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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: Long-term DBT example posted by andy_c on April 28, 2008 at 08:54:48:
"My point is that harmonics are a symptom of nonlinearity, and that it's not sufficient to use harmonics to describe a nonlinearity or determine the potential subjective effects of it.
"
Well, if they are the AUDIBLE side effects of non-linearity then they are sufficient to describe the nonlinearity from a subjective perspective. In this case the symptom is what you hear (just like a symptom of a cold is headached, fever etc. It may not be the root cause but it sure is what you sense)
"You can see that because K1 is different from K2, there is a discontinuity in the first derivative of this function at the origin, or a "kink" as it were. It's not a smooth function. As I mentioned in my other post to Rick, if we call this function f(Vin), it has the property:
"
I understand but we have to look at the "symptom" of such a discontinuous function because this is what you might be able to hear and that is a burst of some kind of distortion (have no idea what exactly) when that kink in the function is reached. If it behaves like crossover distortion (also a "kink" in the transfer function of an amplifier) or not is open to plenty of speculation.
"this should raise concern that the nature of the distortion could be more subjectively nasty than one might initially think if one only considers that it produces only even-order harmonics.
"
I agree, but this can be found out with a spectrum analysis of the signal after it has passed through the device. My guess is that you are right it will degrade the sound subjectively more than a simple even order harmonic series. It again depends on the level of the kink. In some cases with real amplifiers, the crossover distortion is severe enough to be easily audible and more so at lower levels than at higher ones (as this kind of distortion is basically level independent).
"If you get hit in the head by a rock, it is not the velocity of the rock that hurts you, it is the rock"
This is not true in fact. It is the Kinetic Energy with which the rock hits you that causes the damage. KE = 1/2 MV^2 and as you can see that the rock itself is only a part of that equation. Water can kill you if you hit it at a high enough velocity (by say jumping off of the Golden Gate Bridge). Enter at a lower velocity and no pain.
I know what you are saying, a smooth transfer function with no kinks should be a goal to getting a good sound, discontinuous transfer functions indicate severe non-linearties and I would agree that this is the underlying cause but it is not what we are hearing. We are hearing the overlay of harmonic and non-harmonic (ie. noise or perhaps these discontinuities) distortion frequencies with varying amplitudes that are clouding up and hardening the sound of our music.
There are two goals as I see it: 1) Identify sonically what types of distortions (harmonic and non-harmonic) are unacceptable in amplification and 2) What kinds of circuit design practice lead to the non-linearities that cause these unacceptable distortions. IMO, we are still primarily at point 1 and only have vague ideas about how to approach point 2.
For myself, using Keith Howard's software was enlightening in that by adding quite low levels of harmonic distortion (obviously I could not add noise and other aspects of the amplifiers I was "modeling") I could hear differences and formed preferences. If you could make the model frequency dependent it would be even more interesting. For example, most tube amps have a sharp rise in distortion in the bass, which I am convinced leads to the "tubby" warm and loose sounding bass most people associate with tube amps. Tube amps that don't exhibit this distortion increase also exhibit very good bass control despite having a low damping factor. Also, a majority of high negative feedback SS amps show a sharp increase in distortion in the high frequencies (sometimes by a 100x between 1Khz and 20Khz). Using these harmonic patterns along with 1Khz (sufficient for most of the midrange, ie. a three point model) and perhaps these three regions under dynamic conditions as well could give a decent picture of the sonic character of an amp.
If you can, as Keith Howard did with his simpler model, imprint this on a previously undistorted recording, then I think you can really evaluate many amplifiers under very controlled conditions in a virtual sense with the same recording at the same level etc.
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Follow Ups
- RE: Long-term DBT example - morricab 04/29/0804:13:50 04/29/08 (1)
- RE: Long-term DBT example - andy_c 07:44:10 04/30/08 (0)
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