Principleof High Frequency Circuit

The capsule 01a RF condenser microphone presents, contrary to low Irequency circuits, a low impedance output. Instead 01 the high polarization voltage normally required, a high Irequency capsule needs only a high Irequency voltage 01 about 10 volts, wh ich is produced bya built-in low noise oscillator (8 MHz). The low capsule

impedance leads to a high performance reliability 01 the micro- phones.

Powering and Connection

Sennheiser electronic introduced A-B powering, which was then standardised in DIN 45595. As with dynamic microphones, only . two wires are required to connect the microphone when this powering system is being used. The operating current is led along the same wires as the audio Irequency signal, so that the circuitry in the microphone does not have to be connected to earth. Because

01this earth Iree technique the highest possible values 01 immunity Irom noise or disturbance are achieved.

The connection 01 Sennheiser condenser microphones and dynamic microphones as weil is carried out using the principle 01 voltage matching. The advantages 01this system are that neither impedance vBriations 01 the microphone output nor 01the amplilier input exercise a noticeable inlluence on the total Irequency response. The source impedance 01the Sennheiser condenser

microphones with A-B powering is so low (approx. 8 Q at 1000 Hz) that an amplilier input with an impedance 01 at least 400 Q will be suitable. This is usual in the majority 01 cases. However, il the input impedance is smaller than 400 Q, a resistor 01 appropriate value should be placed in series with the microphone so that it

"sees" at least 400 Q. The voltage division caused by this series resistor must 01 course be considered. The same method can be

used when a highter output impedance 01the microphone is demanded. In this case again aseries resistor can be used to provide correct matching.

Sennheiser condenser microphones produce relatively large output voltages, these can be up to 1 volt with maximu[TI sound pressure levels. This has the advantage that even with long cables induced interference signals can be disregarded. Also the internal noise produced by the microphone does not contribute to the total noise level. The microphones are litted with high Irequency lilters, which ensure that no high Irequency signals Irom the microphone can affect the external circuitry, and also that the microphone itsell is protected lrom high Irequency disturbance. It is, therelore, not necessary, even under the most difficult conditions, to take special precautions such as double screening 01 the cables or the provision 01 high Irequency lilters.

Sennheiser condenser microphones are polarised according to DIN' standard i. e. when pressure signal strikes the capsule Irom the Iront, pin 1 (pin 2 lor MKH 416 TU-3) goes positive with relerence

to pin 3. This should be considered when the amplilier input plug is being wired.

Connection to balanced, floating microphones inputs

In this case the microphone is simply connected to the input 01 an

amplilier via a powering adapter MZA 16 T or the power supply unit MZN 161.

Connection to unbalanced microphone inputs

In many cases, lor example most tape recorders, the input socket is unbalanced. In this case one pin 01the balanced microphone output has to be earthed (prelerably pin 3). When using the power supply unit MZN 16 T an unbalanced operation only is possible after a modilication 01the power supply unit. Apart lrom cases where the

microphone is being used for professional studio purposes, this is not critical, as the large output voltage of the microphone combined with its low output impedance provides a large signal to noise ratio. Care should be taken, however, that no multiple ground circuits are lormed when the microphones are mounted on tripods etc.

Connection to microphone inputs with high sensitivity

If the unit being used has a very high input sensitivity, i. e,

when it is normally intended lor use with dynamic microphones, it can be necessary to reduce the output voltage Irom the microphone by means 01 a voltage divider, wh ich should be built into the microphone cable at the amplilier input. By this means the large signal on the microphone cable is maintained up to just belore the amplilier, which results in a increased signal to noise ratio.

Connection to mixer and sound recording equipment with powering facilities

IIan appropriate voltage source is available the condenser microphone can be powered directly. The voltage should

be 12 volts :t 2 volt. It should be so stabilised and liltered, that the unweighted noise voltage is less than 5 flV and that the weighted noise components are less than 2 flV. The current consumption 01

the microphone is approximately 6 mA. According to the DIN standard the resistors should be 2 x 180 Q. This means that the

voltage drop across the resistors is approx. 2 V.

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