## An RF-Circuit for Condenser Microphone Capsules

This RF-circuit is operating at 7.68 MHz and 48 V phantom power supplied. It is tested with a large diaphragm cardioid capsule from Ralf Falk, who himself is designing a LF tube microphone and with a small diaphragm capsule from a Sennheiser MKH415. (Do not mix up: "RF" stands for "Radio-Frequency", not for "Ralf Falk"!)

Lots of experiments, gathered experiences and improvements during the development - with quite satisfactory results, to my point of view.

### Introduction to the RF Principle

There are two basic ways to implement an RF condenser microphone.

-    Either the capsule's capacitance is part of an oscillator's resonant circuit, so that the oscillator's frequency is modulated with the audio frequency and can be demodulated with a usual frequency demodulator or discriminator.

-    Or a fixed oscillator generates a fixed frequency and the capsule's capacitance is part of the frequency demodulator, so that with the audio frequency it detunes the demodulator and thus produces the audio signal.

### Description of the Actual Circuit

My circuit is of the latter type. "My circuit" - no, it is not invented by me. It is very similar to the one of the Sennheiser MKH-series introduced some 10 years ago (MKH406-P48, MKH416-P48 and so on). I believe this concept to be quite ingenious and I am convinced not to have any chance to surpass it, so I made mine a little different only.

#### Oscillator

The oscillator runs with a crystal of 7.68 MHz. Other frequencies would do, too, if they were within the tuning range of the succeeding inductors. The oscillator's low voltage output at pins 2, 1 and 6 has one or two windings resp., and with J4 to J6 it is possible to test the effects of different input voltages to the FM-demodulator.

#### Demodulator

The demodulator circuit (C5 - C7 and L2 and, most important, the capsule) is fed by C5 with a 90° phase shifted reference signal, which is phase shifted another +/-90° by L2 and CCapsule. Depending on the overall phase shift the demodulator's output voltage across C6 and C7 is either positive, zero or negative. J1 and J2 allow to select between different capsule capacitances. Opening J3 allows to introduce voltages or capacitances for further experiments.

For both, the oscillator and the demodulator inductor, I went over to use high quality ferrite cores (Epcos RM5, K1, AL25) after experiencing poorer results with cheaper cores.

#### AF-Amplifier

The AF signal across C8 is amplified by T2, T3 and T4 to become a low-impedance output signal. By means of C9 - C11 and R5, low and high frequencies can be amplified additionally. This is required in case of a capsule with a non-linear frequency response, like the one of the MKH415 I have. With C11 of e. g. a couple of µF and without C9, C10 and R5 a linear frequency response is achieved.

Usual push-pull power output stages have a constant current consumption over a wide range of supply voltage. In contrast to that, this output stage, formed by T3 and T4, has a constant voltage consumption over a wide range of supply current, i. e., it behaves more like a Zener diode. Not really easy to understand is why the output stage, though it is an unbalanced concept, produces a balanced output signal. The key to understand this is that it's supply practically (by the constant current source and the transformers L1 and L2) is floating as if the output amplifier was supplied by a separate battery.

#### Power Supply

The whole circuit is not, as usual, fed by a constant voltage with all sub circuits connected in parallel to that supply. It is rather fed by a constant current of 2 mA with all sub circuits connected in series to that supply. The constant current source is formed by T5 and T6. Practically I experienced a more linear demodulator characteristic by omitting ZD1.

Click to enlarge this picture