Audio Engineering

272 Chapter 8

only be turned with a screwdriver. This avoids not just casual tampering, but knobs
being moved (and settings lost) by accidental brushing, sweeping, or knocking. A collet
nut may be included. When tightened, the setting will then be immune to attack by a
screwdriver, as well as vibration creep. As a further discouragement to “ let’s turn this up, ”
such controls may be placed on the rear panel of the amp or hidden behind cover plates.

8.6.2 Remotable Gain Controls (Machine Control)

Pots are mostly made to interface with human fi ngers via knobs. When a sound system
moves past the point where a single driver in each band can handle the power required or
whereAmbisonic or other multichannel sound is contemplated, remote control opens the
door to “ intelligent ” control of loudspeaker systems and clusters, including balancing and
tweaking directivity, imaging, and focusing, by machines and via wires and radio links.
The gain of an amp can be controlled by a variety of electronic means ( Figure 8.11 ). The
purely electronic means are fast enough to perform additional, true processing functions,
for example, limiting.

VCRs

Stepped finite resolution

Continuous infinite

resolution

Log-antilog

Variable

transconductance

J-FET

LED/LDR

Lamp/LDR

Cermet

CP

Multiplying

DACs

Discrete switches or FETs

Motorised pots

Motorised

pots

VCAs

Figure 8.11 : The family tree of electronically controllable gain and attenuation devices.