Dual grounds?

[Mark McLemore]

Hi folks,

I'm about to put together the wiring harness/power supply for a homebrew
modular and have some thoughts and questions about grounding that I'd
like to get advice on before I start soldering.  I'm building with Paia
FracRack panels and interconnecting signals via 4mm banana jacks.  The
FracRacks are mounted in an oak cabinet to eliminate ground loops, and
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.  I'm likewise
considering the use of two grounds, especially since I'm using standard
4-pin disk-drive-style molex connectors and have an extra conductor:

             V+                   +------+
              +-------------------|      |---  +15 V
              |    G              |      |
         +--------+N   +----------|      |---  Reference Ground
         | PWR    |D  /           |      |
         | SUPPLY |--*------------|      |---  Earth Ground
         +--------+  |            |      |
              |      |            |      |
              +----- | -----------|      |---  -15 V
             V-      |            +------+
                     |            Module      -> To module
                  +--+--+         Connector
                 /  /  /

              Earth Ground

The idea here is to attach Earth Ground to the FracRack chassis and PCB
"surround-ground" trace, but use Reference Ground for all pots, switches,
LED's and board electronics.  The single point of ground (shown by
the asterisk, above) ties together both grounds on the back panel of
the system along with earth ground from the AC plug and the ground
connections from 1/4" jacks on the back panel.  I intend to connect
the rear 1/4" jacks to front panels using shielded cable, but with the
shield connected only at the 1/4" jack end so as to avoid a ground loop.

Since I'm not using shielded patch cables, I want to get all the noise
immunity that I can get.  I think I remember someone on this list
suggesting this method several years ago.  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?

    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?

    3. If I use the dual-ground method, which ground do I attach
       bypass capacitors to?

    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?

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

I'd appreciate any advice or comments from those of you with insight
into the black magic of effective grounding.

Thanks in advance,
Mark McLemore                   mgm at eden.com