1298 Chapter 34
load impedance required by a high-impedance micro-
phone is many times higher than the load impedance
required by a low-impedance microphone. High-imped-
ance microphones, therefore, can only be used with
mixers having special inputs designed for these high
impedances.
34.5.1.7.5 Active Devices
An active device is one that uses batteries or ac power
and has one or more tubes, transistors, or ICs. Imped-
ance watching for an active device means not overload-
ing its output, that is, not connecting too low a load
impedance to the output of the active device. A too-low
impedance is an overload because the lower the imped-
ance, the closer it is to a short circuit.
It’s usually very easy to follow this rule because
almost every active device comes with a set of specifi-
cations that indicates the value in ohms of the lowest
allowable load impedance. This is usually called the
rated or minimum load impedance. Incidentally, in
almost every case, it’s acceptable to connect a higher
than rated load impedance to any active device.
For many modern solid state power amplifiers, for
example, the minimum load impedance is 4:. That
means any impedance down to 4: may be connected to
this power amplifier. Since an 8: loudspeaker is
greater than 4: , it is an acceptable load; a 16: loud-
speaker is also acceptable. Two 8: loudspeakers in
parallel equals a 4: load so this arrangement is also
acceptable. Four 4: loudspeakers in parallel equal a
1 : load; this is definitely not acceptable. Connecting a
too-low load impedance to a power amplifier will cause
the protection circuits of the power amplifier to operate,
which increases distortion, and may, in extreme cases,
cause damage to the power amplifier or loudspeakers.
For a line-level active device, like a limiter, the same
rule applies. If the limiter has a rated minimum load
impedance of 600: , the output of the limiter may be
connected to the input of any device whose input
impedance is 600: or higher. (The input impedance of
most active devices is considerably higher than 600: .)
Some professional power amplifiers, on the other
hand, have input impedances of 5 k: or lower.
Connecting a hi-fi-type tuner, with a 10 k: minimum
load impedance to the professional power amplifier,
with its 5 k: input impedance would reduce the output
level from the tuner and might also cause an increase in
distortion.
34.5.1.7.6 Active SourcesActive sources like battery or phantom-powered con-
denser microphones should receive the same treatment
as any other active device although most battery or
phantom-powered microphones are designed to act like
conventional low-Z dynamic microphones from the
point of view of their desired load impedance.34.5.1.7.7 Impedance and Cable LengthOne more aspect of impedance watching involves the
effect of cable length on the frequency response of
high-impedance microphones. From the following
information, we can see that a too-long cable on a
high-impedance microphone will cause a loss in
high-frequency response; that is, the sound from the
microphone will be dull, and voices will lack intelligi-
bility. This results from the interaction between the
capacitance in the cable and the high impedance of the
microphone, which form a low-pass filter. The lower
impedance of a low-impedance microphone also inter-
acts with the capacitance of the cable, but the effect is
noticeable only at very high frequencies (out of the
audio range). A good rule of thumb is to avoid cables
longer than 15 ft with a high-impedance microphone
(some high-impedance microphones will tolerate cable
lengths up to about 25 ft). A low-impedance micro-
phone, on the other hand, will perform properly with
cables as long as 200 ft or more.
This same cable length consideration applies to
line-level devices. The hi-fi tuner mentioned previously,
for example, should not be used with a cable longer than
about 15 ft. (The cable should be shorter if possible.)34.5.1.7.8 Signal-Level CompatibilityAchieving level compatibility between devices means
two things: avoiding too-high levels, which cause clip-
ping distortion, and avoiding too-low levels, which
allow electronic noises (usually hiss), as shown in Figs.
34-62A and 34-62B.
There are three basic classifications of level in
analog professional audio devices, Fig.34-63:- Low-level devices (microphones, pickups, and so
on). - Line-level devices (limiters, graphic equalizers,
and so on). - High-level devices (the output from a power ampli-
fier).