"gndloops"

[Ingo Debus]

Martin Czech wrote:
> I want to propose some idea and let me precisely say: it is about ONE
> (big) modular system. I'm not talking about different separate machines
> tied together, just ONE (in the case of a large distributed system,
> with different power supplys etc etc. you won't get away without
> balanced connections).

But some day you might want to connect your synth to an amp, mixing
console, etc. won't you? ;-)

> The case is connected to power-gnd, but the jacks
> are isolated, power-gnd may be connected only at the star point to signal gnd.

You mean Protective Earth (PE) with power-gnd? I would not connect this
directly to signal ground. There should be *one* connection between the
two in the whole system. If you have another device (amplifier, mixer,
PC or whatever) with this connection in it, the problems begin...
Somebody suggested here to put a low-R resistor between signal ground
and PE. This sounded very reasonable to me. You can use a switch instead
(I did so in my mixing console).

> TO SAY IT LOUD AND CLEAR: I THINK THE GND LOOP BY CABLE SHIELDS
> CONNECTED AT BOTH ENDS IS NOT HARMFULL IN THE SPECIAL APPLICATION I'M
> TALKING ABOUT.  IT IS USEFULL.

100% agree. Disconnecting the shield only makes sense when the shield
does not carry signal current. If it does, it is most important to keep
the shield impedance as low as possible. A ground loop is not as bad as
an uncontrolled signal path via ground.

Ingo