[sdiy] Better testing techiques for audio circuits

[Neil Johnson]

Hi,

> Neil - can I just clarify...
>
>>1/ The nulling technique, where you apply a stimulus (e.g., a sine
> wave) to a circuit and then measure the difference between the input
> and output using a true RMS AC voltmeter.
>
> I'm measuring the stimulus at source (a) - e.g. a sine wave generator at its output, not connected to anything (except the meter), then I'm taking a second measurement at the output of the circuit under test (b) with the sine wave as an input to that circuit and from there I can assume that the difference between (a) and (b) is 'stuff' (noise?) that's introduced by the circuit?

No.  You need to build a high-quality difference amplifier and a
phase/gain compensator.  Its described in this paper (see figure 2.2):

http://www.angelfire.com/ab3/mjramp/distortiontest.pdf

There's also a good simple diagram here:

http://www.daqarta.com/dw_0d0e.htm

The technique is sometimes described as "distortion magnification"...

http://www.cordellaudio.com/instrumentation/distortion_magnifier.shtml

> How much more detail can I potentially get out of that other than e.g. seeing that a 5V AC input is coming out at 5.2V and therefore the circuit is adding 0.2V of 'stuff'? Does that make sense?

But then you're assuming that the test setup is exactly the same in
both cases.  Unless your circuit is distorting really badly that you
see it that way, then you just won't see what you're trying to see.
You're also assuming that your circuit has a gain of 1.0.  If it has
any other gain then you're not seeing distortion as such, just gain.

Cheers,
Neil
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