Tube DIY Asylum

RE: Slew rate, TIM hogwwash

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This Post Has Been Edited by the Author

In Reply to: RE: Slew rate, TIM hogwwash posted by Paul Eizik on July 10, 2017 at 17:46:51

I never spoke of "FM distortion" It is called TIM or Transient Inter-modulation Distortion" And you have provided no evidence TIM is audible. Just because you can simulate the problem with test equipment doesn't mean you can hear it with normal program materiel. The subjective camp is always saying how they can hear things no test equipment can measure. But here's the truth with TIM, even legacy test equipment can measure distortions that cannot be normally heard.
How about my test example. Let me refine it further:
Capture a 15khz square wave from a hobbyist quality signal generator on an audio workstation. Then record any random sampling of music or acoustically generated sound effects, gunshots etc, and compare the rise-times. Now tell me if any of those natural rise-times are a steep as the 15khz square wave? Further analyze and measure the rise-times. Then tell me if the path delay through a typical DC coupled amplifier is anywhere near the rise time of the test material? One popular claim is that dues to the path delay of the amplifier, the feedback fails to "catch the leading edge" and that is TIM. That sounds quite plausible to a non-engineer. But enter mathematics and we see how minuscule the problem is. The leading edge is so slow in relation to the amplifiers path delay that all that is missed is a minuscule fraction of the leading waveform edge. Now in light of all the other larger distortions, mainly from the speaker system, that tiny fraction of a waveform edge distortion is audible?
This proves TIM is a theory exercise. It can easily be simulated with even cheap test gear. But it's never happens in the real works of audio reproduction. Also note by specifying hobbyist grade, I am putting the test in the TIM camp's favor. A pro signal generator would have much steeper rise times.
My professional background is the design of broadcast television equipment. Today it's mostly digital but in the late 1970s when I started it was mostly analog. I have designed many analog video amplifiers during that era and we used negative feedback extensively. We never worried about TIM and note that TIM should be a much larger problem with video frequencies, eg 6mhz. So if a video amplifier circuit which is very similar to an audio version is fast enough for TIM to be irrelevant, how can it possibly be a problem with audio amplifiers?
I am not interested in articles or AES paper paraphrases from Strereophile, Absolute Sound, or similar consumer product sales magazines. Lets look at this from a solid engineering perspective and analyze the problem. When you do it becomes very clear.
Again I'm not saying Mr. Otala's results are wrong. They are quite correct from a pure technical perspective. But what is wrong is the conclusion that this phenomena is detrimental in modern DC coupled amplifiers.

Edits: 07/10/17

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