Owner's Manual

Switchable
Impedance:
In
Depth Explanation
Dynamic moving coil and condenser microphones
Almost all professional dynamic and
condenser
microphones
are
designed to have a relatively low nominal output impedance
of
between
1500
and
3000
when
measured
at 1kHz. Microphones
are
designed to have such low output impedance
because
the
following
advantages
result:
•
They
are
less
susceptible to noise pickup
•
They can drive long
cables
without high frequency roll-off due
to
cable capacitance
The side effect
of
having such
low
output impedance
is
that the mic preamp input impedance has a major effect
on
the output
Level
of
the microphone.
Low
preamp
impedance loads down the microphone output voltage, and
emphasizes
any frequency-related
variation
in
microphone output impedance. Matching the mic
preamp
resistance
to the microphone output impedance !e.g. making
a preamp input impedance
2000
to match a
2000 microphone) still
reduces
the microphone output and signal
to
noise ratio
by
6d8,
which
is
undesirable.
To
minimise microphone loading, and
to
maximise signal to noise ratio,
preamps
have traditionally been designed to have an input
impedance about ten
times
greater
than the
average
microphone, around
1.2k0
to
2k0.
!The originaliSA
110
pre-amp
design
followed this convention and has an input impedance
of
1.4k0
at
1kHz.)
Input
impedance
settings
greater
than
2k0 tend to make the
frequency-related variations
of
microphone
outputs
less
significant than at low impedance
settings
. Therefore high input impedance
settings
yield a microphone
performance
that
is
flatter
in
the
low
and
mid
frequency
areas
and boosted
in
the high frequency
area
when compared to low impedance settings.
Ribbon microphones
The impedance
of
a ribbon microphone
is
worthy
of
special
mention,
as
this type
of
microphone
is
affected enormously
by
preamp
impedance. The ribbon impedance within this type
of
microphone
is
incredibly
low,
around 0.20, and requires an output
transformer
to convert the extremely
low
voltage
it
can
generate
into a signal capable
of
being amplified
by
a pre-amp. The ribbon microphone
output
transformer
requires
a ratio
of
around 1:30
[primary: secondary) to
increase
the
ribbon voltage
to
a useful level, and this
transformer
ratio
also
has
the effect
of
increasing the output impedance
of
the mic to around
2000
at 1kHz. This
transformer
impedance, however,
is
very dependent upon
frequency-
it
can
almost
double at
some
frequencies !known
as
the
resonance
point)
and
tends
to roll off
to
very
small
values at low and high frequencies.
Therefore,
as
with the dynamic and
condenser
microphones, the
mic
preamp
input impedance has a massive effect
on
the signal
levels and frequency
response
of
the ribbon microphone output
transformer,
and thus the 'sound
quality"
of
the microphone.
It
is
recommended
that
a mic
pre-amp
connected to a ribbon microphone should have an input impedance
of
at
least
5
times
the nominal
microphone impedance.
For a ribbon microphone impedance
of
300
to
1200,
the input impedance
of
6000[Low)
will work
fine. For
1200
to
2000
ribbon
microphones, the input impedance setting
of
1.4kO!ISA
110)
is
recommended.
Impedance
Setting Quick Guide
In
general,
the
following
selections
will yield
these
results:
High mic
pre-amp
impedance settings
• will
generate
more overall level
• will tend to make the low- and mid-frequency
response
of
the microphone flatter
• will improve the high-frequency
response
of
the
microphone.
Low
pre-amp
impedance settings
will reduce the microphone output level
•
will tend to
emphasise
the low- and mid-frequency
presence
peaks
and
resonant
points
of
the microphone
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