[NJARC] Power Supply Question

[njarc@mailman.qth.net]

Joe and the group
I did some reading on electrodynamic speakers and learned some
interesting things about them.  Initially, I couldn't understand how
reversing the field coil could reduce the power supply hum.  On the face
of it, what difference would a north pole facing the speaker cone have
over the south pole?

What I didn't realize was that the field coil, acting as the power supply
choke, would have a magnetic field that varies in strength at the ripple
current frequency.  This would affect the voice coil just like the
amplifier's output.  To counteract this unavoidable coupling, the voice
coil has two windings on it.  One is for the audio amplifier as expected.
 The other is a hum-bucker winding which is wired in series opposition,
with the voice coil, and cancels the induced hum.

So when properly connected, the winding is series subtracting and the hum
is reduced.  When reversed, just as Joe reported, it is series aiding and
more hum in produced.

Always something new to learn about old radios!
Regards,
Jim