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

[ShedSynth]

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.

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


-----Original Message-----
From: Synth-diy <synth-diy-bounces at synth-diy.org> On Behalf Of rsdio at audiobanshee.com
Sent: 06 January 2020 01:11
To: synth-diy mailing list <Synth-diy at synth-diy.org>
Subject: [sdiy] Module power - regulated or filtered (passively)?

Hello all,

The idea of a couple of dozen modules that each have their own ±10 V regulators somehow seems wrong. I realize that it’s fully necessary for some modules, but is it really necessary for every kind of module?

A good op-amp should have an excellent power supply rejection ratio, meaning that a nominal amount of power supply noise should not appear in the audio (or CV). In addition, you can’t really get 20 V peak-to-peak, input to output, from an op-amp that only has a ±10 V supply, so there’s a real advantage to giving the op-amp a few Volts more than it’s peak signal. I realize that most modular gear rarely deals with 20 Vpp (±10 V) signals - it’s usually more common to see 10 Vpp (±5 V).

So, I’m thinking, what’s wrong with simply connecting the ±12 V supply from the Euro power to the op-amps without precise regulation (linear or switched)? I’d want to at least have LDO diodes for protection against polarity reversal, and passive RC filtering to reduce the noise cheaply.

On the subject of passive filtering of the supply, even a linear regulator will dissipate 4 mW per 1 mA of current draw. So, if a module draws 20 mA to 30 mA, then on-board linear regulation will dissipate 80 mW to 120 mW. In contrast, using 10 Ω resistors for passive filtering would only dissipate 25 mW for a 50 mA draw. 10 Ω resistors would not even catch up to linear regulator dissipation until the module is drawing 100 mA, which is quite a bit for most modules. Using a separate passive filter for each op-amp might mean 200 mW dissipation for a 50 mA module, but that’s still on par with a linear regulator at the same current draw.

I’m not going to think about switching regulation options, because if a module has the board space and needs a switching regulator then that’s probably not the kind of module I’m asking about in this thread.

Another question is whether the passive filter should be referenced to ground if the op-amp doesn’t even connect to ground. It seems possible to just put 10 Ω of resistance (or less) on each supply pin of the op-amp with a single capacitor between the + and - power pins. Some op-amp configurations do reference ground, or at least some sort of virtual ground, so it might make sense to reference a pair of caps to that ground, although it shouldn’t be necessary.

Any comments?

At the moment, I’m thinking of Euro modules (±12 V), but if anyone has thoughts of MOTM (±15 V) or other modular standards, I’d also like to hear them.

Brian


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