Audio Engineering

786 Chapter 27

Imagine that Z1 is a small resistance. When the slider of VR1 is at position A, the
input is heavily attenuated and the lower feedback limb of the op-amp is at its greatest
value (i.e., the gain of the stage is low). Now imagine moving the slider of VR1 position B.
The situation is reversed; the input is much less attenuated and the gain of the amplifi er
stage is high. This circuit therefore acts as a gain control because point A is arranged
to be at the extreme anticlockwise position of the control. As a tone control this is
obviously useless. However, in the second part of Figure 27.18(b) , Z1 is replaced with
a tunable circuit, formed by a variable inductor and a capacitor. This tuned circuit has
high impedance—and therefore little effect—except at its resonant frequency, whereupon
it acquires very low dynamic impedance. With the appropriate choice of inductor and
capacitor values, the midfrequency equalizer can be made to operate over the central
frequency range. The action of VR1 is to introduce a bell-shaped EQ response (as
illustrated in Figure 27.12 ), which may be used to attenuate or enhance a particular range
of frequencies, as determined by the setting of the variable inductor.

R 1

Input from600 R

previous stage

VR1

Treble boost

SW1

5 kHz 12 kHz

R 2

2K

L 1

100 mH

L 2

39 mH

C 1

10 n

C 2

4n7

R 4

2K2

R 3

47R

Bass boost

VR4

SW3

100 Hz

C 6

470 n

C 5

3u3

20 HZ

22 R

220 n

100 n

47 n 20 Hz

SW4

100 HzSW2

10 n

R 5

C 4

C 8

C 7

C 3

10 kHz

20 kHZ

VR2

Bass cut

R 6

18 K

Output to following

24 dB amplifier

VR3

Treble cut

(d)

Figure 27.18(d) : Older, passive equalizer circuit.