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Re: test for transients

Several comments.

First off, I am not sure if a simulation will show what is going on fully, it would have to be a real dandy simulator, with gobs of secondary and parasitic circuit aspects modeled in properly.

Assuming you have such a dandy sim:

I would try a higher frequency, say 5 kHz, to see what effect this had.
I am assuming that your simulator can show the time domain, and can arbitrarily crank the gain on the waveforms so you can look at the leadng and falling edges 'microscopically'. A typical 'scope display is visually limited to about 40 dB of dyanmic range or so, and if you can add simulator 'gain' to the transistion regions, this will increase what can be seen.
It is my firm belief that signal distortions down to -90 dB are relevant to what we hear when listening to a home playback event.

The other comments about some of the distortion products ending up buried by the original square wave harmonics is correct, BUT, even if the amplitude is hardly affected, and the cancelled signal null is way down, there should be some effect due to a phase shift of the distortion products relative to a "perfect" series of square wave harmonics, so if you can look at the phase of the harmonics, this might be more senisitive. The only problem with this is that the perfect square wave will not be bandlimited, while the amp one will. Perhaps you can use the perfect square wave with the proper linear bandlimiting applied to it BEFORE you use it to subtract out from the simulated amp output. This would of neccessity be the small signal bandwidth limiting of the amp only, but it sure would show up when the amp got non-linear at higher levels of output very pointedly.

My own observations of power amp behavior is that many amps sound OK at low levels, but start to change their signal behavior at higher output levels, well below clipping, and I have in the past been able to hear that certain digital based power amps seem relatively impervious to this problem. Their character does not change significantly with output level.

Then there are other forms of transient distortion, what many would call envelope distortion, which has to do more with the DC biasing and DC coupling throughout the amp.

See:
http://www.AudioAsylum.com/audio/general/messages/219136.html
for details and some more comments on this subject.

I feel that this is at least as relevant as the HF transient response of an amp, and often is ignored or overlooked.


Jon Risch

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