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Old 4th November 2008, 07:01 PM   #21
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Default Re: IM distortion

Quote:
Originally posted by SCD
Well now I am really thinking. Is there a way to invert the IM distortion and use it as feed back.
If I understand the concepts behind John Broskie's Aikido topology correctly he developed a way to take the power supply noise, invert it and inject it at two points so it would cancel.
This concept sounds desirable. Can the concpet be applied to IM distortion?
If you can isolate distortion from signal, you can inject it to get some
advantage (read the paper on SuperSymmetry).

Or you can apply feedback, but then you knew that already.

Alternatively, I did play with inversion of coefficients of successive stages,
so that each cascaded stage has the "opposite" distortion of the previous.
It reduces the total distortion, but the increase in complexity remains.

This phenomenon has been well known since before I got involved in audio,
and I seem to recall Len Feldman writing about it in the 60's.

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Old 5th November 2008, 01:14 AM   #22
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Mr. Pass,

If IMD only can be cured by feedback, will putting local feedback (resistor from C-B or D-G) on every transistor in the circuit helps lower IM before global feedback applied? It will cut total gain alot , but help linearity, I think.

Or is this suggestion inferior to ordinary global feedback method (where we build up OL gain with many stages, then apply global feedback like usage of opamps).
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Old 5th November 2008, 02:20 PM   #23
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IMD is curable or preventable by all the same mechanisms that
cure or prevent THD, at least as far the article is concerned.

Resistors D-G, etc form local loop feedback, and we clearly can see
that they while they lower total distortion, they contribute to higher
orders of harmonics and sidebands.

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Old 5th November 2008, 03:02 PM   #24
SCD is offline SCD  Canada
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Baby steps
So If I am getting this corrrect.
Relatively simple circuits with consistent, predicable, quality parts that are operated in their linear ranges should produce fairly low levels of IM distortion. Or at least levels that keep the music in the listenable range. Feed back should be used to adjust rather than correct. IE season to taste
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Old 5th November 2008, 06:30 PM   #25
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Quote:
Originally posted by SCD
Relatively simple circuits with consistent, predicable, quality parts that are operated in their linear ranges should produce fairly low levels of IM distortion. Or at least levels that keep the music in the listenable range. Feed back should be used to adjust rather than correct. IE season to taste
Simple circuits with simple low harmonic distortion seems to be the best
approach. Listening to simple music is also a big help.

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Old 5th November 2008, 06:36 PM   #26
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Ive always been interested in IMD effect on complex music but was unsure to what degree.
Thanks for the great article Mr Pass I learn so much from you.
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Old 5th November 2008, 07:27 PM   #27
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Quote:
Originally posted by leapcat
Question
Would a fast amp lower the Im distortion of a complex
signal?
There are some articles that dig into amp bandwidth and distortion.
http://waltjung.org/Classic_Articles.html
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Old 5th November 2008, 07:30 PM   #28
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At intervals I have suggested ideas for testing methods--most recently the idea of using two square waves instead of sine waves for IM testing. Nelson's seven tone test signal is a giant step in that direction (if you do a Fourier breakdown on a square wave, it has a mathematically infinite sequence of components, but the first few are the highest proportionally; they diminish in amplitude as the frequency rises). I, for one, am grateful that he took the time to do so. I don't have enough hardware to do that sort of thing at this time. I can generate three signals, but only two of them are of decent quality.
Nelson,
I may have missed it in the article, but what frequencies/amplitudes did you use?

Grey
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Old 5th November 2008, 08:56 PM   #29
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I wanted to minimize overlap of sidebands, so I chose ratios of
1, 3, 6, 10, 15, 21, 28. It was perfectly arbitrary, and as stated
in the article, they were equal amplitude - again arbitrary.

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Old 5th November 2008, 09:34 PM   #30
leapcat is offline leapcat  United States
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Thanks! Audio se
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