Audio Engineering

788 Chapter 27

27.6.5 ‘ Q ’

Often it is also useful to control the Q of the resonant circuit so that a very broad, or a
very narrow, range of frequencies can be affected as appropriate. Hence the inclusion of
the Q control as shown in Figure 27.12. This effect is often achieved by means of a series
variable resistor in series with the inductor–capacitor (or gyrator–capacitor) frequency-
determining circuit.

27.6.6 Effect Send and Return

The effect send control feeds a separate mix amplifi er. The output of this submix circuit
is made available to the user via an effect send output on the back of the mixer. An effect
return path is also usually provided so that the submix (suitably “ effected ” —usually with
digital reverberation) can be reinjected into the amplifi er at line level directly into the
main mix amplifi ers.

27.6.7 Faders and Pan Controls

Beneath the effect send control is the pan control and the main fader. Each channel fader
is used to control the contribution of each channel to the overall mix as described earlier.
A design for a fader and pan pot is illustrated in Figure 27.19. The only disadvantage of
the circuit is that it introduces loss. However, its use is confi ned to a part of the circuit
where the signal is at a high level and the Johnson noise generated in the network is very
low since all the resistances are low. The circuit takes as its starting point that a semilog
audio-taper fader can be obtained using a linear potentiometer when its slider tap and
“ earthy ” end is shunted with a resistor one-tenth of the value of the total potentiometer

Input

Main fader

100 K

lin.

10 K

VRb lin.

VRa

Rc

Rd

15k

15 K

Pan

To right

mix amp.

To left mix amp.

Figure 27.19 : Fader and pan pot circuit.