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2. If your transceivers operate off of a +12V power supply, then use that same power
supply to power your StationPro. If your power supply has “floating” positive and
negative output terminals, do
not
tie the negative terminal to the power supply chassis or
to the AC ground terminal on the power supply.
3. Be careful if using a “wall wart” to power your StationPro, because these often have
high AC ripple voltages on their +12V outputs. Obviously, to minimize hum, you need a
clean +12V power source.
4. Bond the case of the StationPro to the cases of all of your transmitters, receivers, linear
amplifiers, and power supplies with short braided ground straps. It is best not to “daisy
chain” these ground straps from one rig to another, and it is important to use short lengths
and as large a conductor as possible. (For a given ground loop current, a large diameter
conductor will result in a lower hum voltage than a small conductor.) Listen to your
transmitted signal (on a dummy load!) and pick the configuration of ground straps that
minimizes hum. Because all station layouts are different, trial and error is the only
practical way to find the optimal configuration.
Steps to Take If All Else Fails
If the above steps don’t reduce hum to acceptable levels, and you still want to route a
vintage microphone through your StationPro, then you will have to take additional
measures to isolate the microphone audio from the hum voltage produced by ground
loops. Here are suggestions for isolating the microphone audio. Some experimentation
will undoubtedly be required to see which gives the best results.
1. Rewire your microphone (and mic connector) so that the microphone element does not
use the braided shield for its audio return. Unfortunately, this usually entails replacing the
mic cable and connector.
2. If you want to use a “Collins” type two-circuit 3/16 in. mic plug, or a 1/4 in. “stereo”
mic plug, then you can electrically insulate the mic jack body from the StationPro front
panel by using fiber or nylon insulating washers. (Recall that that these jacks mount in a
3/8 in. hole, whereas the StationPro front panel has 5/8 in. holes for microphone jacks.)
The Collins-type 3/16” mic jack is insulated from the
front panel using homemade fiberglass washers on
the front and back sides of the panel
