Neumann.Berlin Condenser Microphones Owner's manual

Neumann-Information 183 August 1983
Some informations on RF-interferences in the Studio
Neumann condenser microphones are protected through various
means very effectively against RF interferences so that they
may be used also in those cases where other microphones due
to heavy RF radiation do not perform satisfactorily. Some of
our past years experiences pertaining to this Problem have
been collected here for the purpose of helping to identify
this type of interference and to simplify its removal.
1. RF frequencies always enter through the microphone cable
and never through the capsule housing.
2. All microphone cable couplings must be arranged so that
the cable shield is connected to the connector Shell at
both ends of the cable. It is not sufficient to connect
theshield only through the relevant plug Pins. The
shield must be soldered to the connector Shell. In case
of heavy RF rad'iation it is not sufficient to only clamp
the shield by means of a screw. All connecting cables
supplied by Neumann comply with these requirements. Very
often when our customers prepare their own cables the be-
fore mentioned interference Problems appear, because the
prerequisites are not adhered to during cable assembly.
3. Microphone cables with a woven shield have a remarkably
lower degree of covering an area than such cables using
counterrotating double reusen shielding. For this reason
cables with reusen shielding are remarkably better pro-
tecting against RF interference. Neumann microphone
cables are using this counterrotating double reusen
shielding. Cables which are shielded only through conduc-
tive plastic material are not RF-proof.
4. Only demodulated high frequencies appear as interference.
Semiconducter circuits, especially the gates of the
fieldeffect transistors, may therefore not have any
capacitive coupling to RF frequency Signals.
5. If it is not sure whether the demodulation happens in the
microphone or in other equipment to which the microphone
is connected, it is recommended to test this by means of
a 200 Ohms equivalent resistor built into a plug which is
connected instead of the microphone, or with a dynamic
microphone.
GN1
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6. Only amplitude modulated RF leads to interference. Inter-
ferences caused by FM-transmitters are not known to
us.
7. For interference caused by Standard broadcast and short
wave transmitters the audio program becomes audible.
With TV-transmitters the frame frequency (50 Hz or 60 Hz)
is perceived and is easily confused whith AC power line
hum. If the interference level fluctuates strongly with
movement of the microphone or cable then a TV-transmitter
is the probable Source of the interference.
8. Because TV-transmitter antennas are usually placed at
high elevations and most often radiate with horizontal
polarization, antenna masts located in the immediate
proximity often do not present the danger that those
located further away do.
9. Older microphones of the type U 87 tan, if necessary, be
modified to up to date circuitry and thereby become vir-
tually as RF-prooved as the other microphones. This
applies mainly for the exchange of the transformer 09013
against the type 90415 (with electrostatic shield win-
ding, which is Standard beginning with circuit Status 19)
and the installation of the RF-rejection filter in the
base plate of the microphone (Choke 1 and 2 with C 16 and
C 17, which is Standard beginning with circuit Status
17).