power supply noise

I'm getting a considerable amount of 60 hz hum from my power supply. it's light so I think it might be a switching power supply which apparently are noisier than old power supplies. I'm wondering if there's an easy way to reduce this hum. I tried putting a 10uf capacitor between the 12 volt and ground but there was no effect. if not, can anyone reccomend a 120VAC 60hz to 12VDC wall plug? Thankyou

Which theremin are you using? What current rating is the 12 volt side of your existing adapter?

Modern switch-mode power packs create considerable electrical noise across a range of frequencies. The older style adapters used heavy transformers and when the output is over 1 amp or so they are too big for "wall warts" so are usually in a plastic box with a mains cord and plug attached to them.

The older style are becoming harder to find these days; most I see advertised are second hand.

the theremin is one I built from plans at strangeapparatus.com but I didn't make the power supply shown there. I bought one that is rated for 100mA. the circuitry apparently loads the power supply less than expected becuase it actually is putting in about 16V but that's fine. It's a new one so I think that it is probably switch mode. if possible I'd like to avoid buying another source, and just filter out the hum (which appears to only be a 60hz saw wave). but i don't know how that would be done, so i guess I would consider buying one too.

Okay, assuming it's 60 Hz that you're hearing, I'd go for a much bigger shunt capacitor. Your 10uF at 60Hz has an impedance of around 260 ohms whereas 1000uF at 60Hz is down around 2.5 ohms.

A better solution would be an L-C filter.

(BTW: There's a Chinese supplier selling new/unused old style adapters on eBay for $5 plus shipping. Item # 390142370177)

If you have 60Hz sawtooth, that means that it is NOT a switched mode PS, they operate at much higher frequencies. It's rather an unregulated one and what you are getting are the peaks of the capacitor's charge/discharge curve. You should just add a secondary regulation. I'll draw it and post it here in a short time.

Edit: Here, it is: click

^ that's another solution but if you go to that effort you may as well build the regulated power supply described in the article, except use a 120v to 12v transformer.

Due to oscillator drift, possible output hum, etc. I think ALL Theremins should have internally regulated power supplies.  The 78L series that Thierry has provided a schematic for is entirely adequate for this task (good regulation, low noise) and quite inexpensive.

So, nickexists, what have you decided to do?

I just added the voltage regulator as Thierry suggested and it works perfectly. Thanks, Thierry.

Glad to hear you got it sorted.