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

[music.maker at gte.net]

This is probably one of those "religious" subjects, but I've seen numerous posted schematics that
have 10R or 20R in series with the rails.  I've never done that, but I understand why some (sellers
of modules often) put them in.  IMO, it's a dummy-proof thing.  The 10R resistors are expected to
act like a fuse (so one designer told me) and will burn out if you do something stupid.  
For my money, I think an actual fuse is a better idea.  But yes, even though 10R isn't much, it
still limits current and a voltage appears across it that varies depending on the load driven by
the 10R.

It's a choice you can make, those resistors don't make it work, they help protect you from
accidents or whatever.  If you short those resistor, I would expect the problem to stop or be
reduced substantially.


"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. 
>
>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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-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
-- http://scott.joviansynth.com/
-- When the going gets tough, the tough use the command line.
-- Matt 21:22