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Curious sample & hold behaviour
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author Curious sample & hold behaviour
terjewinther
I was checking all functions on all my modules today, as a follow-up from the last concert where I throught one of the VCOs malfunctioned, and sure enough; one of the outputs on one of the classic VCOs were dead. Probably an op-amp or something, so I will check for that. I have my ICs socketed.

As I was checking the sample & hold module, I noticed this:
Using a square LFO for trigger, and noise for signal in, I got the expected random CV output. However, this only happend if I was using pink noise; if I was using white noise the S&H module stopped working and only output a very low (negative) voltage. Patching source signal back to pink noise or another source, like another triangle LFO made the S&H start working proper again. I repeated this many times, with exactly the same result every time.

I check the raw output from the noise module, and both pink, white and infrared is working perfect.

So what is happening?

(no, it is not the patchcable, it is not the trigger, and it happens regardless of slew and restriction pot settings, and yes, it happens with different triggers/LFOs).

My Sample/slew module is version 1.
ersatzplanet
Many new noise modules are using digital noise chips for the white noise. These chips produce noise by making a very random PWM square wave. The harmonic content may be very noise like, but the waveform is a square wave as far as a S&H is concerned. That might be the problem.
terjewinther
Interesting about noise, and I am familiar with the effects of various form of noise-creation. However, the Oakely noise/dual filter module use a transistor for noise. I remember testing various transistors until I found one with good noise. Ended up using an old japanese transistor I had in my spare bin.

But I have not tested/seen the white noise output on the scoop, so it could be some strange behaviour there. I need to check that.

Thanks for the idea.
Synthbuilder
I think perhaps the white noise output is too loud.

The original issue 1 sample/slew module has the sample input going via a resistor into an op-amp's input. It is possible that a sufficiently large input signal will cause the op-amp to latch up. Latch up in this case means the op-amp's output fails to do what it should do and is stuck at either + or -13V or so. It takes a small amount of time for an op-amp to come out of latch up even if the offending signal level is removed. And it may be that once the white noise has forced the op-amp into a latch up state, the noise signal doesn't fall low enough for long enough to correct it.

So either replace the op-amp, U1, in the sample/slew module for one that doesn't latch up. The 4558 would do.

Or, turn the noise level down a bit, using the LEVEL trimmer, on the Noise module.

I maybe wrong but this is what I'd do first.

Tony
terjewinther
You were so right!

Attentuating the white noise signal into the sample/slew solved it.
So I swapped out the TL072 with the 4558, and now it behaves much better.

Thanks a lot.
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