[Hammarlund] SP-200 Series

[Glen Zook]

One thing to be aware of when replacing an
electrodynamic speaker field winding with a choke is
the fact that in many receivers the field winding also
serves as a "hum buck" coil.  The phase of the AC
component of the B+ is 180 degrees out-of-phase with
AC component within the field winding.  This serves to
"cancel" the "hum" component of the audio which comes
from the speaker.

If you get the connections to the winding of the field
coil reversed, the components are "additive" and the
hum from the speaker will be considerable, whereas if
the connections are correct, the hum will be very
greatly eliminated.  When you replace the field
winding with a choke and use a permanent magnet type
of speaker you also eliminate the hum reduction
effects of the "hum buck" coil.  The result is a fair
amount more hum coming along with the desired audio.

It takes a fair amount of additional filtering in the
power supply to reduce the residual AC component on
the B+ so that the "ripple" or hum on the high voltage
is reduced to the level which will not produce
objectionable hum in the audio output.  The use of the
"hum buck" type of circuitry was very common in the
receivers of the late 1920s and of the 1930s.

Frankly, I do not know if the Physics behind the "hum
buck" coil was understood by many of the designers of
the 1920s and 1930s or if it was just one of those
things that was discovered "by accident".  However,
considering the knowledge that most of these designers
had at the time, I think that the Physics was
definitely understood.  Also, the cost factor was also
present.  It was cheaper to use an electrodynamic
speaker to provide the B+ filtering and the "hum buck"
action than it was to provide a permanent magnet
speaker with the additional B+ filtering that was
required to reduce the hum to a reasonable level.

By the way, the mechanical effect of the AC component
in the field winding is causing the cone of the
speaker to be "pulled" in one direction and then the
other as the "ripple" is superimposed on the B+.  The
"hum" or "ripple" that is present on the voltage that
is applied to the voice coil is 180 degrees out of
phase with that in the field winding.  As the field
winding pulls the cone one direction (which will
produce hum), the voice coil is pulling the cone in
the opposite direction.  Thus, the effects of the two
voltages cancel each other.  If you get either the
voice coil or the field coil windings reversed. then
the actions enhance each other (both pull the cone in
the same direction) and the hum is much worse than
would be caused by either action by itself.  Thus, the
actual effects of the "hum buck" coil are mechanical
and not electrical.  However, the results are very
effective in eliminating objectionable hum from the
audio output of the receiver.

Glen, K9STH


--- Kl0nr@aol.com wrote:

Is the field coil of the speaker used as a choke in
one of the B+ supplies? What would be the choke rating
for replacing the field coil and using a PM speaker,
that is, XX Henrys, XXXXXohms, etc.

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