HF Mic

Description: This RF circuit operates at 7.68 MHz and is supplied with 48V phantom power. It has been tested with a large diaphragm cardioid capsule designed by Ralf Falk, who is also known for creating a LF tube microphone, and with a small diaphragm capsule from the Sennheiser MKH415. It is important to note that "RF" refers to "Radio-Frequency" and not to "Ralf Falk." The capacitance of either capsule is part of an oscillator's resonant circuit, allowing the oscillator's frequency to be modulated with audio frequency, which can then be demodulated using a standard frequency demodulator or discriminator. Alternatively, a fixed oscillator can generate a constant frequency, with the capsule's capacitance integrated into the frequency demodulator, causing detuning with the audio frequency to produce the audio signal. The circuit described follows the latter approach. It resembles the design of the Sennheiser MKH-series microphones introduced approximately a decade ago (such as the MKH406-P48 and MKH416-P48). The circuit employs a crystal oscillator operating at 7.68 MHz, although other frequencies within the tuning range of the subsequent inductors are also acceptable. The oscillator's low voltage output at pins 2, 1, and 6 features one or two windings, and connectors J4 to J6 allow testing of different input voltages to the FM demodulator. The demodulator circuit, comprising components C5, C6, C7, L2, and the capsule, is supplied by C5 with a reference signal that is phase-shifted by +/-90° through L2 and the capsule. The output voltage across C6 and C7 varies based on the overall phase shift, resulting in either a positive, zero, or negative output. Connectors J1 and J2 enable selection between different capsule capacitances, while J3 permits the introduction of voltages or capacitances for additional experimentation. High-quality ferrite cores (Epcos RM5, K1, AL25) are utilized for both the oscillator and demodulator inductors, following subpar results with lower-quality cores. The audio frequency (AF) signal across C8 is amplified by transistors T2, T3, and T4 to produce a low-impedance output signal. Capacitors C9, C10, C11, and resistor R5 allow for additional amplification of low and high frequencies, which is necessary for capsules with non-linear frequency responses, such as the MKH415. A linear frequency response can be achieved with C11 set to a few microfarads and without C9, C10, and R5. Unlike typical push-pull power output stages that maintain constant current consumption across a wide supply voltage range, this output stage, formed by T3 and T4, maintains constant voltage consumption over a wide supply current range, functioning similarly to a Zener diode. Despite being an unbalanced concept, this output stage produces a balanced output signal due to its floating supply, provided by the constant current source and transformers L1 and L2, which creates an effect akin to separate battery supply. The entire circuit operates on a constant current of 2 mA, with all sub-circuits connected in series to this supply. The constant current source is constructed using transistors T5 and T6. Omitting ZD1 has been observed to yield a more linear demodulator characteristic. Various standard instruments were employed for testing and optimization of this circuit, including two capsule simulators: one being a small capacity switcher operated by a relay, and the other a continuously tunable capacitor made from 28 varicap diodes. Although this diode-based circuit is not highly linear, it significantly aids in optimizing the dynamic range of the demodulator and determining the overall frequency response.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 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 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.

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. 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 2mA 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. For testing and optimization of this circuit I of course used several usual instruments. Very helpful were these two capsule simulators: One is a small capacity switcher, where the capacity is switched by a relay, and the other one is a continuously tunable capacitor build out of 28 Varicap-diodes.

Though I used a lot of diodes, this circuit is not too linear, but it helps very much to optimize e. g. the dynamic range of the demodulator or to determine the overall frequency res