510 Chapter 16
tive feedback circuit, or by reducing the polarizing
voltage to one-third its normal value and by using a
resistive voltage divider in the audio line. Fig. 16-44 is
the schematic for the AKG C-460B microphone.
16.3.4.2 Phantom Power for Capacitor Microphones
A common way to supply power for capacitor micro-
phones is with a phantom circuit. Phantom or simplex
powering is supplying power to the microphone from
the input of the following device such as a preamplifier,
mixer, or console.
Most capacitor microphone preamplifiers can
operate on any voltage between 9 Vdc and 52 Vdc
because they incorporate an internal voltage regulator.
The preamplifier supplies the proper polarizing voltage
for the capacitor capsule plus impedance matches the
capsule to the balanced low-impedance output.
Standard low-impedance, balanced microphone
input receptacles are easily modified to simplex both
operating voltage and audio output signal, offering the
following advantages in reduced cost and ease of capac-
itor microphone operation:
- Special external power supplies and separate multi-
conductor cables formerly required with capacitor
microphones can be eliminated. - The B+ supply in associated recorders, audio
consoles, and commercial sound amplifiers can be
used to power the microphone directly.- Dynamic, ribbon, and capacitor microphones can be
used interchangeably on standard, low-impedance,
balanced microphone circuits. - Dynamic, ribbon, and self-powered capacitor micro-
phones may be connected to the modified amplifier
input without defeating the microphone operating
voltage. - Any recording, broadcast, and commercial installa-
tion can be inexpensively upgraded to capacitor
microphone operation using existing, two-conductor
microphone cables and electronics.
- Dynamic, ribbon, and capacitor microphones can be
Phantom circuit use requires only that the micro-
phone operating voltage be applied equally to pins 2 and
3 of the amplifier low-impedance (normally an XLR
input) receptacle. Pin 1 remains ground and circuit
voltage minus. The polarity of standard microphone
cable wiring is not important except for the usual audio
polarity requirement (see Section 16.5.3). Two equally
effective methods of amplifier powering can be used:- Connect an amplifier B+ supply of 9–12 V directly
to the ungrounded center tap of the microphone
input transformer, as shown in Fig. 16-45. A
series-dropping resistor is required for voltages
between 12 and 52 V. Fig. 16-46 is a typical
resistor value chart. A chart can be made for any
microphone if the current is known for a particular
voltage. - A two-resistor, artificial center powering circuit is
required when the microphone input transformer is
not center-tapped, or when input attenuation
Figure 16-44. Schematic of an AKG C-460B microphone. Courtesy AKG Acoustics.