Handbook for Sound Engineers

Consoles 893

shifted around to the noninverting input of the op-amp.
At minimum, the input attenuation is quite vicious
while the feedback leg is long, making the op-amp
deliver only a small amount of gain. When the attenua-
tion characteristics are reversed for maximum, the
op-amp works at a high loop gain, while the input is
only slightly attenuated. Unity is achieved at control
center where the input attenuation equals the make-up
gain of the amplifier.

There is a fascinating tradeoff between noise mecha-

nisms in this circuit arrangement. Assuming a

maximum of three controls (for fairly basic

high-frequency, low-frequency, and midsweep curves)

before interaction becomes a major hassle, the amplifier

can have between 10 dB and 20 dB of fairly

frequency-conscious background gain (i.e., with all

controls flat) rendering it at first sight significantly

noisier than a Baxandall. However, the impedances

around the amplifier are around a decade lower. This

considerably reduces thermal noise generation due to

resistive elements and op-amp internal mechanisms.

In addition, the noise generated by the active

frequency-determining filters is, with the controls

neutral, injected equally into the inverting and nonin-

verting inputs of the op-amp. Differential amplifiers

being what they are, common-mode signals (such as

this equally injected filter noise) get canceled out and

do not appear at the output.

Interaction can still intrude, and care is required to

prevent excessive frequency band overlap.

Center-tapped pots (the tap grounded) eliminate many

interactive effects but at the cost of increased invariable

background gain (noise) and peculiar, almost intrac-

table, boost-and-cut gain variation linearity versus

control rotation.

Figure 25-69. Swinging output equalizer.

Vin

Vin

Vin

Vo

Vo

Vo

Feedback

chain Outputattenuator

chain

= =

A. The two chains have equal attenuation for unity gain.

Max Min

Stopper resistor

B. Unity at the center, variable gain around unity.

HF

LF

MF

C. Multiband equalizer system approach.

Figure 25-70. Swinging input equalizer.

A. Unity at center.

Vo

Vo

Vin

Vin

Min Max

Stopper

HF

LF

Mid

Real or

actively

generated

reactances

B. A three-band equalizer configuration.