AudioBanter.com - View Single Post - Audio Ground 10 ohms above powersupply ground??

wrote:

I've noticed some designs place their audio ground 10 ohms above power
supply (chassis) ground.
Sometimes this includes all the grounds of the preamp and driver circuitry
as well as the
input (RCA) jack. Anyone know the theory behind this? In a typical
input/driver stage drawing say 20 mA,
the 10 ohm resistor will float "0" volts aprox .2 volts above power supply
ground. Can this be a good thing?
Any comments appreciated.

Depending on the exact implementation, this may be done for a variety of
possible reasons.

One very sensible application of this method is to avoid low level signal
currents sharing the same path as high level output currents for example where
the IR voltage created by the output due to the finite resistance of
conductors might otherwise create crosstalk type interference in the input
stages.

Another reason would be to avoid the ( possibly surprising to you ) problem
that not all points in the chassis are at equal potential. This is because the
stray flux from power transformers will induce small voltages in the chassis.
It may only be in the tens of microvolts but you'll see it for sure if you're
working at low noise levels.

There's no excuse for a DC potential though. I'd always return the power
supply zero volts to the audio ground, so you shouldn't really see this. The
chassis should always be connected directly to mains earth btw for safety
reasons.

Graham