Impedance Setting Quick Guide - Focusrite ISA428 MkII Mode D'emploi

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 200Ω to match a 200Ω microphone) 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 1.2kΩ to 2kΩ. (The original ISA110 pre-amp design
followed this convention and has an input impedance of 1.4kΩ at 1kHz.)
Input impedance settings greater than 2kΩ tend 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 200Ω 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 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 30Ω to 120Ω the input impedance of 600Ω (Low) will work fine and for 120Ω to 200Ω ribbon
microphones the input impedance setting of 1.4kΩ (ISA110) 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
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