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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: kind of a long answer posted by morricab on August 17, 2006 at 12:39:51:
HiGood eye, here is what you are seeing.
The negative feedback is what lowers the amplifiers output impedance as well as "fix" distortion.
More or less, the shape of the damping factor curve is the inverse of the shape of the open loop bandwidth, same for the THD curve.
What you see is that above some frequency, more and more of the feedback is used to extend the frequency response, past the open loop corner.
As more of the gain is used for extending the response, less is available for lowering the output Z and THD.
For a hifi amp, a 1 KHz open loop bw is not at all unbelievable, some old op amps had an open loop corner at 10 - 100Hz (only) but went to 20+KHz with huge fb.
For a 1KHz open loop corner, a mere 30 dB (1000:1) of feed back is needed to extend the closed loop response to 32 KHz.Another "curious thing" about large fb is what happens to simple order distortion.
Your ears hear a 2nd harmonic poorly a 3rd much more easily and the higher you go in order, the more audible the harmonic is (except for when the harmonic fall above say 5KHz where ones hearing is falling off). With music, even harmonics are less detectable than odd harmonics which are dissonant.
This is why "THD" is nearly meaningless so far as what it sounds like, it has no weighting to account for how audible each harmonic is relative to the fundamental.
Take an amplifier that has say a THD of 10%, but is 10% of the 2nd harmonic only, that may not be audible, add 20 dB of -fb and reduce the THD to say .2% but now instead of only a 2nd harmonic, the 5th, 7th, 9th and 11th harmonics are the highest level, in a range (harmonic number) where your ears are Vastly more sensitive.
Consider given the way your ears hear, at 20Hz, a 3rd harmonic of only 7% has an apparent loudness EQUAL to the fundamental. Similarly, a HUGE level of THD can be tolerated above say 5KHz as everyone’s hearing has fallen off the cliff by say the 4th harmonic.
Best,Tom
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Follow Ups
- Re: kind of a long answer - tomservo 13:26:15 08/17/06 (1)
- Re: kind of a long answer - morricab 02:13:05 08/18/06 (0)
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