chassis grounding

Tom May ftom at netcom.com
Sat Aug 10 01:43:30 CEST 1996


Gene Stopp writes:

>Within a self-contained system such as a synthesizer I always just 
>ground everything together - the circuit board ground, the panel, the 
>jacks, the box, and the third prong of the AC power cord. Works fine 
>for me so far. I think that you only run into ground loop problems 
>when interconnecting machines together whose AC power comes from 
>different and/or distant outlets.

I had ground loop problems on a tube amplifier that had effects loop
jacks grounded to the steel chassis.  I got a lot of hum reduction by
replacing them with nylon-bushing jacks and grounding them at the
power supply.  On the other hand, Mackie highly touts their metal
jacks screwed directly to the steel chassis.

>You can lift the synthesizer housing from the third prong if you want 
>to avoid potential ground loop problems. What I would do in that case 
>is make sure that the power supply ground terminal (usually the center 
>tap of the bipolar supply transformer winding) is connected to the 
>box, jacks, and circuit board ground.

That's just like using a three-to-two prong (ground lift) adapter,
which is a reasonable fix for something you didn't build yourself,
although you lose the safety advantage of having the chassis directly
grounded.  For DIY, I have thought the better solution is to use the
chassis as a shield and a safety by connecting it to the middle prong,
then ground all the signal stuff (power supply, jacks, circuit board)
together and call it a signal ground, then ground the signal ground to
the chassis ground through a ground lift switch.  That way the chassis
always has a beefy ground connection, but ground loops can be
minimized by opening the ground lift switch if necessary.

There are a couple books I'm planning to look at some day that should
be more authoritative: "Grounding and Shielding Techniques in
Instrumentation" by Ralph Morrison, published by Wiley, and "Noise
Reduction Techniques in Electronic Systems" by Henry Ott, also
published by Wiley.

Tom.



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