Posted by Lew on October 05, 2005 at 07:18:49:
In Reply to: Backplate Controls posted by Mike Currie on October 05, 2005 at 06:20:56:
The bass control chooses which of three primary windings in the bass/midrange audio
transformer are engaged. This seems to affect efficiency and probably affects the impedance in
the extreme low frequency range. The mid-range control chooses one of 3 possible inductances
vs no inductor (4 positions, I think) that will be seen by the driving amplifier in series with the
bass/midrange transformer. Any of the inductors effect a 6db per octave low pass filter with a
-3db point dependent upon the choice of the inductance. The frequency of the cross-over to the
treble transformer is fixed with respect to that tranformer by an RC network (hi-pass filter),
typically at about 550 Hz. The Brilliance control is not a "brilliant" idea, but it places a resistance
in series with the treble transformer and thereby cuts gain to the treble. Unfortunately, the
Brilliance control is an L-pad type of potentiometer and cannot sound good. I and others have
long advocated bypassing it OR choosing a setting which pleases you, measuring R across the
control at that setting, and then replacing the control with a single high-quality resistor of like
value. Even if you open up the Brilliance control all the way, so that there is effectively zero
resistance across it, there is still an audible benefit to bypassing it or removing it, because
otherwise the audio signal has to pass across it.