coherent noise and chaotic distortion

[smoking-amp]

Reading some old and recent threads which mention chaos in feedback systems and the limitations of FFT distortion analysis:

http://www.diyaudio.com/forums/showt...718#post418718

http://www.diyaudio.com/forums/showt...636#post711636

Got me thinking a bit on this. Intermodulations of complex signals can lead to an apparent increase in the noise floor on FFTs. But this correlated or coherent "noise" may be quite audible in contrast to just ordinary noise.

Varying device capacitances with signal level may cause phase intermodulation distortions. (which normally would appear as spectral lines in an FFT, similar to amplitude distortions)

Usually we think of a varying capacitance as just affecting signal phase and impedance. But looking at the equation for group velocity on a transmission line shows group velocity as proportional to 1/SQRT(L*C) (L = inductance, C = capacitance) So group propagation velocity thru the amplifier may be varying slightly.

High bandwidth feedback amplifiers allow very high harmonics to circulate. Varying delay time thru the amplifier may cause high harmonics to recirculate in non periodic times, ie chaotic. The inverse of time delay also acts as another source for intermodulation distortion. Intermodulation between these higher harmonics will bring some artifacts back down into the audible range.

Chaotic recirculation times will likely cause spectral lines to widen. Since when has anyone bothered to look at the WIDTH of FFT spectral lines in distortion plots? This may be a very audible distortion to the ear, since the spectral lines are generally well above the noise floor. (ie, similar looking FFTs to sine wave testing may sound quite different with sounds)

We need some test procedure to isolate what is left in the signal after the primary harmonic distortions and the signal are removed. Listening to the remaining grunge would give some indication as to its harmfulness.

The obvious fixups would be operating devices in regions where capacitance is either minimized or stable.

And how about some Static Induction Transistors with linear transfer functions by the way too. (and slightly OT, since these somehow manage linearity and are SS versions of tubes, what needs to be fixed with tube design to make them linear instead of 3/2 power? - - well, must be the varying electron velocity in tubes versus saturated velocity in SS)

Another idea might be some way to limit the bandwidth of Neg. feedback signals. Who needs out of audio band NFB anyway? (OK, I'm waiting for mikeks usual lengthy reply on this one: "no" )

Don

[smoking-amp]

One way to test for non-harmonic distortions might be to use a delay line oscillator to generate a picket fence harmonic signal spectrum across the audio spectrum. (Maybe an adjustable windowing wide bandpass filter too.) Then a similar delay line filter would generate a picket fence filter to remove all of these from the amplifier output. Anything left would be non-harmonic chaotic distortion which could be listened to or put onto a FFT plot.

Don

[MikeB]

Originally posted by smoking-amp

Another idea might be some way to limit the bandwidth of Neg. feedback signals. Who needs out of audio band NFB anyway? (OK, I'm waiting for mikeks usual lengthy reply on this one: "no" )

Don

No.

The reason: The distortion products have much higher frequency than the original frequency, especially crossover distortion artefacts.

For example, a 5th order harmonic from a 10khz will be 50khz. If you limit feedback to 20khz, you will only be able to attenuate the 2nd harmonic from a 10khz signal.
These harmonics above 20khz will not be audible directly, but only for a single sinewave. IM products will fold down these distortions again into audible range, when amp is fed with complex signals like music.

Intermodulation is not a product from NFB, it always happens when a complex signal gets distorted.

Mike

[MikeB]

Originally posted by MikeB

Intermodulation is not a product from NFB, it always happens when a complex signal gets distorted.

Sorry, not completely correct, NFB can also create IMs additionaly...

[smoking-amp]

"NFB can also create IMs additionaly..."

My point exactly! The crossover HF distortions are generated in the last stage of the amplifier (no non-linear stages left to affect them), why let them recirculate to intermodulate back down to the audio band. Once an ultrasonic distortion gets generated, just kill it with a filter.

Maybe could put the filter after the input diffl. stage, since the amplifier would be effectively operating open loop above the lowpass filter frequency if placed in the feedback only.

Don

[theChris]

as for OP post, this is interesting. Are you concidering the effects of changing capacitance. it does require energy to change capacitance, so i'm not sure the linear equations can be used.

IMD can take higher frequencies and convert them back down, as it is essentailly caused by mixing. this is a known phenomenon in inamps where high RF signals become LF noise on the inamp.

[smoking-amp]

Checking the data sheets for most any bipolar or mosfet device shows varying capacitance across junctions or terminals versus applied voltages. An unavoidable effect. In the amplifier case I would expect this to mainly affect phase of high frequencies, particularly ultrasonic ones, as large amplitude low frequencies swing voltages around. If delay time is also affected via the dielectric affect on signal velocity, then further changes in phase/freq. of the HF gremlins will occur. Making more annoying possibilities for IMD to generate audible products.

Maybe one could put back to back varicaps across offending junctions and control them with opposite phase audio to compensate capacitance. Probably difficult due to non-linear cap. curves.

Don

[MikeB]

Originally posted by smoking-amp
Once an ultrasonic distortion gets generated, just kill it with a filter.

That was exactly my point, this thinking works only for single sinewaves. For complex signals this "ultrasonic distortion" creates IM that folds down to audible distortions. This effect has NOTHING to do with feedback. You can't kill them with a filter.
You need to avoid/reduce ANY distortions, they become all audible with complex signals because of IM. Even in the Mhz range.

Mike

[smoking-amp]

"creates IM that folds down to audible distortions"

So you are saying that a single transistor stage could generate some HF harmonics which could immediately intermodulate to cause audible IMD. I was hoping that 1st pass products would have to re-enter the input to interact again to generate 2nd order IMD products. (I'm not saying that higher order products couldn't be generated by a bad transfer curve straight away, I'm just looking at say signal1 -> 2nd H1, and signal2 -> 2nd H2, then interacting down to 2nd H1- 2nd H2)

If so, then we need to kick some semi manufacturers B--ts! Where are some linear silicon SITs to work with!

Don

[smoking-amp]

I should clarify the last model scenario, since a quartic transfer function could obviously generate 2nd H1 -2nd H2 straight away.
(sig1 + sig2)**4

Lets say we have a square law transfer function like Mosfets. Then (sig1 + sig2)**2 can generate 2nd H1 and 2nd H2. Would it not require a second pass thru the device then to get (2nd H1 + 2nd H2)**2 --> 2nd H1 - 2nd H2.

Since varying junction capacitances would mainly affect HF signals, one could hope that such 2nd harmonics and above would be out of band, and hence filterable. Of course this still would not prevent Sig1-Sig2 intermod.

Likewise, varying propagation delay effects that widen FFT lines would also become serious only if multiple passes thru the amplifier are permitted.

Don