[sdiy] Why dual grounds?

[David G. Dixon]

> > Since many of the modular synth standard power supply connectors
> > (MOTM, Oakley, Blacet etc) have two ground wires, is one of these
> > used for a "clean" ground and the other "dirty ?
> 
> No. Usually, both ground pins are shorted. This is true for all
> MOTM, Tellun, Blacet, Patchell and Ruberto boards I've seen.
> 
> On Oakley boards, all signal and power ground traces go to the
> ground pin that's on the +15 V side. The ground pin on the -15 V
> side is only used to ground the sleeve contacts of the jacks. It's
> not clean vs. dirty or signal vs. power, it's frame ground vs.
> everything-else-ground.

Why?  Ground loops?
 
> More modules should use the 6-pin power connector and power LEDs
> etc. off the +5 V line. If nothing else, it would mitigate the
> derating of the power supply caused by asymmetrical loads (most
> modules draw more current from the positive rail).

Most schematics I've seen specify current-limiting resistors for LEDs which
are too small.  I typically use at least 2.2k for an LED powered off of a
15V rail, while most schematics specify 1k.  My LEDs are plenty bright, and
draw less than half the power.  Of course, I tend to use blinking lights
fairly sparingly...