Dual grounds?

[Mark Amundson]

Mark McLemore wrote:
> I'm about to put together the wiring harness/power supply for a homebrew
> modular....
> I've been thinking of using a "star" grounding system where the ground
> to each module is tied directly to a single point near the power supply.
> 
> I've seen schematics for old Moog modulars where TWO grounds are being
> used: a reference ground and a "dirty" ground -- the latter appears
> to be intended to eliminate noise from switch triggers....
> My questions are:
> 
>     1. Does this work or does it create more problems?  Does it prevent
>        ground loop hum and RF interference any better than a single
>        ground?

I would recommend chassis and signal ground stars where both separated
by a 100 ohm, 1/2 watt safety resistor. The supply drives the signal
ground, leaving the chassis and line cord grounds for safety only.

>     2. Would it be better to just use the extra wire as another
>        conductor for the same ground and tie the chassis and signal
>        grounds together within each module?

Use the spare connection as a "supply return to ground (dirty)" and use
the other ground as a signal "ground (reference)".
 
>     3. If I use the dual-ground method, which ground do I attach
>        bypass capacitors to?

The supply return to ground except if you have the need for RF shield
caps.

>     4. I used to have a problem with VCO's locking due to intermodulation
>        over the power supply lines, does the dual grounding system
>        help this problem at all?

Yes.

>     5. Does anyone see a pitfall or disaster waiting to happen by using
>        the above approach?

In comparison to others, no. Module interconnects use the reference
ground. In a perfect world I would use balanced line interconnects with
one-side grounded shield connections between modules.

> Thanks in advance,  Mark McLemore         mgm at eden.com

Mark Amundson,