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Appendix A: Avoiding Ground Loop Complications
Until about 1975, most amateur microphones such as the venerable Astatic D104
used a two-conductor shielded cable – one conductor for the microphone audio, and one
for the push-to-talk line. For such microphones, the grounded shield was connected
directly to the transmitter chassis, and both the microphone audio and PTT return currents
flowed through the shield.
This arrangement generally worked satisfactorily so long as the microphone was
plugged directly into a transmitter or transceiver. However, if used with any kind of
switching device, or with push-to-talk circuits that used an AC control voltage, these
microphones were found to be susceptible to hum pickup.
This problem was solved with the introduction of microphones that use two
conductors for the audio, typically called “mic+” and “mic-,” and today virtually all
amateur microphones use this configuration. By using a dedicated “mic-“ wire for the
return audio currents, the low-level microphone voltage is not affected by hum currents
on the shield or chassis of connected equipment. (A move toward low impedance
microphones – 200 ohms is common – also provided enhanced hum immunity and also
allowed longer cable runs than is possible with high impedance microphones.
Understanding How Ground Loop Currents Cause Hum
Numerous currents flow along the chassis of any transceiver or transmitter. In
vintage vacuum tube transmitters, these might include several amperes of 50/60 Hz
filament current, ripple currents from power supply filter capacitors, and 50/60 Hz AC
currents from bypass capacitors on the AC line. When a chassis also carries the
microphone audio return current, the voltages produced by these AC currents can be a
significant fraction of the microphone audio voltage. This AC voltage adds directly to the
microphone audio and appears as hum on the transmitted signal.
When a microphone attaches directly to the mic jack in vintage transmitters this
hum voltage does not cause a problem, because there is typically only an inch or two
between the transmitter’s grounded microphone jack and the grid of the first audio
amplifier stage. However, when a vintage microphone is routed to the transmitter through
a piece of peripheral equipment, such as a VOX adaptor, audio speech processor, or a
switching device such as the StationPro, then the AC hum voltage can be quite
substantial. In these cases the hum voltage is caused by the AC currents that inevitably
flow on the outer surface of the shielded cable that ties the peripheral equipment to the
transceiver. The following diagram shows how these shield currents cause hum when the
StationPro is used in connection with a vintage vacuum tube transmitter that uses its
chassis for the microphone audio return.
