Dual Power Supply Question For Dummies

[Paul Marossy]

OK, so I'm done with my new pedal board project (http://www.diystompboxes.com/smfforum/index.php?topic=74667.new#new), and I had a thougt to try and use one wall wart  (14VDC 350mA) for a dual power supply on the pedal board- one supply going into an LM317 adjustable voltage regulator which is supplying all of my 9V pedals and one unregulated supply for my ADA Flanger clone, which has its own internal voltage regulator. When I try to do this, however, the ADA Flanger clone has massive hum but if I power it with a seperate wall wart, there's no problem. Not exactly what I expected to happen. 

So, my question is: how come this doesn't work? 

[R.G.]

There is no way to do anything but wildly speculative guessing from the available information. Not having available the schematic for the ADA Flanger clone power supply would help clear things up.

Being inspired by the challenge of being asked to make a wildly speculative guess, I would say that the ADA flanger with its own regulator is somehow not sharing the same "ground" as provided by the other effects, perhaps having a diode drop or so between it and the ground from the single supply.

Something like this happens when people try to feed unregulated DC to a Neovibe clone and also power their other effects from the same power supply.

[Paul Marossy]

There is no way to do anything but wildly speculative guessing from the available information. Not having available the schematic for the ADA Flanger clone power supply would help clear things up.

Being inspired by the challenge of being asked to make a wildly speculative guess, I would say that the ADA flanger with its own regulator is somehow not sharing the same "ground" as provided by the other effects, perhaps having a diode drop or so between it and the ground from the single supply.

Something like this happens when people try to feed unregulated DC to a Neovibe clone and also power their other effects from the same power supply.

Thanks for the wild speculative guess R.G.! That is what I suspected, but I thought I'd get a second opinion on that.
Here is the schematic, in case you want to have a look at it: http://www.moosapotamus.net/IDEAS/ADAflanger/ADA_MN3007/ADAflangerSCHmn3007.GIF

[R.G.]

OK, that's why they call them wild speculative guesses - they're frequently wrong.  and this one was. There's no ground funnies going on, postulating only good construction.

However the schematic does show up what may be the problem. The innards of the ADA run on a 7815 and the schemo wants +18V in. You're feeding it from a nominally 14V power supply. True, that's probably higher than 14V at normal operation, but a 7815 wants at least 17V to work right. So there's one place to check to start with.

You say "hum". Is this power line hum (60Hz), rectifier ripple hum (120Hz) or other low frequency (motorboating)? You can get a sample of power line by touching the end of a guitar cord plugged into an amp. It's an octave lower than 120.

[Paul Marossy]

However the schematic does show up what may be the problem. The innards of the ADA run on a 7815 and the schemo wants +18V in. You're feeding it from a nominally 14V power supply. True, that's probably higher than 14V at normal operation, but a 7815 wants at least 17V to work right. So there's one place to check to start with.

That's a good point. The wall wart actually puts out 20.2V with no load on it, so I thought that between the six pedals operating at 9V that there would probably be enough current left over to supply those pedals and the ADA Flanger clone and still have a high enough voltage. I guess I should try it again and this time measure the voltage going to the flanger. Duh. I've been up late several nights tweaking stuff, sometimes the obvious escapes you when you're tired! 

You say "hum". Is this power line hum (60Hz), rectifier ripple hum (120Hz) or other low frequency (motorboating)? You can get a sample of power line by touching the end of a guitar cord plugged into an amp. It's an octave lower than 120.

I think it was 120Hz going off of memory, because it sounded a little more like a buzz vs. a hum. I think it was a little of both, actually - but more buzzing than humming. I'm at work now, so I can't recreate those conditions at the moment...