HF Mic

Posted on Feb 6, 2014

This RF-circuit is operating at 7. 68MHz and 48V 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`!) - Either the capsule`s capa

HF Mic
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citance is part of an oscillator`s resonant circuit, so that theoscillator`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. 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. The oscillator runs with a crystal of 7. 68MHz. 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. 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...

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