[sdiy] Opinions on Switchmode PSUs for analog synths?

Mattias Rickardsson mr at analogue.org
Thu Oct 31 19:50:29 CET 2019

Den tors 31 okt. 2019 13:02 <rburnett at richieburnett.co.uk> skrev:

> ...  Although most SMPSU
> output noise is well above the audio band where it can't be heard
> directly, it has the potential to makes it's presence audible by some
> strange effects.  For instance, consider a standard saw-core analogue
> VCO, based around an expo-converter feeding an integrator that is reset
> periodically by a comparator.  This circuit normally performs very well
> in an RF-free environment.  Now think what might happen if you introduce
> noise in the form of a 200kHz pulse train of tiny RF pulses.  You won't
> hear the 200kHz directly.  That much we know.  But those little pulses
> could easily find their way onto the output of the integrator due to
> poor PSRR of the integrator op-amp, or they could find their way onto
> the reference voltage for the comparator.  If you get tiny pulses
> present at the inputs of the comparator, then the comparator is likely
> to trip early and terminate the integrator's charging cycle early.  The
> effect of this is a shift upwards in pitch!!!  Likewise if RF hash finds
> it's way into the highly non-linear expo-converter circuit, it's likely
> to also cause a shift in pitch.  So in summary, I wouldn't be at all
> suprised if RF hash from a poorly filtered SMPSU made an analogue VCO
> play out of tune, even though the average supply voltage is fine, and
> there's no audible ripple on the supply itself.  This is how I think
> problems would manifest themselves.

Aha, unstable VCO even if its reset mechanism is "not dependent on the
supply voltages"! :-O

Thanks for the meaty post, Richie. :-)


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