>"I put a pair of small pie wound chokes in series with the + and return
lines at the box's keying jack and that fixed the problem."
That often works fine on control circuits but for audio circuits and other
circuits that require maintaining signal waveform integrity, I now use
common-mode chokes and abandoned differential-mode filtering that uses
Pi-section L and bypass C components. For example, pass a square wave, or
any other fast, complex waveform through a Pi-section choke and observe the
result. Even when properly terminated, large overshoot spikes and ringing
results.
By contrast, a CM choke does not affect differential mode signaling as
there's no differential series L due to inductance cancellation. A CM choke
yields 2*L on common-mode currents, or twice the inductance of a single
winding.
We also need to be mindful of series resistance when using simple series
chokes in power supplies and high-power audio output circuits. We can
mostly ignore that with CM chokes except for the added wiring resistance
required with each turn through a core.
Paul, W9AC
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