Noob alert.

I'm about to build a pretty simple PSU for my modular synth project. I'm looking at a few schematics from popular sites and I've previously built a simple wall-wart one based on this from MusicFromOuterSpace, using LM7912 and LM7812.

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There are also lots of kits with these 7812/7912 pairs like

But then I watched this video on capacitance multipliers:

and just don't understand why this isn't used everywhere? I guess I'm missing something.

What I mean is, it seems to be a really big thing to really reduce ripple on power for synth applications. The schematics I'm looking at are sticking really really big electrolytics in there to do that - it's like that's probably the big material cost for these PSU's. Like 10kuF or so. Wouldn't it be a very simple win to just use a mosfet capacitance multiplier just before the voltage regulator?

  • Your schematic shows single-diode (half-wave) rectifier. That gives 50/60 Hz ripple. You can reduce ripple by half with full-wave rectifier (the extra diode(s) are cheaper than monster electrolytics) now have 100/120 Hz ripple. Mind you, for audio work, some may complain that 50/60 Hz is less objectionable than 100/120 Hz, if it gets into the audio signal path. – glen_geek Dec 6 at 16:02
  • yes, sure, this was just because it was a simple ac/ac wall-wart supply with dual rail output. you'd need center-tapped transformer for full-wave dual rail output, right? – Viktor Hedefalk Dec 6 at 19:30
  • A 12V RMS centre-tapped transformer (with 2 diodes) works. But the whole idea of reducing ripple going into the regulator seems like a pedantic notion...groundloop currents may have a bigger impact on audible hum. – glen_geek Dec 6 at 19:59
  • The point he is making in the video about here is that regulators are really not very good at mitigating ripple from the input. But I guess this is all about higher frequencies on switched supplies, not mains ripple… – Viktor Hedefalk Dec 6 at 23:08
up vote 2 down vote accepted

A capacitance multiplier can mimic a capacitor of large value but it can't hold-up the supply voltage from a bridge rectifier because it doesn't store the energy that can be released to the load when the AC voltage drops to zero every 8 to 10 milliseconds. There are no free meals in electronics.

  • Oh, ok. So this would be more usable for a switching power supply? – Viktor Hedefalk Dec 6 at 19:34
  • 1
    No, because (1) switching power supplies don't require a lot of smoothing capacitance due to their high operating frequency and (2) adding a pass transistor to facilitate a capacitance multiplier means input to output voltage differential just like a linear regulator and hence power loss (so why use a switcher if you are going to add-back power loss using a linear circuit?). – Andy aka Dec 7 at 8:18

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