[sdiy] Powering a second bit of kit from same PSU(wiringoptions)

[Richie Burnett]

Hi Tom,

> From: Tom Wiltshire
> Subject: Re: [sdiy] Powering a second bit of kit from same PSU(wiring 
> options)
>
> Can you explain a bit more about this ground loop situation, Richie?
> I'm not seeing what's going on, and I don't want to be encouraging someone 
> to try something if it's genuinely risky.

Taking the TB-303 as an example, in a momentary stroke of insanity Kikumoto 
put the voltage regulator (pass transistor Q45) for the 5V logic in the 
negative side of the supply from the external power connector, instead of 
the more common (and more sensible) positive rail.  See top right corner of 
schematic here:

http://www.freeinfosociety.com/electronics/schematics/audio/pictures/rolandtb303.gif

This means that the negative terminal of the power jack and the sleeve of 
the audio output jack are not at the same potential!  And this is the 
dangerous part, if the same power supply is also used to power a piece of 
equipment that does have it's negative power rail connected to the ground of 
its audio jack, this will effectively *bypass* the voltage regulator inside 
the TB-303 when their audio outputs are connected to the inputs of a mixer 
for example.  As you can imagine the 5V logic chips don't like 9 to 12V 
across them.

The same "trick" appears here in the TR-606:

http://machines.hyperreal.org/manufacturers/Roland/TR-606/schematics/roland.TR-606.schem-6.gif

And I've seen guitar pedals that generate a split-rail supply from a 
single-rail DC supply using a virtual ground at half the DC supply voltage 
for op-amps.

My point was, don't assume that the supply negative rail and the audio 
ground are at the same potential.  They could be at different potentials for 
different pieces of equipment, and trying to use one power supply to power 
lots of things in parallel could attempt to force "supply negative" and 
"audio ground" to be at the same potential for all pieces of equipment. 
Possibly with disastrous consequences.  I'm sure anyone who blew up the CPU 
in their TB-303 by trying to power it from the same adapter as a distortion 
pedal, really wishes they'd just shelled out for another wall wart :-(

In an ideal world, there would be no electrical connection between power 
jacks and audio jacks, and they would be galvanically isolated from each 
other, like MIDI ports.  This would eliminate ground loops, USB power 
"whine", and regulation issues like the one we've just been discussing.  But 
DC-to-DC converters are expensive so designers compromise.  Kikumoto made 
many other compromises! ;-)

-Richie,