Preamp Input Impedance; Switchable Impedance - In-Depth Explanation - Focusrite ISA Two User Manual

Preamp 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 preamp 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 Two mic preamp.
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 Two preamp
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 preamp input impedance interact.
Note
Impedance Setting – Quick Guide
In general, the following selections will yield the following results:
High mic preamp impedance settings:
• Generate more overall level
• Tend to make low- and mid-frequency responses of the microphone flatter
• Improve the high-frequency response of the microphone.
Low preamp impedance settings:
• Reduce the microphone output level
• Tend to emphasise the low- and mid-frequency presence peaks and resonant points of the
microphone.

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 150 Ω and 300 Ω when measured at 1 kHz. Microphones are designed to have
such low output impedance because of 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 (eg., making a preamp input impedance 200 Ω to
match a 200 Ω microphone) still reduces the microphone output and signal-to-noise ratio by 6 dB, 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 1.2 kΩ to
ISA Two User Guide
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