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2. Pre Amp Impedance...
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 1.2 kΩ to 2 kΩ. (The original ISA 110 preamp design followed this convention and has an
input impedance of 1.4 kΩ at 1 kHz.) Input impedance settings greater than 2 kΩ 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 very low, around 0.2 Ω, and requires an
output transformer to convert the low voltage it generates into a signal capable of being amplified by
a preamp. The transformer uses a ratio of around 1:30 (primary:secondary) to increase the ribbon
voltage to a useful level. This transformer ratio has the effect of increasing the output impedance of
the mic to around 200 Ω at 1 kHz.
The 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, in common with dynamic and condenser microphones, the mic
preamp input impedance has a significant effect on the signal level and frequency response of the
ribbon microphone output transformer, and the associated '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
well. For 120 Ω to 200 Ω ribbon microphones, the input impedance setting of 1.4 kΩ (ISA 110) is
recommended.
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