SCHOEPS CMIT 5 U - Handbuch (Seite 20)

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Phantom Powering
20
English
Phan tom Po we ring to Stan dard
DIN EN 61938
“Phantom” powering is the standard way to
power condenser microphones. It works using
a two-conductor, shielded cable. Both conduc-
tors are under the same voltage, and exactly
the same current flows through both of them.
Fig. 1 shows the only valid 48 V phan tom
powering circuit (abbreviation: P48) that can
be built using resistors as opposed to a center-
tapped input transformer. This illustration is
based on the international standard document
EN 61938 from 1997.
The permissible tolerance of the feed resistor
values as such is ±20%. However, the differ-
ence between the resistors of any one pair
should be less than 0.4% (i.e. 27 Ohms). This
close matching is necessary to maintain ade-
quate impedance balance for the sake of com-
mon mode rejection. It also avoids the flow of
DC in an input transformer should one be
present, which could lead to distortion or a
reduced dynamic range.
A microphone designed for 48 V phantom
powering may draw as much as 10 mA accord-
ing to the standard; a SCHOEPS CMIT 5 U will
draw about 4 mA. This falls well within the limit
set by the prevailing standard.
Although there is nothing particularly compli-
cated or demanding about phantom powering,
there are certain commercially available power
supplies, preamplifiers, and mixing desks –
mostly older, but some more recent – which
fail to meet this standard and hence may not
be able to power SCHOEPS microphones ade-
quately. If in doubt, equipment should be
checked to verify its suitability for professional
work with SCHOEPS microphones. A quick and
easy way to check a phantom supply is
described on page 26.
Unbalanced Operation
You may encounter a situation in which there
is no balanced input with phantom power
available, but only an unbalanced input –
probably even unpowered. If the latter is the
case, a standalone P48 supply with balanced
in-/ouputs should be used before this input.
If the input has got phantom powering but
is unbalanced, the temptation may be
great, but we strongly advise against using
it on the CMIT 5 U, since this microphone
is not designed for such operation, and
the advantages of phantom powering
(noise and interference rejection) will be
completely lost. Instead, a high-quality
microphone input transformer should be used
to turn an unbalanced into a balanced input.
This will allow the signal leads from the micro-
phone to be kept balanced, for best rejection
of interference.
If you have no choice but to go ahead
despite the disadvantages of an unbalanced
signal, then you can set up your system for
unbalanced operation by uncoupling the signal
on pin 2 using a capacitor (rating: 100 μF, 63 V
at R= 22 k
Ω). There is no signal on pin 3.
Nevertheless, it must still be ensured that the
microphone is fed over all three pins. With the
CMIT 5 U, this operating method does not alter
the signal-to-noise ratio.
Simultaneous Connection to Multiple Inputs
If a microphone has to be connected to multi-
ple inputs simultaneously, an active micro phone
splitter should be used in order to preserve
the loading and powering conditions for the
microphone, and to prevent interference.
Ma xi mum Cable Length
In the typical application of a shotgun micro-
phone, namely on a boom, cables of just a
few meters are used. The CMIT 5 U can be
connected to cables up to 300 m in length.
The practical limit depends on the electrical
capacitance of the cable, which is sometimes
an unknown quantity. The lower this capaci-
tance is per unit length, the longer the cable
can be. All SCHOEPS cables have very low capaci-
tance (100 pF/m between the conductors).
The main risks with excessively long micro-
phone cables are gradual losses at high fre-
quencies due to the cable capacitance, some
reduction in ability to handle very high sound
pressure levels, and increased pickup of inter-
ference.