Audio Engineering

186 Chapter 7

7.8.2 Diode Switching

Diode switching of the form shown in Figure 7.15 , while employed very commonly in RF
circuitry, is unsuitable for audio use because of the large shifts in the DC level between
the “ on ” and “ off ” conditions, which would produce intolerable “ bangs ” on operation.

For all switching, quietness of operation is an essential requirement, and this demands
that care shall be taken to ensure that all of the switched inputs are at the same DC
potential, preferably that of the 0-V line. For this reason, it is customary to introduce DC
blocking capacitors on all input lines, as shown in Figure 7.16 , and the time constants
of the input RC networks should be chosen so that there is no unwanted loss of low-
frequency signals due to this cause.

VOLTAGE AMPLIFIERS AND CONTROLS

7.9 Preamplifi er Stages ...............................................................................................

The popular concept of hi-fi attributes the major role in fi nal sound quality to the audio
power amplifi er and the output devices or output confi guration that it uses. Yet in reality
the preamplifi er system, used with the power amplifi er, has at least as large an infl uence
on the fi nal sound quality as the power amplifi er, and the design of the voltage gain stages
within the pre- and power amplifi ers is just as important as that of the power output





33 K

Output

A

B

Inputs

33 K

33 K

R 1

R 1 R 2

0.1μ

1 μF

1 μF

0.1μ

100 K

100 K

2N5459

2N5459

‘On’

‘On’

10 V ‘Off’

10 V ‘Off’

0V

Figure 7.14 : Junction FET input switching circuit.