[sdiy] Module power - regulated or filtered (passively)?

[rsdio at audiobanshee.com]

On Jan 6, 2020, at 1:16 AM, ShedSynth <shedsynth at gmail.com> wrote:
> Hi Brian,
> I'm making a Eurorack-sized modular synthesiser, built mainly with op-amps on perfboard.
> Mostly analogue but some modules (MIDI input, ADSR, LFO) have Arduino Nano 8-bit microcontrollers to generate control voltages, and those have status LEDs.
> 
> One issue I struggled with: the regulated supply has some resistance so a few milliamps drawn by the flashing LEDs causes fluctuations of a few millivolts on the +12V rail, and my 3340 VCOs are very sensitive to that fluctuation so the pitch wobbles.
> My problems were mostly cured by providing a second regulated +12V supply for the VCO modules only.

If you were building a discrete VCO instead of a 3340, you might be able to use a precision reference like the Analog Devices REF02DN8 (0.3%). Such references are common for analog-to-digital converters. They’re a bit expensive at $3.87, but while searching for a part number I found a number of cheaper options like the Texas Instruments LM385Z-1.2 at $0.72 with 2% accuracy.


> While it seems to be true that op-amp gain isn't affected by the supply voltage, my summing amplifier modules include an offset voltage pot between -12V and +12V rails which causes obvious fluctuations if the output is used as a pitch bend CV.
> There might be room for me to add regulators on the summing amplifier modules just to supply those offset pots.
> 
> Alasdair

I was going to suggest buffering the output of your pot with a voltage follower, but that won’t help when the input voltage is fluctuating. Instead, use one of the precision voltage references mentioned above along with the necessary voltage follower buffering. If your pitch bend only needs an octave range, then you should be able to manage ±1 V from a common precision voltage reference.

Brian