TECHNICAL NOTES
High frequency circuit
The capsule 01a RF condenser microphone presents, contrary to
low Irequency circiuts, a low impedance output. Instead 01the 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 01the micro-
phones.
Powering and connection
Sennheiser electronic introduced A-B powering, which was then
standardised in OIN 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 01immunity
lrom noise or disturbance are achieved.
The connedion 01Sennheiser condenser microphones, as also
dynamic microphones, is carried out using the principal 01voltage
matching. The advantages 01this system are that, neither impedance
variations 01 the microphone output, nor 01the amplilier input exer-
cise a noticeable inlluence on the total Irequency response. The
source impedance 01the Sennheiser condenser microphones with
A-B powering is small (approx. 8 11at 1000 Hz), which means that it
need only be demanded 01the 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, which
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 01the 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 01the
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 01the 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 sensitivity
II the amplilier being used has a very high input sensitivity, i.e.
10
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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 micro-
phone 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 impedanceis largerthan 60011. This
is usual in the majority 01cases. Should, however, the input imped-
ance be smaller than 600 11, a resistor 01appropriate value should
be placed in series with the microphone so that it "sees" at least
600 11.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
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 that
approx. 2 volts are lost across the resistors.
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according to OIN 45595
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A-B powering
unbalanced connection
11
