B&W electronics 70 User Instruction Handbook - page 18
-18-
region was the correct one in terms of minimal intermodulation
distortion within the all important mid frequency band. Due
to the successful completion of the 701 design, a crossover
frequency of 500 Hz was possible and proved successful in all
respects.
The high pass filter of an electrostatic unit requires
special consideration if the full potential of the unit is to
be realised. Attention must be paid to tailoring the stop
band response to the required slope rate. The natural response
of an electrostatic assembly is to rise very steeply immediately
at diaphragm resonance and produce an almost impossible matching
characteristic for the bass radiator. This rise is controlled
partially by accurate diaphragm tension but one must also
restrict input within the first octave above resonance and,
coupled with the falling impedance characteristic of the
capacity load, the E.L.S. presents different and somewhat
exacting requirements in terms of filter network.
The remainder of the crossover unit on Model 70 is
conventional but it is worth mentioning that no less than 74
mfd. of paper is used in this circuit - at a cost of one
component that would almost buy a cheap loudspeaker system.
Reversible electrolytic capacitors whilst satisfactory at A.C.
powers for a limited period of use become conductive and render
the filter network useless under the stringent conditions laid
down for a system such as Model 70.
One final component worthy of mention is the series feed
inductor on the high pass section. This component can
seriously modify the controlling feedback applied to the voice
coil and, because it handles considerable current, we designed
a special ferrite inductor which reduced the D tot. at 25 watts
input to well below 0.5% at all frequencies from 30 H z .
Conventional components sampled showed figures in the region of
2.5%