Audio Engineering

252 Chapter 8

8.2 Radio Frequency Filtration ...................................................................................

8.2.1 Introduction

Music starts out as air vibrations. These are not directly affected by electromagnetic (EM)
waves, except while they are passing through an audio system in the form of electronic
signals. Planet Earth has long had natural EMI, in the form of various electric and magnetic
storms; both those occurring in the atmosphere and those occurring on the “ surface ” of the
Sun and Jupiter in particular. Since 1900, the planet has increasingly abounded in human-
made EMl babble, comprising electromagnetic energy fi elds and waves, some continuous,
some pulsed, and others random. As stray signals nearly always have nothing to add to the
music at hand, and most are profoundly unmusical, and as EMI permeates almost everywhere
above ground unless guarded against, music signals require “pro-active ” protection.

EM waves used for radio broadcasting and communications mainly start in earnest at
150 kHz (in the United Kingdom and continental Europe) and above, and continue to

1

(v(Zin))

10 100 1 K 10 K

1K

10 K

100 K

Zin

ohms

Frequency (Hz)

100 K 1 M

• Change with gain knob setting Not monotonic

Impedance variation in an archetypal unbalanced power amp’s input

UNBL3ZIN.CIR Temperature  15 Padj.DC.value  0. 001000002

Mid and min settings

Max and near min settings

Audio band

Figure 8.4 : Impedance variation in a typical unbalanced power amplifi er’s input stage

as the gain control is swept.