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Principleof High Frequency Circuit
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 01 cases. However, if the input
impedance
is smaller than 600 Q, a resistor 01 appropriate 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 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 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 16 T 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 01 the 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 01 the 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
If the 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 01 a 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
If an 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 f1V and 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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9
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