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MFOS Synth 1: Built the Power Supply

Pete Brown - 02 December 2011

Over the next several months (3? 6? 10?) I'll be building what is definitely my most complex electronics project to date: the MFOS (Music From Outer Space) Sound Lab Ultimate, Ultimate Expander and (if Santa brings one) Sound Lab Mini-Synth Mark II, likely all in the same home-made wooden case, side by side. The Ultimate and Expander are together a 3 oscillator monophonic true analog synthesizer with filters, envelopment generator, ring modulator, sample and hold and more. You patch between the different logical modules using banana cables, so it's a bit of a self-contained modular synthesizer. The Mark II is smaller, newer, and has a few fewer features, but a sound of its own. You also patch that with banana cables, and can integrate the two.

Mouser made a huge delivery today, so I finally have most of the parts I need. I'm still waiting for a shipment of wire from All Electronics, my black banana jacks from ebay, and the potentiometers from Small Bear Electronics. However, I did have enough for the power supply.

Today's project was building the +12v/-12v bipolar wall-wart-based power supply. This power supply uses an AC (not the usual DC) wall-wart transformer to convert from 120v AC to 12vAC. It then takes that AC power and transforms it into +12v/-12v and ground to power the synthesizer modules.

The AC power supply I used is from an old router I no longer use. I hate wall-warts, especially ones that don't have a pigtail for mains power. They always sit oddly on the power strip (never as firmly as you'd like) and take up two or three spaces. You can order a new one, of course, but I'm always happy to recycle old working equipment. This one does 12vAC at 1000mA

Ray has a great set of information on his site, so I won't repeat that here. However, I thought I'd share a few photos.


I made some ghetto heat sinks from aluminum angle. The Al was a bit thicker than made sense for this operation, so I had to bend the 7812/7912 leads down at an odd angle. It worked, though. They're attached with screws and non-conductive thermal grease. I'm not sure how necessary the heat sinks will be given the voltages being fed in, but I don't like to go without. The reason I made those instead of picking some up is because almost every commercial TO-220 heat sink I've seen is wide enough to short to the pads in between the two regulators. Also, as Ray points out on his site, the two regulators can't share a heat sink as they aren't both at ground potential. I may have been able to finagle something by standing the regulators up, but space is pretty tight in there as you can see:


I do have to admit to a tiny bit of fear when I plugged it in. I double checked everything, and checked for continuity between points where I wouldn't want any. Still, let's just say that I kept my face away from the capacitors when plugging it all in :)

Here I'm testing for +12v


And here I'm looking for -12v


The values are fine; you're not typically going to get perfect voltages using a power supply like this. It could also have to do with the quality of the LMs I'm using. If I took time to test a bunch I could probably get this a lot closer to +/- 12v, but it's likely not worth the trouble. I'll double-check with Ray just in case.

My potentiometers shipped today, so I'll likely work on the front panel for the synth after I return from VSLive next week.

posted by Pete Brown on Friday, December 2, 2011
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