Audio Engineering

Interfacing and Processing 251

Figure 8.3 shows how the same input stage’s impedance varies (without changing
anything else) as temperature is changed from 15° to 85°. In other words, what can
happen to the input impedance when an amplifi er is “ cooked? ” For the most part,
impedance increases, which will do no harm. However, in live work it might just alter a
howl round threshold, as the higher load impedance allows the signal voltage to rise ever
so slightly.

Figure 8.4 shows how the input impedance typically varies as the gain is adjusted.
Because the change with each 30° rotation step is nonmonotonic,Zin goes up and then
comes down, as you might expect. A 10k Ω log pot is assumed.

Ideally, an amp’s input impedance would remain constant despite these changes. In
unbalanced circuits, there is not much harm as long as any change in impedance is
gradual and stays above certain limits, and anything that isn’t like this happens well
above (or even further below) the audio band. Staying constant is far more important in
balanced circuits.

1

(v(Zin))

10 100 1 K 10 K

1K

10 K

100 K

Zin

ohms

Frequency (Hz)

100 K 1M

• Change with temperature

Impedance variation in an archetypal unbalanced power amp’s input

85

 C

15

 C

Z

Audio band

Figure 8.3 : Impedance variation in a typical unbalanced power amplifi er input stage

as the amplifi er warms up.