736 Chapter 21
then 20 × log 0.91, which is 0.8 dB. Generally, loading
loss is negligible (under 1 dB) if load impedance is ten
or more times the source impedance. Therefore, as
discussed in Chapter 16, it is neither desirable nor
necessary to “match” the impedances of the preampli-
fier and microphone. If impedances are matched, half
the available output voltage from the microphone is
lost, degrading signal-to-noise ratio by 6 dB. Although
impedance matching transfers maximum power, this is
not what we want.
When a microphone is connected to a cable and a
preamplifier, a passive two-pole (12 dB/octave)
low-pass LC filter is formed as shown in Fig. 21-4. The
behavior of LC filters as they approach their cutoff or
resonant frequency is controlled by resistive elements in
the filter. This resistive damping is largely provided by
the input resistance RL of the preamplifier. Fig. 21-5
shows the deviation in frequency response of a Shure
SM57 microphone loaded by the 2.5 nF capacitance of
150 ft of typical microphone cable and three different
values of preamplifier input resistance. The upper
curves, 10 k: and 3 k: are typical of preamps that
don’t use an input transformer. Note the high-frequency
response peaking caused by insufficient damping. The
lower curve, 1.5 k:, is typical of a preamp using an
input transformer.Capacitance of shielded twisted pair cable is usually
specified as that from one conductor to the other
conductor and the shield. Belden 8451, for example, is
listed at 34 pF/ft. However, the differential signal is
affected by the capacitance between the conductors,
which is about half that, or 17 pF/ft. With dynamic
microphones, high cable capacitance causes
high-frequency roll-off. For the SM57 microphone,
about a thousand feet of this cable (about 17 nF) will
limit high-frequency bandwidth to about 15 kHz.
Because condenser microphones use internal amplifiers
to drive the output cable, high cable capacitance can
cause distortion. If the amplifier has limited output
current, it will distort or clip high-level, high-frequency
(i.e., high slew rate) signals such as vocal sibilance or a
cymbal crash. “Star-quad” cable, although it offers
amazing freedom from magnetic pickup problems, has
about twice the capacitance per foot of standard cable.
This fact must be seriously considered for long cables.
Keep in mind, however, that other types (or even
models) of microphones may behave quite differently,
depending on their exact equivalent circuit. For
example, some condenser types have low (around 30:)
and almost purely resistive output impedances while
some dynamic types can have actual midband imped-
ances over 600:.Figure 21-2. Measured output impedance of the Shure
SM57.
Figure 21-3. Microphone and preamplifier voltage divider.
Figure 21-4. Low-pass filter formed by microphone, cable,
and preamplifier.
1000
900
800
700
600
500
400
300
200
100
0Shure SM57 Impedance– 7 vs Frequency–Hz20 200 2k 20k
Frequency–HzImpedance–7ZLInput voltage at preamp EP = EM x ZL/ZS + ZLMicrophoneEM ZSPreampSM57 Microphone
CablePreamp2.5 nF RL3007EM6 mHFigure 21-5. Effect of load resistance on damping.2k 20k 200k
Frequency–HzAmplitude–dB+9+6+30
3
6
910 K 7
3 K 7
1.5 K 7