This is yet another informational post to save somebody a lot of headaches. It's about trying to convert a Digitech guitar effects board to work on native (220-240V 50hz) power.

In this case the model is a Digitech RP-12, which isn't made anymore, however this applies to most of the large pedal boards that Digitech has made in the last ten years. The actual power supply is a separate component that likewise isn't made anymore. They do offer a replacement (the PSS3), with an adapter (PS4BC) to fit the non-standard DIN power socket on the pedal board. These run about $110 AUD. Yikes. It's really a $20 power supply, and there's plenty of profit selling them for $20. I refuse to pay over a hundred. It's a freaking hobby transformer, fercrysakes.

It turns out there are reasons they charge so much. Nothing else seems to work. The ratings as specified are 9VAC, 2.1A. Simple enough. I can do this - yes? 

Not quite. The commercial 9VAC wall-warts usually stop at 1 amp. A couple of months ago I tried to get around this by purchasing an adequate transformer and building a box for it. After much effort, I finally plugged it in and tried it. HUM and BUZZ. Loud and nasty. Not something you want for studio recording. I scratched my head a while and went back to using the original (110V/60hz mains) unit through a step-down transformer. No hum. So clearly it's not the 50hz that's causing me grief. I brought the original supply into work and put it on an oscilloscope to see what was special about the Digitech branded supply. Couldn't find anything obvious, except that it was a cheap transformer with a lot of inductance artifacts and the waveform was skewed a bit - pointy instead of pure sinusoidal. 

So I took one of my 9VAC 1A wall-warts (that I have several of to power other equipment) and tried it briefly - maybe there was something wrong with my wiring. The 1 amp unit likewise gave me nasty HUM and BUZZ. I didn't leave it plugged in long enough to cook it.

There's no ground and no shielding in the Digitech supply, but I even tried grounding and shielding the lines to reduce the hum - no luck. I tried phase reversing both the primary and secondary (one at a time) in case it was phase sensitive. Nope. There's something obviously different about the Digitech supply.

Hmmm. Back to the drawing board. OK, looks like I just have to use the original 110V supply through a step-down transformer. It's the only thing that makes it work. I gave up on the project and that got me by for the last month or two.

Then last weekend, I turned everything on and the pedal board was dead. Defunct. After some probing it looks like the Digitech transformer just decided it was time to die. Open primary. This gave me an excuse to open the hermetically sealed enclosure to find out what was magic about the Digitech unit. 

Absolutely nothing. It's got a transformer and a fuse. Period. (And the fuse was still good.) OK. So I pulled out my homebrew transformer again. It's only a transformer. What's wrong here? Why is my solution so darned noisy? Why is a generic 9VAC supply so noisy? How do I get rid of the noise?

I tried to order a new 110V supply from the states (to save $50), but nobody will sell me one (even with the huge profit they get). They are forbidden from sending new Digitech products overseas. Even if it's a just a wall-wart that I need to make my old pedal work.  

So getting desperate, I tried a 9VDC supply. The logic boards work and the display lights up, but there's no audio signal at all. So obviously the audio circuitry requires AC. That's weird and I can't imagine what the engineers were smoking when they made that decision, but it doesn't matter - that's just the way it is.  

So I started trying different voltage taps on my homebrew transformer box. I went down to 3V. Nothing worked anymore. Not enough voltage to drive the logic boards. 6V, logic boards work, but the noise is back. 8.5V, noise. 9V, noise. 9.5V, noise. But now it's getting dangerous. Too much voltage can cook something. However, I've got nothing to lose. Right now it's all just an expensive paper weight. The absolute worst that could happen is that I'd be forced to buy a new pedal board, which wouldn't be horrible but the good ones run about $1000 here. Yeah, a new PodXT Live would be ok, as would a GNX4. But maybe, just maybe - my old board just needs a little bit more umph than they claim it does.  

So anyway I cringed (as one normally does when doing something that could end up being a thousand dollar mistake) and tapped into 10.5V. Bingo. Everything works, no noise, no hum. I breathed a sigh of relief. I finally found the issue. They lied. It doesn't take 9VAC, 2.1A. It requires something above 9.5, and probably about 10V for everything to work properly.

So that's the bottom line. If you need a replacement Digitech power unit, ignore the 9V rating. It won't work worth a darn. Get 10.5V (to be sure, since I don't have a variac handy to figure out exactly where it starts working).