how to mod an AC power supply to make it less noisy

[Zilla]

So i was picking thru the electronics recycling pile at work and scored a few dozen 9-12VDC wall wart power supplies.

Naturally, I'm thinking that my power supply worries are over.

No such luck. I tried all of them and they introduce hum into any stomp box i use.

Which brings up the question:  is there any way that I can make these less noisy and "usable" to run stomp boxes off of? Would a big ferrite bead on the power cable reduce the noise?  Is there any way to open the wall wart up and beef up the cheap parts?

Thanks

Tim

[Jdansti]

> is there any way that I can make these less noisy and "usable" to run stomp boxes off of?

You'd have to place some filtering on the output to reduce ripple. Not hard to do, but it would have to be done external to the wall wart.

The other problem you may have but might not be aware of is that the wall warts are probably unregulated. Place a meter on them and check the voltage.  If it's > than the stated voltage on the label, it's probably not regulated.  You'd only be able to regulated it to 9V if the unregulated voltage is > ~10.5 - 11V (regulator inputs have to be at least 1.5-2V > the output and no more than the maximum rating for the particular regulator).

Also, watch out for the polarity of the DC output.

>Would a big ferrite bead on the power cable reduce the noise?

That would be a nice easy solution if it worked, but I think that Fe beads only suppress AC noise at much higher frequencies than you'd hear through a stompbox. Maybe someone else can confirm or refute this.

>Is there any way to open the wall wart up and beef up the cheap parts?

No. See the answer to the first question above.

[garcho]

Read this, courtesy of Jack Orman.

I wouldn't open up any wall warts. It is mains wiring after all, and you can kill yourself, or even worse, someone else.
Besides that, even new, they're so cheap; buying parts for them and resoldering the thing is a waste of time and money.
I believe ferrite beads are there to prevent digital devices with lots of oscillators, clocks, processors, et al. from interfering with your all your other junk. More of an RFI filter.

[J0K3RX]

Look here
http://www.beavisaudio.com/projects/Huminator/index.htm

[Mark Hammer]

The Huminator, and indeed, most simple passive wallwart smoothers, provide what is essentially a "treble cut".  A 100-ohm resistor in series, with a 100uf cap to ground, provides a "rolloff" of higher-frequency content starting around 16hz.  That rolloff is 6db/oct, such that anything on the power line, coming in from the wallwart, that is 32hz is 6db lower, and 64hz is 6db lower still.  In effect, the hum coming from whatever ripple is on the power line is not eliminated, but reduced enough to be bearable.

But, much like filters on the output of distortions where we want to take the fizz away, we can apply more than one pole of lowpass filtering to power lines as well.

Keep in mind that R1 in the Huminator limits how much current will be able to pass.  Let's take that 100 ohms and divide it in half, using two 47R resistors in series.  Run a 220uf cap from their junction to ground (as in the schematic), and a second 220uf cap to ground from the output.  We now have a 2-pole lowpass filter with a rolloff that starts around 15.4hz, but with a 12db/oct slope.  By the time we get out to 60hz, any ripple on the power line will be reduced by 24db, rather than 12.  I'm pretty confident it would not pass muster in The Absolute Sound magazine, but it will provide a noticeable improvement in hum reduction coming from a less-than-stellar wallwart.

[Jdansti]

What they said...

Edit:

...but make sure you check the actual output voltage with a meter. You might be able to make it perfectly quiet and not have the proper voltage for your pedals.

Another thing: Check the output current spec on the label to make sure you've got enough juice for your pedals.

[Kesh]

Just to add it is easier to make effective RC smoothing for lower current pedals. And with the 12v warts as you can drop  some more volts.

[caspercody]

Check out page 8:

http://www.aronnelson.com/DIYFiles/up/12AU7-6111_Valve_Caster_Summary_Rev002.pdf