Handbook for Sound Engineers

Consoles 891

around today’s more familiar op-amp technology rather
than discrete transistors or tubes. Fig. 25-67A shows a
virtual-earth-type inverting amplifier with the gain
(being equal to the ratio of the feedback resistor RF to
the series resistor RS) continuously variable from
near-infinite loss (min) to near-infinite gain (max) with
unity in the middle. If a fixed-gain-determining leg is
introduced and the variable leg is made frequency
conscious, as shown in Fig. 25-67B (in this instance by
crude single-order high-pass filters—the series capaci-
tors), the gain swing only occurs within the passband of
those filters. The through gain for the rest of the spec-
trum is determined by the two fixed resistors. If this
fixed chain is replaced by a second frequency-conscious
network that does not significantly overlap the original
one in bandwidth, the two chains independently modify
their frequency areas, Fig. 25-67C. The fixed chain is
only necessary where the gain is otherwise unpredict-
ably defined by a frequency-conscious network.
The belt-and-braces low-pass arrangement (for
low-frequency boost and cut) of Fig. 25-67C can be
rationalized into the more elegant circuit of Fig.
25-67D. This circuit more closely resembles the defini-
tive Baxandall circuit. Rather than isolating the
low-frequency boost-and-cut chain with increasing
inductive reactance, the control is buffered away with
relatively small resistances and bypassed to high
frequencies by capacitance. The control takes progres-
sively greater effect at lower frequencies as the rising
capacitative reactance reduces the effective bypass. A
further refinement is a pair of stopper resistors, small in
value, that define the maximum boost and cut of the
entire network.

Naturally, a more complex EQ can be configured
around the same arrangement. A midfrequency bell
curve is easily introduced by any of the means in Fig.
25-68, giving a good hint on how to avoid using a real
tuned circuit using inductors.
A variable signal either positive or negative in phase
to the source Vin can be picked off from a pot straight
across the existing high-frequency and low-frequency
chains, taken to an active filter arrangement to derive
the needed amplitude response shape. The signal is then
returned to the loop at either the virtual-earth point (to
which the high-frequency and low-frequency chains are
tied) or to the noninverting reference input, Fig.
25-68D, depending on whether the absolute phase of the
filter is positive or negative. Industry favorites seem to
be this approach using either a Wein Bridge bandpass or
a state-variable integrator-loop type.
Any number of such active chains may be introduced,
provided two great hangups don’t intrude excessively:

Figure 25-68. Resonant frequency selective elements in the

Baxandall equalizer.

C. Single series tuned bypass filter.

B. Single parallel tuned element.

Bypass

technique

Vin Vo

MF

Vin Vo

Series technique

These resistors

can also signify

existing HF/LF chains

MF

Vin Vo

A. Series tuned symmetrical bandpass.

D. Using an active filter element.

Vin Vo

Response

shaping

active

filter

(inverting)