Audio Engineering

Microphone Technology 669

that with the induced voltage in the white conductor in the lower right in which the
induced voltage is oppositely directed.

The same analysis applied to the two similar segments of the black conductor yields
identical results. There is no voltage induced in either conductor in the transposition
region as the arrows in the adjacent circles are oppositely directed. In practice, the
magnetic fi eld alternates but as it changes its direction of growth, the induction in the
loops reverses direction also while the net voltage induced in the transposed conductors
remains at zero. Static magnetic fi elds are of no consequence unless a conductor is
moving through them. Even so, a twisted pair translated through a magnetic fi eld that
is static in time will experience a net-induced voltage only if the magnetic fi eld varies
rapidly with position in space.

Air capacitor microphones require a source of polarization voltage, as well as a DC
power source for operating the source follower that handles the microphone signal.
Electret capacitor microphones are self-polarized but still require power for the source
follower signal circuitry. This power is usually supplied by the microphone mixer via the
cable connecting the microphone to the mixer. The circuitry employed for accomplishing
this must maintain balance of the microphone signal circuitry. DC circuits that perform
this task are called phantom power supplies. One such arrangement is depicted in
Figure 22.16.

The arrangement of Figure 22.16 features an output transformer internal to the
microphone housing, as well as an input transformer internal to the mixer. The DC
voltage is applied equally to the microphone signal conductors at pins 2 and 3. The DC

2

1

3

Mic signal

Power return

Phantom supply

To preamp

Mic power

Figure 22.16 : Phantom power arrangement for capacitor microphones.