Mic Pre-Amp Input Impedance - Focusrite ISA 430 MKII User Manual

Mic Pre-amp Input Impedance

A major element of the sound of a mic pre-amp is related
to the interaction between the specific microphone being
used, and the type of mic pre-amp interface technology to
which it is connected. 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, so more level can be achieved by
selecting the higher impedance positions of the ISA 430
MKII mic pre-amp.
•
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 improved ambient
information and high end clarity, even from average-
performance microphones.
Various microphone/ISA 430 MKII pre-amp 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 pre-
amp input impedance interact.
Switchable Impedance: In Depth
Explanation
Dynamic moving coil and condenser mics
Almost all professional dynamic and condenser
microphones are designed to have a relatively low nominal
output impedance of between 150Ω and 300Ω when
measured at 1kHz. Microphones are designed to have such
low output impedances because the following advantages
apply:
•
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 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
ISA 110 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
ISA 430 MKII's input impedance of 600Ω (Low) will work
well, and for 120Ω to 200Ω ribbon microphones the input
impedance setting of 1.4kΩ (ISA 110) is recommended.
Impedance Setting Quick Guide
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
16