Audio Engineering

Preamplifi ers and Input Signals 219

This is because:

● increased applied potentials produce higher thermal dissipations in the

components for the same operating currents;

● device performance tends to deteriorate at higher interelectrode potentials and

higher output voltage excursions; and,

● available high/voltage transistors tend to be more restricted in variety and less

good in performance than lower voltage types.

7.18 Controls

These fall into a variety of categories:

● gain controls needed to adjust the signal level between source and power

amplifi er stages;

● tone controls used to modify the tonal characteristics of the signal chain ; and,

● fi lters employed to remove unwanted parts of the incoming signal, and those

adjustments used to alter the quality of the audio presentation, such as stereo

channel balance or channel separation controls.

7.18.1 Gain Controls

These are the simplest in basic form and are often just a resistive potentiometer voltage
divider of the type shown in Figure 7.50. Although simple, this component can generate
a variety of problems. Of these, the fi rst is due to the value chosen for R 1. Unless this is

0V 0V

C 3 C 4

C 2

C 1

R 1

Eout

Ein

Zsource

(Emax)

Figure 7.50 : Standard gain control circuit.