TECHNICAL NOTES

High Frequency Circuit

The capsule 01 a 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, which is produced bya built-in low noise oscillator (8 MHz). The low capsule impedance leads to a high performance reliability 01 the microphones.

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 01 these

earth Iree techniques the highest possible values of 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 01 this system are that neither impedance variations 01 the microphone output nor of the amplilier input exer- eise a noticeable influence on the total frequency response. The source impedance of the Sennheiser condenser microphones with

A-B powering is so low (approx. 8 n at 1000 Hz) that an amplilier input with an impedance of at least 400 n will be suitable.

Senn heiser condenser microphones produce relatively large output voltages, these can be up to 1 volt with maximum 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 frequency filters, which ensure that no high frequency signals from the microphone can al-

lect the external circuitry, and also that the microphone itsell is pro- 1 tected from high frequency disturbance. It is, therelore, not necess-

ary, even under the most difficult conditions, to take special pre-

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cautions such as double screening 01 the cables or the provision of high frequency lilters.

Sennheiser condenser microphones are polarised according to DIN standard i. e. when apressure signal strikes the capsule from the front, Pin 1 goes positive with reference to Pin 3. This should be considered when the amplilier input plug is being wired.

Connection to Amplifiers with balanced Inputs

In this case the microphone is simply connected to the input of an amplifier via a battery adapter or the power supply MZN 16 T.

Connection to Amplifiers with unbalanced Inputs

In many cases, for example most tape recorders, the input socket is unbalanced. In this case one side of the balanced microphone output has to be earthed. 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 formed when the.microphones are mounted on tripods etc.

Connection to Amplifiers with High Input Sensitivity

In the amplifier being used has a very high input sensitivity, i.e. when it is normally intended for use with dynamic microphones, it can be necessary to reduce the output voltage from the microphone by means of a voltage divider. This should be built into the microphone cable at the amplifier input. By this means the large signal on the microphone cable is maintained up to just before the amplifier, which helps to increase the signal to noise ratio.

Connection to Amplifiers with Defined Input Impedances

Sennheiser studio condenser microphones can be connected directly to all amplifiers whose input impedance is larger than 400 n. This is usual inthe majorityofcases. However, if the input impedance is smaller than 400 n, a resistor of appropriate value should be placed in series with the microphone so that it "sees" at least 400 n. The voltage division caused by this series resistor must 01 course be considered.

The same method can be used when a higher output impedance of the microphone is demanded. In this case again aseries resistor can be used to provide correct matching.

Connection to Amplifiers with Powering Facilities

If an appropriate voltage source is available in the amplifier the condenser microphone can be powered directly. The voltage should be 12 volts :t 2 volt. It should be so stabilised and filtered, that the un- weighted noise voltage is less than 5 /LV and that the weighted noise components are less than 2 /LV. The current consumption 01 the microphone is approximately 6 mA. According to the DIN standard

the feed resistors should be 2 x 180 n. This means that approx. 2 volts are lost across the resistors.

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