Legend Audio ISA 828 - Manual (Page 13)

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 828 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 you to get improved ambient information and high end clarity, even from average-
performance microphones. Various microphone/ISA 828 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 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 150Ω and 300Ω when measured at 1kHz. Microphones are designed to have such low output
impedance 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 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 emphasizes 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 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 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 a 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 input impedance of 600Ω (Low) will work fine. For 120Ω to
200Ω ribbon microphones, the input impedance setting of 1.4kΩ (ISA 110) is recommended.
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