On 7/8/2017 7:18 PM, Michael Walker wrote:
> Ground can be optional since the DC ground for the amp will be completed via the coax cable.
NO! NO! NO! That's a recipe for RFI. EVERY path, even when it's DC,
should be a transmission line so that it can reject RFI and other noise.
A single conductor from one RCA to the other combines with the shield
return to form a wonderful LOOP that couples RFI and noise two ways --
first, inductively, and second as an antenna.
Many years ago, at an IEEE EMC engineering conference, I saw a wonderful
demonstration of this problem. They had an HP oscillator and a voltmeter
connected two ways. First, a wire from hot to hot with a very large
conductive plate serving as the return. Second, coax. There was an
current sense in both returns.
At low AF, the current was all in the big plate, but through the audio
spectrum, it shifted to the coax, and by about 10 kHz, was almost all in
the coax. The principle is that current follows the lowest IMPEDANCE
path. At low AF, the inductance of the loop with the plate is low and
the coax is Zo (which varies with frequency, is very high at low audio,
falling rapidly). As frequency increases, XL of that loop increases, Zo
falls, and all the current is in the coax.
73, Jim K9YC
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