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Old 11th May 2009, 09:56 PM   #1
tbrooke is offline tbrooke  United States
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Default Null Test

I know this is off topic but I saw the article below that basically said that by connecting two amplifiers to a common load and measuring the differences you could adjust them until they became identical and therfore indistinguishable from each other. Their outputs produce a null.


http://www.stereophile.com/features/...ver_challenge/

In this case Bob Carver did it to create a Carver amp identical to an undisclosed "expensive" amp. they talked about the implications of this in that one could "steal" the sound of expensive amps and make cheap clones.

Just thought I'd throw this out because the whole thing seemed absurd to me.

Tom
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Old 11th May 2009, 11:11 PM   #2
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It didn't seem absurd to Bob and the guys at Stereophile. Personally I
don't do such things, but it's generally worth exploring just what makes
different amplifiers sound different. If two really dissimilar designs can
really be made subjectively identical, then that says a lot about the whole
endeavor of high end audio.

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Old 12th May 2009, 12:16 AM   #3
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Warning! Not for beginners!!!

One more approach: to connect a resistor from 120V phase to output of the amp loaded on a speaker and listen to what it produces. Very revealing test.


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Old 12th May 2009, 03:32 AM   #4
flg is offline flg  United States
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"...you could adjust them until they became identical and therfore indistinguishable from each other..."
Just what is it you think might be "adjustable" ???
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Old 12th May 2009, 03:31 PM   #5
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You can adjust the gain, frequency response, and output impedance
without much difficulty. If you get more sophisticated you probably
could also tweak the distortion for a better match.

As I recall the test, Bob probably tweaked the first three, and after
several such the listeners were not able to distinguish the difference.

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Old 12th May 2009, 03:44 PM   #6
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I remember reading this article and Bobs Hocus Pocus. I would be real curious to know bobs method!
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Old 12th May 2009, 03:51 PM   #7
PB2 is offline PB2  United States
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Quote:
Originally posted by Nelson Pass
You can adjust the gain, frequency response, and output impedance
without much difficulty. If you get more sophisticated you probably
could also tweak the distortion for a better match.

As I recall the test, Bob probably tweaked the first three, and after
several such the listeners were not able to distinguish the difference.


Yes, and as I recall from that write up, finally he had to limit power, and add some distortion as the final correction.

Also, the AC line sag was different from his test bench to the listening area, so he finally had to account for that also to get the best match.

Hmmm, this is what engineering teaches, amazing when reality follows the theory.

The article in .pdf:
www.carveraudio.com/CarverChallenge.pdf

Pete B.
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