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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Re: 3rd harmonic distortion posted by Soundmind on December 31, 2004 at 07:17:57:
Soundmind, this is where you are in error. It is not necessary to measure the distortion of a process at ALL operating levels if it is operating class A. What you need to know is the order of the distortion at one level and you can predict the drop in distortion at lower levels. For example magnetic tape.
If O Vu = 0.5% 3rd harmonic Vu=dB in this case
then -10 Vu = 0.05%
and -20 Vu = 0.005%
etc
Second harmonic drops more slowly.
For example 1% second harmonic at 0dB gives .1% at -20dB and .01% at -40dBMany open loop systems operate class A or at least class A at low levels. This includes many tube circuits, analog magnetic tape, vinyl discs, loudspeakers, and many discrete component solid state designs.
The practical actual listening level might have very low to vanishingly small distortion, except for perhaps some residual 2'nd harmonic, since it falls relatively slowly. A little 2'nd harmonic is practically indistinguishable from the original source material when listening anyway, so a little won't have much effect.When we discuss subtle interactions of negative feedback, we are trying to explain why negative feedback can be a potential problem in some cases, because of our direct experience with using it, and finding the subjective results less acceptable than predicted by the static measurements.
At an advanced academic level, we find that negative feedback can actually produce distortion of a higher order. This is interesting stuff to experienced designers often who wonder why their 'perfectly measuring design' was rejected by the users of their product, and something else was preferred.
Of course, this factor is but one of several. Others might include TIM (SID), IIM, PIM, Hiraga distortion, peak current limitations, power supply ringing, etc.
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Follow Ups
- Re: 3rd harmonic distortion - john curl 13:17:22 12/31/04 (0)
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