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Old 27th September 2007, 04:57 PM   #1
MBK is offline MBK  Singapore
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Default Geddes on distortion measurements

In a variety of threads, Dr. Earl Geddes has presented his takes on more meaningful methods to test amplifier distortions beyond HD. This is a spin off thread to discuss the subject in detail. I am putting this in the solid state forum because it likely applies more acutely here, where high level HD is a non issue in most designs, but low level audible problems are a constant matter of discussion.
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Old 27th September 2007, 07:30 PM   #2
gedlee is offline gedlee  United States
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Crossover distortion is a particularly insideous form of nonlinearity because it happens at all signal levels and there is no comparable mechanism in a loudspeaker to mask it. The question was asked if I have a way of identifying crossover distortion in an amplifier.

Yes, I do.

You see the situation with crossover distortion is that the % distortion increases with falling signal level. This is exactly why it is so audible since this is directly opposite to our hearing.

One could therefor ***** crossover distortion by looking at THD as the signal level goes lower, which is a typical measurement. The problem is that virtually all of these THD versus level measurements are THD + noise. When this is the case, the rise in THD at lower signal levels is actually the noise and NOT the distortion, but it is impossible to tell which is which. SO this test actually masks the real problem. One would have to track the individual harmonics of the waveform, but then the noise floor is still an issue.

Hence the measurement problem is one of noise floor and how to measure distortion products down below this floor.

This is done by averaging. But normal averaging can only lower the noise floor so much - down to the noise power. But if I have a signal and I average this signal sychronously then I can raise the net signal to noise level. This too is common. But if the signal does not exactly fit the time base then I need to window it and the resultant spectral leakage makes this sychronous averaging less effective.

I use a signal that exactly fits into the time base of the A/D taking the data. This means that I don't have to use a window and I can sychronously average a signal to noise ratio that is about 20 dB better than a simpler test could achieve. This means for example that the input signal needs to be something like 976 Hz, not 1000 Hz, which doesn't exactly fit the window.

I actually had to generate the input wav file in FORTRAN using quad precision, special random number generators and rounding techniques, because the test signals needed to have a 120 dB dynamic range - very difficult with 16 bits.

I use a signal that starts out low and goes up in level. I plot out the results as the signal drops into the noise floor. This test shows vast differences in amps that measure identical with standard tests.

It also shows that my Pioneer amp - you know the "really crappy" one that I get crticized for using at RMAF - is an extremely good amplifier. As good as the best that I have tested with this technique.
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Old 27th September 2007, 07:37 PM   #3
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I know the presentation, reasoning and the files. When I explored them in Cool Edit about 2 years ago, I found parts of the signals with very low level missing, just "zero". Of course , THD would be still low. If this was the case, I would consider such a test very very unfair. Could you explain??
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Old 27th September 2007, 07:37 PM   #4
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Quote:
Originally posted by gedlee
[snip]I use a signal that exactly fits into the time base of the A/D taking the data. This means that I don't have to use a window and I can sychronously average a signal to noise ratio that is about 20 dB better than a simpler test could achieve. This means for example that the input signal needs to be something like 976 Hz, not 1000 Hz, which doesn't exactly fit the window.
[snip]

Mr Geddes,

Have you published more details about your test tsetup, is it available in the public domain?

Jan Didden
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Old 27th September 2007, 07:39 PM   #5
gedlee is offline gedlee  United States
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Quote:
Originally posted by janneman



Mr Geddes,

Have you published more details about your test tsetup, is it available in the public domain?

Jan Didden

No and No
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Old 27th September 2007, 07:41 PM   #6
gedlee is offline gedlee  United States
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Quote:
Originally posted by PMA
I know the presentation, reasoning and the files. When I explored them in Cool Edit about 2 years ago, I found parts of the signals with very low level missing, just "zero". Of course , THD would be still low. If this was the case, I would consider such a test very very unfair. Could you explain??


I don't understand the question. Could you explain?
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Old 27th September 2007, 07:43 PM   #7
PMA is offline PMA  Europe
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The wav files to demonstrate and support unimportance of THD and IMD, that were linked on your website.
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Old 27th September 2007, 08:29 PM   #8
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Quote:
Originally posted by gedlee
This test shows vast differences in amps that measure identical with standard tests.

It also shows that my Pioneer amp - you know the "really crappy" one that I get crticized for using at RMAF - is an extremely good amplifier. As good as the best that I have tested with this technique.

Dr. Geddes,

What is the future of this technique in regards to common use and availability? Will you be patenting it and building a measurement product around it? Or publishing it?

..Todd
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Old 27th September 2007, 08:32 PM   #9
PMA is offline PMA  Europe
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Never found 2 amps with "identical" measurements on my workbench. With a sophisticated set of measurements, NO 2 different circuits measure the same!
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Old 27th September 2007, 08:43 PM   #10
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Hi,

Mr. Geddes,

In an earlier thread as well as now you favor a particular Pioneer amp.
Iíll have to ask out of curiosity, what model?
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