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 polarisation 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 perlormance reliabilityolthe microphones.

Powering and connection

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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 beeing 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 ground. Because 01 these ground-Iree techniques the highest possible values 01 immunity lrom noise or disturbance are achieved.

The connection 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 Ü at 1000 Hz), which means that it need onl~ be demanded 01 the amplilier that its input impedance be larger than 200 Ü. II the microphone is working with sound pressures

exceeding 20 Pa (120 dB) the input impedance should be at least 600 Ü.

Sennheiser 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 induce,d 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 ensures 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 Iilters.

be taken, however, that when the microphones are mounted on tri- pods, etc. no multiple e,arth circuits are lormed.

,Connection to amplifiers with high input sensitivity

If the amplifier being used has a very high input sensitivity, i. e. when I 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 amplilier, which helps to increase the signal to noise ratio.

Connection to amplifiers with defined input impedances

Sennheiser studio condenser microphones can be connected direct- Iy to all amplifiers whose input impedance is larger that 200 Ü. This is usual in the majority of cases. Should however the input impedance be smaller than 200 Ü, a resistor of appropriate value should be placed in series with the microphone so that it "sees" at least

200 Ü. The voltage division caused by this series 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

If an appropriate voltage source is available in the amplifier the condenser microphone can be powered directly. The voltage should be 12 volts::!: 2 volts. It should be stabilised and liltered, that the un- weighted noise voltage is less than 5 /LV and that the weighted noise components are less than 2 /LV. The current taken by Sennheiser

condenser microphones MKH is approx. 6 mA.

According to the DIN standard the leed resistors should be 2 x

180 Ü. This means that approx. 2 volts are lost across the resistors.

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A-B powering according to DIN 45595

Sennheiser condenser microphones are polarised according to DIN 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.

Connetion 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 grounded. Apart Irom cases where the microphone is be!ing 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

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