Ashly Parametric Equalizers SC-63 - Manual (Page 16)

CIRCUIT DESCRIPTION
The heart (and primary expense) of the SC-66A and SC-63 is a unique bandpass
filter circuit. Basically a "state-variable" type, this filter is trimmed and
optimized to provide excellent transient response and a wide-range bandwidth
adjustment. Each filter in the SC-66A and SC-63 can be tuned over a 50:1
frequency range (about 5 1/2 octaves) and a 70:1 bandwidth range with no more
than a 2 dB amplitude error at center frequency. At its sharpest setting, the
filter has a "Q" of about 35 and generates a response curve with 3 dB points
only 1/20 octave apart, making feedback control possible with no audible side
effects. The filters are manufactured as individual, plug-in units to make
servicing easy.
Each filter is placed in the feedback loop of a summing amplifier to produce
the desired frequency response. Since a separate summing amplifier is used
for each band, no interaction between bands occurs.