Handbook for Sound Engineers

Consoles 845

solid, infinite, immovable, dependable ground. It has
many other names too: earth, 0 V, reference, chassis,
frame, deck, and so on, each of differing interpretation
but all, ultimately, alluding to the great immovable
reference.

Electrons could not care less about all this. They just
go charging about as potentials dictate; any circuit will
work perfectly well referred to nothing but itself. (Satel-
lites, cars, and flashlights work, don’t they?) Ground in
these instances is but an intellectual convenience.
Interconnection of a number of circuit elements to
form a system necessarily means a reference to be used
between them. To a large degree, it’s possible to obviate
a reference even then by the use of differential or
balanced interfacing, unless, of course, power supplies
are shared.
So, having proved that ground is seemingly only a
mental crutch, why is it the most crucial aspect of
system design and implementation?

25.8.1 Wire

Fig. 25-23A shows a typical, ordinary, long, thin length
of metal known more commonly as wire and occasion-
ally as printed-circuit track. However short it is, it will
have resistance, which means that a voltage will
develop across it as soon as any current goes down it.
Similarly, it has inductance and a magnetic field will
develop around it. If it is in proximity to anything, it
will also have capacitance to it.

So Fig. 25-23A actually looks more like Fig. 25-23B
with resistive and distributed reactive components.
Admittedly, these values are small and seem of little
significance at audio frequencies, but clues have already
been laid (particularly in Section 25.7 on op-amps) that
believing the world ends at 20 kHz is not so much
myopic as naive.
A radio engineer looking at Fig. 25-23B would
mumble things like “transmission line,” “resonance,” or
“bandpass filter,” maybe even “antenna.” RF tech-
nology and thinking may seem abstruse and irrelevant

to audio design until it is considered that active devices

commonly used nowadays have bandwidths often

dozens, sometimes hundreds of megahertz wide. An

even more frightening realization is the enormous quan-

tity of RF energy present in the air as a consequence of

our technological being; never mind the gigawatts of

broadcasting bombarding us, the proliferation of

walkie-talkies, cellular phones, and business radio all

beg mischief in our systems. It even comes from other

continents; the aggregate field strength of international

broadcasters clutched loosely around 6 MHz, 7 MHz,

10 MHz, and 15 MHz is truly phenomenal.

A more obscure collection of equivalents is shown in

Fig. 25-24. Fig. 25-24A represents a wire into a bipolar

transistor input; Fig. 25-24B shows a wire from a

conventional complementary output stage; and, for

reference sake, Fig. 25-24C shows a basic crystal-set

radio receiver. It may seem quaint, but for the presence

today of wildly more volts per meter RF field energy

compared to the heyday of wireless it works just the

same. In all the three circumstances, radio frequencies

collected and delivered by the antenna are rectified

(hence, demodulated and rendered audible) by a diode

(the base-emitter junctions in Figs. 25-24A and B). As

contrary as it may seem for demodulation to occur at an

amplifier output, it is perhaps the most common detec-

tion mechanism with the demodulated product finding

its way back to the amplifier input by means of the

conveniently provided bypassed negative feedback leg.

Making our length of wire fatter and thicker has the

effect of lowering the resistance and inductance while

increasing capacitance (greater surface area exposed to

things nearby). So, although the resonant frequency of

the wire stays about the same, the dynamic impedance

(hence, Q) reduces. Although in general this is deemed

Figure 25-23. What is a length of wire?

B. The wire actually has resistive and distributed

reactive components.

A. Long, thin length of wire.

Figure 25-24. A collection of equivalents.

Bipolar

input stage

A. A wire into a bipolar

transistor input.

Complementary

output stage

Feedback

B. A wire from a conventional

complementary output stage.

Antenna

Basic

crystal

set

Demodulated

audio out

C. A basic crystral-set receiver.