Radio Shack Wall Wart problems

[javacody]

I bought a Radio Shack Wall Wart with variable voltage. It makes the most annoying, ear-shattering, vomit inducing, high pitched note which gets lower in pitch as I lower the voltage. I've tried using caps, and a 1000uF cap seems to lower the pitch, but not eliminate it. I used a 47uF cap with the wrong polarity  (accidentally) and it eliminated the sound part of the time, but it got real hot, and the top puffed up, like it was almost ready to blow.

Anyway, I'm on a very tight budget and need to get this thing working. Any ideas? I've also tried several caps in parallel (.001 uF, .01 uF, 10 uF, and 100 uF). Nothing seems to help.

[petemoore]

What ckt are you using the RS PS for?

[javacody]

I'm using it for a Ruby Amp, but I had the same problems with a Little Gem.

[smoguzbenjamin]

I had the same problem with a similar wallwart, but not from the radio shack. I couldn't figure out what was wrong with it and returned it to the shop. It took me ages to find a suitable wallwart.

[javacody]

Well, there has to be some way to filter this crap out.

[mattv]

Rob Strand has advised on a few occasions to put a 100 ohm resistor in line with the V+ lead and a 100uF (give or take a multiple of ten) cap to ground. Check out the archives.

This problem is why filtered supplies are recommended.

[eliktronik]

Along w/ the 100uf cap across the supply, you could also add .1 and/or a .01 caps in parallel. This is supposed to help reduce hum. Worst case, neither of those work, you could add a regulator. TI has a good sample program and you should be able to get one or two for free. That way, you could set your supply to 12v or so, and use a 9v regulator to get the 9v that you need. Good luck!

[javacody]

The 100 ohm resistor did the trick, mostly. However, when I have the gain all the way up and the volume all the way up, I get a lower tone now. Will adding another cap help this?

[brett]

If it's a hiss or high tone, I suggest adding a 0.1uF cap very close to the wall wart.  If it's a hum or low tone, the 100uF cap might help, and can be placed anywhere in the supply line.  For effective filtering of an unregulated supply, a much bigger cap (1000uF+) is usually needed to get ripple down to respectable levels.

[petemoore]

Only take a few buxx and a few nodes.
 You might like to have variable voltage these amps for different tones at different and higher volumes ... pushing them to see when they start getting warm and blowing up 386's for fun.
 I think my little Gem works right...it definitely amplifies and seems to get octave down [dependant on a certain supply voltage].

[phillip]

The problem with wall warts that you can get from electronics supply places like RS, Jameco, Mouser, etc. is that they're not intended for use with an audio device like a stomp box, and so they're not very well regulated and can be quite noisy.

I would just buy a Danelectro "Zero Hum" or the Boss PSA-120.  Those two (and a few others) are intended for use with audio circuits and I've never had any sort of noise problems with them when used with my effects pedals.  

Phillip

[Paul Perry (Frostwave)]

If the pitch is lowering with voltage, it must be a switch-mode power supply. Get one of those 'heavy' suckers instead! (how to tell a switchmode: it is suspiciously light)

[tomtom]

I understand now, as I was a battery user I bought one from my electronic supplier with switchable plugs (horrible). It sounded like a dream at first (light, plug convertible, 1A max, use only one space...) but plugged into a gain box, I have a tunable oscillator !!

Still, you may use that kind for tests and try to cure the hum down. You'll be sure to be dead quiet with any others PSU.

Cheap 200mA Ibanez/Boss/Danelectro always worked fine.

Tom