Audio Engineering

250 Chapter 8

Figure 8.1 shows how the input impedance of a typical, minimal power amplifi er with an
unbalanced input ( Figure 8.2 ) varies across the frequency range. A 10k Ω gain control is
assumed and is here backed off just ldB. Note how the impedance in most of the audio
band is almost constant at the scale used. Then notice how the impedance drops off (so
the loading increases) at high audio frequencies, and more so at higher radio frequencies
(Y ). At low frequencies, if anything, the load impedance increases ( X ).

100 m

(v(Zin))

1000 m 10 100 1 K 10 K

1K

10 K

100 K

Zin

ohms

Frequency (Hz)

100 K 1 M 10 M

Infrasonic range Ultrasonic range

Radio frequencies

Impedance variation in an archetypal unbalanced power amp’s input

X

Audio band Y

Figure 8.1 : Input impedance (load) variation in a typical, simple unbalanced

power amplifi er input stage.

22 K

680 p

Set at1dB

10 μ

Pot 4K7

Input

Figure 8.2 : A typical unbalanced input stage.