Handbook for Sound Engineers

Consoles 859

seemingly endless number of variables. A dynamic
microphone may be represented (a little simplistically)
as a voltage source in series with a fairly lossy induc-
tance representing a midband impedance typically of
150–300 : Fig. 25-36. Being a transducer and, of
necessity, mechanical in nature, many complex varying
motional impedance effects contribute to the overall
scene, as do the effects of matching transformers used
in many microphones. For most design purposes,
however, this simplistic electrical analog can suffice.
The low impedance commonly and conventionally used
is primarily to mitigate high-frequency attenuation
effects due to inevitable cable capacitance. Despite the
fact that the characteristic impedance of microphone
cable is not too far removed from that of our typical
sources, the runs are so short in wavelength terms that
transmission-line “think” is not really applicable and the
cable just looks like a distributed capacitance. This, in
practical circumstances, amounts to a large value of
capacitance that the transducer must drive along with its
load. Unfortunately, the impedance is not low enough
that it may be treated as a pure voltage source; therefore
a tiny signal at a finite impedance must be ferreted out
and treated with care for optimum performance.

25.10.1 Power Transfer and Termination Impedance

Textbooks on electrical theory state that to extract

maximum power from a given source the optimum load

is equal in value to the source impedance. In the

dynamic microphone, it is of doubtful (if any) value.

We’ve squeezed all the energy possible from the gener-

ator, but to what end? Given that most electronic ampli-

fiers of the type useful in low-noise applications are of

relatively high-input impedance (i.e., voltage ampli-

fiers), then the terminating resistance that largely

defines the load of the microphone would, in fact, dissi-

pate most of our hard-won power. It is the output

voltage capability of the source that is of greatest use

here, not the power. So, as can be seen in Fig. 25-37,

matching source and load impedances does a very effec-

tive job of sacrificing 6 dB of signal level that has to be

made up in the succeeding amplifier. This does not

imply that the noise performance is 6 dB worse than

possible, since the source impedance as seen by the

(assumedly perfect) amplifier is now a parallel of the

microphone and its matching load; hence, it is about

half the value of either. The thermal noise generation of

this combined source is consequently 3 dB less; so

although the voltage is down 6 dB the noise perfor-

mance is only degraded 3 dB by such a termination.

Still, it is better not to throw away a good 3 dB before

even starting to hassle with the amplifier itself.

Figure 25-35. Switcher audio path.

Other switches in package Mixbus

CTrim

Input

4.7 k 7 3.9 k 7 1 k^7

¾ 300 7 RS

100 MF

100 7

Adjust

8.2 k 7 4.7 k 7

To line

amplifier

TL071

Figure 25-36. Simplistic dynamic microphone model.

Coil resistance

Coil inductance

Approx

200 7 midband

VS

Figure 25-37. Matching—how to lose 6 dB.

Effective

series impedance

VS

=

VS /2 or 6 dB

Equal termination

= impedance