To overcome the substantial losses in the passive version of
this circuit, which give a level response (with both controls at mid way
setting) but at -20dB below the input voltage, it is common to incorporate an
amplifier in the designs. Nowadays an op-amp would be a reasonable choice, with
the Baxandall network forming a negative feedback loop to give the required
gain figures over the necessary bandwidth. Various designs are possible with
different values for resistors R1 to R4 and C1 to C4 in the network, depending
to some extent on the output impedance of the previous, and input impedance of
the following circuits.
With active circuits such as that shown in Fig. 0.0.6 the
aim is to have the level response at 0dB so there is no gain and no loss due to
the tone control circuit. The maximum amount of boost possible should not be
sufficient to overload any stage following the tone control if distortion is to
be avoided. The design of such control circuits is usually therefore, an
integral part of the overall design of an amplifier system.
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