subjected to a double blind listening test, they likely may not all sound the same. It remains to be seen.

3. I ran a listening test of 300B PSE and 300B Push Pull at a Hi Fi show. That test showed a lot, but had some test details that needed to be changed, but could not with the equipment that was at hand.
I feel that toobey sound is a defect. A linear amp only makes the output bigger. Not impart an identifiable sound character, toobey or transistory.

Once I heard a report or study, I cannot say from where I got the info, that people that listen to live music prefer SS, and people that listen to recorded music prefer tubes. Consider it an unsubstantiated subjective claim.

I'm just saying that if you tell it's tubes or transistors, then it is a bad design. Or it could be so dirty that no determination can be made. But lets not consider that option.

I'd like to see a controlled valid test of two amps, matched for various factors, then repeat the test with no networks on the input or output, just raw high quality audio direct from the device, then try to determine what, if anything, made them sound different.

I looked for that PE article, I did not find it yet. It was many years ago and I didn't see it on the archive I searched.
How different or how the same a group of amplifiers sounds is often a function of the loudspeakers that load the amp.

All experiments are limited in scope.
Most scientific studies require that, or else there are too many variables to know which one or ones are causing the results.

"All Generalizations Have Exceptions" - Me
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People really need to get their head out of "the tuby sound" galaxy. I'm not against tube equipment, I own several tube amps, but not for the reasons of "having that tube sound". That's just BS.

I focus on accuracy of amplification, what goes in, comes out amplified, with no annoying changes to the input signal. And that can be done with tubes, or transistors, if designed properly.
In the first article below Bob Carver discusses some of the things he did for the Stereophile challenge. Bob had also performed this "transfer function" for "The Audio Critic" magazine several years earlier than he did for Stereophile in October 1985. This null test actually has some history going back even further (see Baxandall article below).

Ultra High Fidelity: Interview: Sounding Off: An Interview with Bob Carver of Carver Corporation by Gordon Brockhouse High Fidelity Oct 1988 pgs A1, A2, A4, A5
(NOTE: These pages appear between pages 48 and 49)

"The Carver Amplifier Controversy Four and a Half Years Later (Will They Ever Learn?)"
The Audio Critic Issue No. 10 Fall/Year End 1987 see pages 40-44

"Audible amplifier distortion is not a mystery" by Peter J. Baxandall Wireless World
November 1977 pgs 63-66 (null test)

"Nulling Out Amplifier Distortion" by David Hafler Audio February 1987 see pgs 46-49

Hafler also did some work with null testing in the Hafler XL-280 and XL-600 amplifiers.
see Hafler XL-600 Power Amplifier Owner's Manual (see pages 6-10 "The Excelinear Conceit" differential/null test)
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