Focusrite AMS-ISA-2, User Manual

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English
APPLICATIONS
Mic Preamp Input Impedance
A major element of the sound of a mic preamp is related to the interaction between the specific
microphone being used and the type of mic preamp circuit it is connected to. This interaction primarily
affects the level and frequency response of the microphone, as follows:
Level
A professional microphone tends to have a low output impedance and so more level can be achieved
with this type of microphone 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 different 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 appropriate
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.
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 at 1 kHz. Microphones are designed to have
such low output impedances 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 a 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 the microphone
output voltage, and emphasises any variation of mic output impedance with frequency. Matching the
mic preamp input impedance to the microphone output impedance (e.g., 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 2 kΩ. (The original Focusrite 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 mic preamp impedance. The ribbon impedance within this type of microphone
is incredibly low, around 0.2 Ω, and requires an output transformer to convert the extremely low