Microphones, preamps and impedance

Microphones, preamps and impedance
Microphones, preamps and impedance
Microphones, preamps and
impedance
By Rob Jones
Every microphone inevitably has an output
impedance. And correspondingly every
microphone pre- amplifier has an input
impedance. These characteristics simply
describe the “resistance” to signal current flow
out of the microphone circuitry and into the
preamplifier. The microphone impedance is
usually made quite low to allow a reason- able
current to flow and the amplifier impedance is
made large enough for that current to develop a
reasonable voltage across its input.
In a simple and ideal world that might be the end
of the matter but, as most users are aware, in
actual practice the input impedance of a
microphone pre-amplifier can have a great
effect on the sound of the recorded signal.
More accurately, the (often quite complex)
impedance of a mic-pre will interact in a unique
way with the out- put impedance of an individual
design of micro- phone. Whether you view this as
a nuisance or an asset depends on your
requirements and taste, but this interaction
explains why certain combinations of
microphone and mic-pre have become revered
within sections of the industry, rather than the
mics or pre-amps alone. The
Neumann U87 and Focusrite Red 7 or ISA series
are a classic example [Figure 1).
Frequency response and level
The main reason why impedance has such an
effect is the significant impact it has on the
frequency response and output level of the
micro- phone. The complete circuit needs to be
considered and a key factor is the mic-pre input
impedance in relation to that of the microphone.
The absolute value of either is less important
than their ratio to each other. Typical figures vary
considerably but some microphones have a
nominal value as low as 30Ω, many are around
150-200Ω and figures as high as 600Ω can be
met. Microphones with low output impedances
are able to drive long microphone lines with
fewer losses and less variation in frequency
response. Ones with higher output impedance
may develop a greater voltage output but are
likely to show more variability in response.
A pre-amp with a relatively low impedance “sees”
a lower voltage level from the mic and is also
Figure 1
likely to emphasize its frequency-related variations,
resonances and dips. However a relatively high
impedance yields a greater overall level and, often, a
flatter frequency response. It may also alter extreme
HF and transient response to give an increase in
perceived clarity and brightness. Dynamic
microphones, without any form of buffering, are
particularly sensitive to the impedance that they see
and may produce very erratic frequency responses
with pre-amp impedances dramatically different from
that which the designer had in mind. This is because
the circuit impedances have a direct effect on the
damping of the voice coil.
Matching microphone and pre-amp impedances to
the same value (power-matching) reduces both the
level and the S/N ratio by 6dB and is not a
technique that is used for those reasons. For
dynamic and condenser microphones, the
preferred pre- amp input impedance is generally
about ten times that of the microphone output;
normally around1.2kΩ or 2kΩ. This figure is partly
chosen because of inherent noise considerations
and also for additional factors such as the effect of
phantom power resistors. However there are a
number of circuit configurations that can, to some
extent, isolate these factors from the actual input
impedance so the situation is more complicated
than it might appear.
Ribbon microphones may require more careful
preamp selection since their fundamental output
impedance is very low indeed (~ 0.1Ω), as is their
volt- age output. In order to bring this voltage up to
a more practical level a transformer is used which
also raises the output impedance. For those microphones that have a nominal output impedance of
30-120Ω a pre-amp impedance of as low as 600Ω
is likely to be a good choice; for nominal output
1
Microphones, preamps and impedance
impedances of 120-200Ω a 1.4kΩ design (such as
the Focusrite ISA 110) is probably a better starting
point.
Transformers
Transformers are rather complex in their AC
behaviour, having capacitance, inductance and
resistance and this leads to an impedance that will
vary with frequency. The characteristics are
extremely de- pendent on the transformer design
and the circuit that it is part of but transformer
output micro- phones will often have a performance
that changes significantly across the audio
spectrum. In the same way transformer-input preamps, as most early “classic” designs were, tend to
have fairly low nominal input impedances that vary
with frequency and even amplifier gain.
The use of transformer inputs, particularly, will
result in a subtle colouration of the signal and this
will be specific for any given microphone and the
amplifier gain setting. Electronic inputs, on the
other hand, mostly have a higher input impedance
which remains substantially constant with
frequency. This type of input, which is relatively
inexpensive and has been fitted to multi-channel
analogue desks of recent years, can be extremely
linear and accurate but often produces a sound
that is judged to be some- what dull and lifeless. If
“character” is what you are looking for then it may
well not satisfy.
External mic preamplifiers
External mic-preamps designs have burgeoned
to resolve this problem and the use of valve
(tube) amplifiers and transformer inputs has (re)
gained considerable vogue precisely because of
the “warmth” they bring to recordings. This is
not due to any innate perfection but to the fact
that such circuit components allow a small level
of desirable harmonic distortion, primarily 2 -,
3 - and 5 -order, to be applied to the signal. This
is partly due to transfer characteristics but also
a result of the amplifier impedance response.
nd
rd
th
While the output impedance characteristic of a
microphone is fixed in manufacture the input
parameters of a microphone preamp do not
have to be. A feature on some newer pre-amps
(eg Focusrite ISA 428 and 430 MKII — figure 2)
is a variable input impedance, which allows the
level of transformer and microphone interaction
to be adjusted. It is therefore possible to create
a variety of mic/mic-pre ‘colours’. It is also
possible to add additional reactive components
that intentionally alter the transformer’s
impedance at HF. An inductor across the
transformer secondary – an “air” circuit – can
produce an attractive shimmer and top-end
boost to the output.
“Impedance” is only of interest in the context of a
complete circuit, and “matching impedance” as
far as microphones and preamplifiers goes is a
Figure 2
2
Microphones, preamps and impedance
widely misunderstood term. Most microphones can
be used very satisfactorily with most preamplifiers and
true “matching” is actually undesirable. However if you
are seeking a particular quality, tone, colouration,
character – the terms are not fixed – then picking the
right combination of equipment can be very helpful.
Using the interaction of a specific microphone’s
output impedance with a specific preamplifier’s input
impedance to create some subtle changes in the
overall response may well produce exactly what you
are listening out for.
Rob Jones deals with technical aspects of
marketing at Focusrite Audio Engineering
Ltd. He is a graduate of the Tonmeister
course, University of Surrey . Once upon a
time he used to be an actor but then found
music to be far more interesting.
© 2010 Microphone Data Ltd
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