ENGLISH
7
Applications
Mic Pre-amp Input Impedance
A major element of the sound of a mic pre is related to the
interaction between the specific microphone being used and
the type of mic pre-amp interface technology it is connected
to. The main area in which this interaction has an effect is
the level and frequency response of the microphone, as
follows:
•Level
Professional microphones tend to have low output
impedances and so more level can be achieved by
selecting the higher impedance positions of the ISA 428
mic pre-amp.
•Frequency response
Microphones with defined presence peaks and tailored
frequency responses can be further enhanced by
choosing lower impedance settings. Choosing higher
input impedance values will tend to emphasise the high
frequency response of the microphone connected,
allowing you to get improved ambient information and
high end clarity, even from average-performance
microphones.
Various microphone/ISA 428 pre-amp impedance
combinations can be tried to achieve the desired amount of
colouration for the instrument or voice being recorded.
To understand how to use the impedance selection
creatively it may be useful to read the following section on
how the microphone output impedance and the mic pre-
amp input impedance interact.
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 150Ωand 300Ωwhen
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 pre-amp input impedance has a major effect on the
output level of the microphone. Low pre-amp impedance
loads down the microphone output voltage, and emphasises
any frequency-related variation in microphone output
impedance. Matching the mic pre-amp resistance to the
microphone output impedance, (e.g. making a pre-amp input
impedance 200Ωto match a 200Ωmicrophone) still
reduces the microphone output and signal to noise ratio by
6dB, which is undesirable.
To minimise microphone loading, and to maximise signal to
noise ratio, pre-amps have traditionally been designed to
have an input impedance about ten times greater than the
average microphone, around 1.2kΩto 2kΩ. (The original
ISA 110 pre-amp design followed this convention and has an
input impedance of 1.4kΩat 1kHz.)
Input impedance settings greater than 2kΩtend to make
the frequency-related variations of microphone output less
significant than at low impedance settings. Therefore high
input impedance settings yield a microphone performance
that is more flat 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 pre-amp impedance. The ribbon impedance within this
type of microphone is incredibly low, around 0.2Ω, 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 200Ωat 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 pre-amp
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 ribbon microphone should have an input
impedance of at least 5 times the nominal microphone
impedance.
For a ribbon microphone impedance of 30Ωto 120Ωthe
input impedance of 600Ω(Low) will work fine and for 120Ω
to 200Ωribbon microphones the input impedance setting of
1.4kΩ(ISA 110) is recommended.
Impedance Setting Quick Guide
In general the following selections will yield the following
results:
High mic pre-amp impedance settings
•Will generate more overall level
•Will tend to make low- and mid-frequency response of
the microphone flatter
•Will improve 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
