Handbook for Sound Engineers

Microphones 511

networks are used across the input transformer

primary. Connect a B+ supply of 9–12 V directly to

the artificial center of two 332:, 1% tolerance

precision resistors, as shown in Fig. 16-47. Any

transformer center tap should not be grounded. For

voltages between 12 and 52 V, double the chart

resistor value of Fig. 16-46.

Any number of capacitor microphones may be
powered by either method from a single B+ source
according to the current available. Use the largest
resistor value shown (Rv max) for various voltages in
Fig. 16-46 for minimum current consumption.

16.3.4.3 Capacitor Radio-Frequency, Frequency-
Modulated Microphones

A frequency-modulated microphone is a capacitor
microphone that is connected to a radio-frequency (RF)
oscillator. Pressure waves striking the diaphragm cause
variations in the capacity of the microphone head that
frequency modulates the oscillator. The output of the
modulated oscillator is passed to a discriminator and
amplified in the usual manner.

Capacitor microphones using an RF oscillator are

not entirely new to the recording profession, but since

the advent of solid-state devices, considerable improve-

ment has been achieved in design and characteristics.

An interesting microphone of this design is the Schoeps

Model CMT26U manufactured in West Germany by

Schall-Technik, and named after Dr. Carl Schoeps, the

designer.

The basic circuitry is shown in Fig. 16-48. By means

of a single transistor, two oscillatory circuits are excited

and tuned to the exact same frequency of 3.7 MHz. The

output voltage from the circuits is rectified by a

phase-bridge detector circuit, which operates over a

large linear modulation range with very small RF volt-

ages from the oscillator. The amplitude and polarity of

the output voltage from the bridge depend on the phase

angle between the two high-frequency voltages. The

microphone capsule (head) acts as a variable capaci-

tance in one of the oscillator circuits. When a sound

wave impinges on the surface of the diaphragm of the

microphone head, the vibrations of the diaphragm are

detected by the phase curve of the oscillator circuit, and

an audio frequency voltage is developed at the output of

the bridge circuit. The microphone-head diaphragm is

metal to guarantee a large constant capacitance. An

automatic frequency control (afc) with a large range of

operation is provided by means of capacitance diodes to

preclude any influence caused by aging or temperature

changes on the frequency-determining elements, that

might throw the circuitry out of balance.

Internal output resistance is about 200:. The signal,

fed directly from the bridge circuit through two capaci-

tors, delivers an output level of 51 dB to 49 dB

(depending on the polar pattern used) into a 200: load

for a sound pressure level of 10 dynes/cm^2. The SNR and

the distortion are independent of the load because of the

bridge circuit; therefore, the microphone may be oper-

ated into load impedances ranging from 30 to 200:.

Figure 16-45. Direct center-tap connection method of

phantom powering capacitor microphones. Courtesy AKG

Acoustics.

Figure 16-46. Dropping resistor value chart for phantom

powering AKG C-451E microphones. Courtesy AKG

Acoustics.

Microphone Cable

Amplifier input

transformer

Rv B+

2

3

1

2

3

1

1 1

0 10 20 30 40 50

14

12

10

8

6

4

2

0

RV max

RV min

RV (k 7 

Figure 16-47. Artificial center tap connected method of

powering capacitor microphones. Courtesy AKG

Acoustics.

Microphone Cable

Amplifier input

2 transformer

3

2 Rv

2 Rv

B+

1 1

3

2