Focusrite ISA Two Rackmount 2-Channel Microphone Preamp - Manual (Page 13)

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voltage it can generate into a signal capable of being amplified by a mic preamp. The ribbon
microphone output transformer typically has 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 200 Ω at 1 kHz. This transformer impedance, however, varies
greatly with 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
an 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 preamp 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 30 Ω to 120 Ω, the input impedance of 600 Ω (Low) will work
fine. For 120 Ω to 200 Ω ribbon microphones, the input impedance setting of 1.4 kΩ (ISA 110) is
recommended.
Impedance Setting Quick Guide
In general, the following selections will yield these results:
High mic preamp 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 mic preamp 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