TECHNICAL NOTES

High frequency circuitThe capsule 01a RF condenser microphone presents, contrary to

low Irequency circiuts, a low impedance output. Instead 01 the high polarisation voltage normally required, a high Irequency capsule needs only a high Irequency voltage 01 about 10 volts, which is produced by a built-in low-noise-oscillator (8 MHz). The low capsule impedance leads to a high perlormance reliability 01 the micro- phones.

Powering and connectionSennheiser electronic introduced A-B powering, which was then

when it is normally intended lor use with dynamic microphones, it can be necessary to reduce the output voltage lrom the microphone by means 01 a voltage divider. This 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 ampli- lier, which helps to increase the signal to noise ratio.

Connection to amplifiers with defined input impedances

Sennheiser studio condenser microphones can be connected direct to all ampliliers whose input impedance is larger than 60011. This

~.is usual in the majority 01 cases. Should, however, the input imped-
standardised in OIN 45595. As with dynamic microphones, only two

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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 01 immunity lrom noise or disturbance are achieved.

The connedion 01 Sennheiser condenser microphones, as also dynamic microphones, is carried out using the principal 01 voltage matching. The advantages 01 this system are that, neither impedance variations 01 the microphone output, nor 01 the amplilier input exer- ci se a noticeable inlluence on the total Irequency response. The source impedance 01 the Sennheiser condenser microphones with A-B powering is small (approx. 8 11at 1000 Hz), wh ich means that it need only be demanded 01 the amplilier that its input impedance be larger than 600 11.

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 interlerence 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, wh ich ensure that no high Irequency signals lrom 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 OIN standard ie. when apressure signal strikes the capsule Irom the lront, Pin 1 goes positive with relerence 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 via the battery adapter MZA 15, or the power unit MZN 16 T to the inputs 01 the amplilier.

Connection to amplifiers with unbalanced inputs

In many cases, lor example most tape recorders, the input socket is unbalanced. In this case one side 01 the balanced microphone output has to be earthed. Apart lrom cases where the microphone is being used lor prolessional studio purposes, this is not critical, as the large output voltage 01 the microphone combined with its low output impedance provides a large signal to noise ratio. Care should be taken, however, that when the microphones are mounted on tri- pods, etc. no multiple earth circuits are lormed.

Connection to amplifiers with high input sensitivityII the amplilier being used has a very high input sensitivity, i. e.

\600 11. The voltage division caused by this se ries resistor must 01 course be considered.

The same method can be used when a higher output impedance 01 the microphone is demanded. In this case again, aseries resistor can be used to provide correct matching.

Connection to amplifiers with powering facilities

II an appropriate voltage source is available in the amplilier the con- denser microphone can be powered direct. The voltage should be 12 volts::!:: 2 volt. It should be so stabilised and liltered, that the un- weighted noise voltage is less than 5 /LV and that the weighted noise components are less than 2 /LVolts. The current consumption

01 the microphone is approximately 6 mA. According to the OIN standard the resistors should be 2 x 180 0. This means thatbe placed in series with the microphone so that it "sees" at least

approx. 2 volts are lost across the resistors.

 

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