Recording Consoles 78727.6.4 Inductor–Gyrators
As shown, the frequency adaptive component is designated as a variable inductor.
Unfortunately, these components do not exist readily at audio frequencies and to
construct components of this type expressly for audio frequency equalization would be
very expensive. For this reason, the variable inductors in most commercial equalizers
are formed by gyrators—circuits that emulate the behavior of inductive components by
means of active circuits, which comprise resistors, capacitors, and op-amps. An inductor–
gyrator circuit is illustrated in Figure 27.18(c). This is known as the “ bootstrap ” gyrator
and its equivalent circuit is also included within the fi gure. Note that this type of gyrator
circuit (and indeed most others) presents a reasonable approximation to an inductor that
is grounded at one end. Floating inductor–gyrator circuits do exist but are rarely seen.
Operation of the bootstrap gyrator circuit can be diffi cult to visualize, but think about the
two frequency extremes. At low frequencies, Ca will not pass any signal to the input of
the op-amp. The impedance presented at point P will therefore be the output impedance
of the op-amp (very low) in series with Rb , which is usually designed to be in the region
of a few hundred ohms. Just like an inductor, the reactance is low at low frequencies.
Now consider the high-frequency case. At HF, Ca will pass signal so that the input to the
op-amp will be substantially that presented at point P. Because the op-amp is a follower,
the output will be a low-impedance copy of its input. By this means, resistor Rb will
thereby have little or no potential across it because the signal at both its ends is the same.
Consequently, no signal current will pass through it. In engineering slang, the low-value
resistorRb is said to have been “ bootstrapped ” by the action of the op-amp and therefore
appears to have a much higher resistance than it actually has. Once again, just like a real
inductor, the value of reactance at high frequencies at point P is high. The inductor–
gyrator circuit is made variable by the action of Ra , which is formed by a variable resistor
component. This alters the breakpoint of the RC circuitCa / Ra and thereby the value of the
“ virtual ” inductor.
In recent years, the fascination in “ retro ” equipment has brought about a resurgence of
interest in fi xed inductor–capacitor type equalizers. Mostly outboard units, these are often
passive EQ circuits [often of great complexity, as illustrated in Figure 27.18(d) ], followed by
a valve line amplifi er to make up the signal gain lost in the EQ. In a classic LC-type equalizer,
the variable-frequency selection is achieved with switched inductors and capacitors.