[sdiy] Low voltage synthesis?

Mike Bryant mbryant at futurehorizons.com
Thu Jan 13 18:07:34 CET 2022


Unfortunately noise creeps in as soon as you reduce the voltage.    However you can reduce the effect by reducing resistor values by more than the ratio of voltage reduction, but resistor noise still eventually wins.   And of course with smaller resistors you'll need larger capacitor values for time constants.   Techniques to look at to reduce problems further include using currents rather than voltages wherever possible.  Of course many synth circuits do use currents - the Moog ladder filter for example - but you'll need to work linearly rather than exponentially.

The other issue you run against is you need to use rail-to-rail op-amps, but most of these are specified for 5V/+-2.5V operation.   Also these opamps are often CMOS based and have what I term "input crossover distortion" as they switch from the N to P type devices conducting.  Negative feedback reduces this, but it's always slightly there.  There are a few bipolar R2R opamps without this problem - Rohm BA2510 is my favourite.

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of cheater cheater via Synth-diy
Sent: 13 January 2022 16:21
To: synth-diy
Subject: [sdiy] Low voltage synthesis?

Synths generally run off 15V or 12V power rails, which among others helps with SNR. However, it creates a bunch of power dissipation, plus you can't use some of the tiniest components. Does it make sense to build synthesizer circuits that run on much lower voltages than that?
Say, 1.5V rails? Are there techniques of suppressing noise that are better than just ramping up the voltage rails? Have the parts progressed enough nowadays that we can do this?

Never mind stuff like interfacing with existing modular synthesizer parts etc. Let's say you're building a modern 8-voice polysynth with 4 VCO, VCF, VCA, ENV, LFO per voice. Obviously that would generate loads of heat and possibly tuning issues given the constraints of a keyboard, plus it would be huge, especially given the size of capacitors vs their rated voltage.

Where would noise creep in first if you started lowering rail voltage in your synthesizer designs? What other disadvantages in terms of signal integrity come from using low voltage rails for synths?

Thanks
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