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Old 18th August 2012, 08:29 PM   #1
Hojvaelde is offline Hojvaelde  Denmark
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Default Distortion


I would like to measure the distortion of my amp.
I have sent with a drawing of my thinking method.

The used opamp is with very low distortion.

But how should I interpret that I get out of the differential amplifier.


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Old 18th August 2012, 09:15 PM   #2
Nelson Pass is offline Nelson Pass  United States
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If the circuit is set up and adjusted correctly, you would see the difference
between the input and the output of the tube amp. After you null out
amplitude and phase, you would be left with distortion and noise.

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Old 18th August 2012, 10:28 PM   #3
tomchr is offline tomchr  Canada
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If your goal is to measure the distortion of your amp, I'd suggest getting a quality external USB or FireWire sound card and some software (TrueRTA comes to mind, though, there are others).

Or... Get your paws on an HP 8903A or 8903B audio analyzer.

However, if your goal is to build a distortion analyzer, I'd get the service manual for the HP8903A and build a similar circuit. Or build the circuit in your schematic. I think your system will be limited by the accuracy of the phase adjustment, though. Recall that the phase of your amp varies with frequency so the phase of the compensation amp will have to vary accordingly.

MOD686: Up to 240W/8Ω, <0.00025% THD. MOD86: 40W/8Ω, 0.000054% THD. HP-1: 3W/20Ω, -130dBc THD, 128dB DNR.
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