Principleof High FrequencyCircuit
The capsule of a RF condenser microphone presents, contrary to
low frequency circuits, a low impedance output. Instead of the high
polarization voltage normally required, a high frequency capsule
needs only a high frequency voltage of about10 volts, which is
produced bya built-in low noise oscillator (8 MHz). The low
capsule impedance leads to a high performance reliability
of the microphones.
Powering and Connection
Sennheiser electronic introduced AB powering, which was then
standardised In OIN 45595. As with dynamic microphones, only
two wires are required to connect the microphone when this power-
ing system is being used. The operating current is fed along the
~ same wires as the audio frequency signal, so thatthe circuitry in the
microphone does not have to be connected to earth. Because of
this earth free technique the highest possible values of immunity
from noise or disturbance are achieved.
The connection of Sennheiser condenser microphones and
dynamic microphones as weil is carried out using the principle of
voltage matching. The advantages of this system are that neither
impedance variations of the microphone output nor of the amplifier
input exercise a noticeable influence on the total frequency
response. The source impedance of the Sennheiser condenser
microphones with AB powering is so low (approx. 8 Q at 1000 Hz)
that an amplifier input with an impedance of at least 600 Q will be
suitable. This is usual in the majority 01cases. However, if the input
impedance is smaller than 600 Q, a resistor 01appropriate value
should be placed in series with the microphone so that it "sees" at
least 600 Q. The voltage division caused by this series resistor
must of course be considered. The same method can be used
when a highter output impedance 01the microphones 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 1volt with maximum sound pressure
levels. This has the advantage that even with long cables induced
Interference signals can be disregarded. Also the internal noise pro-
Juced by the microphone does not contribute to the total noise
3vel. The microphones are lilled with high Irequency filters, whicn
;nsure that no high Irequency signals from the microphone can
3ffectthe external circuitry, and also thatthe microphone itsell is
motected Irom high Irequency disturbance. It is,therefore, not
I.ecessary, even under the most difficult conditions, to take special
['recautions such as double screening of the cables or the provi-
on of high Irequency filters.
. mnhelser condenser microphones are polarised according to OIN
.',3ndard i. e. when pressure signal strikes the capsule from the
, ,nt, pin 1 (pin 2 for MKH 406 TU 3) goe5 positive with relerence to
p',l 3. This should be considered when the amplifier input plug is
being wired.
Connection to balanced, floating microphones inputs
In this case the microphone is simply connected to the input of an
amplilier via a powering adapter MZA 16T or the power supply unit
MZA 16 T.
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 out-
put has to be earthed (preferably pin 3). When using the power
supply unit MZN 16 T an unbalanced operation only is possible
after a modification 01the power supply unit. Apart Irom cases
where the microphone is being used lor prolessional studio purpo-
8
ses, this is not critical, as the large output voltage 01the microphone
combined with its low output impedance provides a large signalto
noise ratio. Care should be taken, however, that no multiple ground
circuits are formed when the microphones are mounted on tripods
ete.
,j
Connection to microphone inputs with high sensitivity
Ifthe unit 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 01a voltage divider, which should be built into the micro-
phone cable atthe amplifier input. By this means the large signal
on the microphone cable is maintained up to just belore the ampli-
fier, which results In an increased signalto noise ratio.
Connection to mixer and sound recording equipment with
powering facilities
Ifan appropriate voltage source is available the condenser micro-
phone can be powered directly. The voltage should be 12 volts::t 2
volt. It should be so stabilised and filtered, thatthe unweighted
noise voltage is less than 5 f1Vand thatthe weighted noise compo-
nents are less than 2 f1V.The current consumption 01the mlcro-
phone is approximately 6 mA. According to the OIN standard the
resistors should be 2 x 180 Q. This means thatthe voltage drop
across the resistors is approx. 2 V.
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Balanced
A-B powering
according to OIN45 595
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Unbalanced AB powenng
9
