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plugger 7th September 2008 12:18 AM

Wall wart PS filtering for wah pedal
I'm trying to convert my Vox V847 wah pedal to be powered from an external DC power supply. Presently, it's powered from a single 9V battery.

I'm having more trouble getting this to work than I would have thought. First thing I tried was to plug in a 9V wall wart via a battery clip connector -- horrible hum.

OK, some filtering and regulation required.

So I got me a 7809, a 15V DC PS (which is supposedly already very well filtered and regulated), and tried that.

Things happen with this set-up I don't understand.

It sort of works, sometimes, but I'm getting grounding problems I don't really understand. Touching my guitar strings is now like touching the tip of a guitar cable while the other end is plugged into an amp. Touching the heatsink on the 7809 (which is electrically connected to ground, which is the -ve side of the DC input going to the battery clip in the wah) has the same effect!

I tried a 470uF electrolytic across the Vin and ground on the 7809, but that didn't seem to help. I was thinking of maybe trying a series 330ohm resistor and 470uF cap across the DC input at the wah pedal, to see if that helps.

But I really don't understand the weird grounding issues. I feel there's probably something basic going on here I don't get. Any advice or insights? Is there anything that makes wah circuits particularly sensitive to main DC power supplies?

plugger 14th September 2008 04:07 PM

Wow -- overwhelmed by all the responses here. ;)

It turns out the basic problem is that the wah circuit is very sensitive to picking up EMI because of the inductor in the crcuit.

So, perhaps I can narrow things down a bit.

What would you try if you wanted to minimise the effect of an inductor picking up residual mains ac frequency hum, after basic filtering and regulation?

I'm now regulating 15 VDC from a switched PS to 9 VDC using a 7809, and providing some additional filtering with a 1000uF cap. Certainly less noisy than an unregulated "wall wart", but still unacceptably noisy and noticable hum when the wah circuit is activated.

richie00boy 14th September 2008 04:23 PM

I'm a little surprised that hasn't cured it. Is the switched adaptor earthed at the wall? Maybe it's a ground loop problem.

plugger 15th September 2008 10:02 AM


Originally posted by richie00boy
I'm a little surprised that hasn't cured it. Is the switched adaptor earthed at the wall? Maybe it's a ground loop problem.
The switched PS is earthed at the wall. It's also properly isolated form the rectified output, i.e., there is no continuity from the wall earth through to the DC ground.

Do you think something as simple as running all the 9 VDC lines inside the (steel) case as twisted pairs would help mitigate the low frequency EMI? Of course, can't do much about the tracks on the PCB...

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