Nagra PORTABLE ANALOGUE AUDIO TAPE RECORDER Instruction Manual - page 35
MAGNETIC INDUCTION
Certain microphones are sensitive to magnetic fields and they should never be placed close to any
motor, or transformer etc. The cables being double and twisted, means that voltages induced
should cancel one another, whether the input is symmetrical or not. It is essential, of course, that
the microphone should be floating, that is to say, that neither of its output wires should be grounded
(except in the case where the shielding should be connected to the shielding of the cable).
The only case where a magnetic induction can be dangerous is where a microphone cable runs
along-side a power cable. The latter radiates a magnetic field, which is not homogeneous, and any
irregularities in the twisting can suffice to induce interference voltages.
CABLE PREAMPLIFIERS
It would seem somewhat illogical to have to take great precautions to transmit a signal as weak as
that given by a microphone when it would be easy to amplify the signal close to the microphone,
and thereby transmit a higher voltage. This can be done with a cable preamplifier. This accessory is
placed close to a dynamic microphone and gives an output voltage similar to that of a condenser
microphone. The power supply requirements are also similar, so that it is possible to interchange a
condenser microphone with a dynamic microphone fitted with a cable preamplifier. Under these
conditions the Nagra should also be fitted with a plug-in preamplifier designed for a condenser
microphone. As the power supply requirements vary according to the type of condenser
microphone, so there exists a corresponding range of cable preamplifiers, to be fed from each type
of preamplifier.
5.4. VOLTAGE OR CURRENT FEED
By altering the negative feedback, it is possible to adjust the input impedance of any preamplifier to
practically any value. If the input impedance is high, the microphone will not supply any current, and
only the voltage will be used to transmit the signal. This is called a voltage feed. If the impedance
seen by the microphone is very low, the voltage at the microphone terminals will remain negligible,
but the microphone will supply a current, which will transmit the signal. This is called a current feed.
A dynamic microphone whose impedance is constant as a function of frequency can be used
indifferently for current of voltage feed. Current feed offers certain advantages: the performance of
the input transformer has much less influence on the overall result, the noise level is minimum when
the input is open etc. This last point ensures that a recording will not be spoilt if an unused channel
is left with the level control open. Therefore, when there is a possible choice, current feed would
seem to be preferable. Unfortunately, microphones with a cardioid characteristic possess an
internal impedance which varies greatly with frequency and they can be used only for voltage feed.
Thus the standard pre-amplifiers for the Nagra 4.2 use the voltage feed method, but current feed
preamplifiers are available for special cases, where desired.
FILTERING PREAMPLIFIERS
In a large number of cases, it is desirable to attenuate very low frequency signals captured by a
microphone. As the human voice contains practically nothing below 50 Hz, a flat frequency
response down to 20 Hz is not only useless, but can be objectionable because low frequency
noises can perturb the recording chain.
The filtering is usually done at the final mixing, but if it is known that there are very low frequencies,
which should be eliminated, it is better to do it within the preamplifier. Several models of plug-in
preamplifier are available for doing this correction.
In the preamplifier code, the figure following the letter Y indicates the attenuation of a signal at 50
Hz in decibels.