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Capacitor microphones


The capacitor mic, formerly known as the ‘condenser mic’, works in
a completely different way to the dynamic. Here, the diaphragm is paralleled
by a ‘backplate’. Together they form the plates of a capacitor. A
capacitor, of any type, works by storing electrical charge. Electrical charge
can be thought of as quantity of electrons (or the quantity of electrons that
normally would be present, but aren't). The greater the disparity in number
of electrons present – i.e. the amount of charge – the higher will
be the voltage across the terminals of the capacitor. There is the equation:

Q = C x V


charge = capacitance x voltage

Note that charge is abbreviated as ‘Q’, because ‘C’ is
already taken by capacitance.

Putting this another way round:

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V = Q/C


voltage = charge / capacitance

Now the tricky part: capacitance varies according to the distance between the
plates of the capacitor. The charge, as long as it is either continuously topped
up or not allowed to leak away, stays constant. Therefore as the distance between
the plates is changed by the action of acoustic vibration, the capacitance will
change and so must the voltage between the plates. Tap off this voltage and
you have a signal that represents the sound hitting the diaphragm of the mic.
(Mic pic: Sennheiser
MKH 40

The great advantage of the capacitor mic is that the diaphragm is unburdened
by a coil of any sort. It is light and very responsive to the most delicate
sound. The capacitor mic is therefore much more accurate and faithful to the
original sound than the dynamic. Of course there is a downside too. This is
that the impedance of the capsule (the part of any mic that collects the sound)
is very high. Not just high – very high. It also requires continually topping
up with charge to replace that which naturally leaks away to the atmosphere.
A capacitor mic therefore needs power for these two reasons: firstly to power
an integral amplifier, and secondly to charge the diaphragm and backplate.

Old capacitor mics used to have bulky and inconvenient power supplies. These
mics are still in widespread use so you would expect to come across them from
time to time. Modern capacitor mics use phantom power. Phantom power
places +48 V on both of the signal carrying conductors of the microphone cable
actually within the mixing console or remote preamplifier, and 0 V on the earth
conductor. So, simply by connecting a normal mic cable, phantom power is connected
automatically. That's why it is called ‘phantom’ – because you
don't see it! In practice this is no inconvenience at all. You have to remember
to switch in on at the mixing console but that's pretty much all there is to
it. Dynamic mics of professional quality are not bothered by the presence of
phantom power in any way, One operational point that is important however is
that the fader must be all the way down when a mic is connected to an input
providing phantom power, or when phantom power is switched on. Otherwise a sharp
crack of speaker-blowing proportions is produced.

A capacitor microphone often incorporates a switched -10 dB or -20 dB pad,
which is an attenuator placed between the capsule and the amplifier to prevent
clipping on loud signals.

David Mellor

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David Mellor