Can I look for (or even measure) distorsion on an amp with a dual scope? - diyAudio
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Old 16th September 2003, 03:21 PM   #1
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Default Can I look for (or even measure) distorsion on an amp with a dual scope?

Is this possible?
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Old 16th September 2003, 03:32 PM   #2
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Yes. If your scope has an A-B setting, you can measure large amounts of distortion. It takes an attenuator and a fiddly hand to null out.

For a better measurement, use a precision instrumentation amp with high CMRR at the frequncies of interest to compare input to (scaled-down) output. Then you only need a single channel.
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Old 16th September 2003, 04:06 PM   #3
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If by A-B, you mean a dual timebase, yes it has one.
It also has little calibration pots for the vertical sensibility

I was trying to play with the CH2 invert, in ADD mode, and adjunsting the sensibilities to cancel the signal, and see if sometimes there is a difference
but it's not easy to adjust the signals to cancel them


Is there a better method? And only with a dual scope, no other lab tools
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Old 16th September 2003, 04:20 PM   #4
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In Nelson Pass' article on the Citation 12 (in his archive section I believe), he describes how to use the amp to "measure itself" which is a slick system. I haven't tried it and havent thought too hard about it (it's a bit of a self licking ice cream cone) but it's something to check out.


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Old 16th September 2003, 04:43 PM   #5
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Quote:
If by A-B, you mean a dual timebase, yes it has one.
No, I meant a setting that would subtract B from A, while letting the voltage scale be different on the two channels. A + B with an invert is just as good. Try scaling down the output externally, with a voltage divider and a 10-turn pot for trim; that will make the null a little less twitchy. But only a little less.
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Old 16th September 2003, 04:53 PM   #6
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Sy,

I gather you propose to oput the input signal on one channel, to put an attenuated version of the output on the other, then subtract and null for min level.
This will indeed let you large amounts of THD only. But the phase shift through the amp will also limit the minimum you can get, unless you also delay the input signal and equivalent amount. It gets messy quickly.

I know that this method has once be proposed by Linsley Hood it was (or maybe Peter baxandall), but he used a series circuit of two adjustable phase shifters for the input network. He then subtracted the scaled down output of one channel of his amp (because the amp was inverting, he just connected the two channels together IIRC). The result (ideally, the THD only) was fed into the other channel, allowing him to listen to the distortion. Slick, but not for the faint of heart.

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Old 16th September 2003, 05:00 PM   #7
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Is the delay really an issue at midband?
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Old 16th September 2003, 05:03 PM   #8
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Hmm, good point. I don't know really. I do remember the involved phase shifters in the Hood/Baxandall example.

What would be a typical delay through a power amp? A microsecond? That would be only a fraction of a degree at 1kHz. Yes, I think you're right. Seems worth a try.


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Old 16th September 2003, 09:06 PM   #9
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Bricolo

I noticed that you insist on using a scope but honestly it's not really suited for distortion measurements. You'll get much better results if you use a decent soundcard, possibly with a buffer pre and some spectrum-analyser soft
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Old 16th September 2003, 09:37 PM   #10
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Quote:
Originally posted by analog_sa
Bricolo

I noticed that you insist on using a scope but honestly it's not really suited for distortion measurements. You'll get much better results if you use a decent soundcard, possibly with a buffer pre and some spectrum-analyser soft
I prefer a distortion analyser with residual output, and I like time and frequency domain both, can't live without each of them

I am in a luxuary position, own an HP339, an HP3580A and daily access to an AP one.....

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