User Guide
5
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 50Ω and 00Ω when measured at kHz. 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 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 (e.g. making
a preamp input impedance 00Ω to match a 00Ω 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, preamps have traditionally been designed to have an input
impedance about ten times greater than the average microphone, around .kΩ to kΩ. (The original ISA 0 pre-amp design
followed this convention and has an input impedance of .kΩ at kHz.) Input impedance settings greater than kΩ tend to make the
frequency-related variations of microphone outputs less signicant than at low impedance settings. Therefore high input impedance
settings yield a microphone performance that is atter 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 incredibly low, around 0.Ω, and requires an output transformer
to convert the extremely low voltage it can generate into a signal capable of being amplied by a pre-amp. The ribbon microphone
output transformer requires a ratio of around :0 (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 00Ω at kHz. 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 preamp 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 0Ω to 0Ω, the input impedance of 600Ω(Low) will work ne. For 0Ω to 00Ω ribbon
microphones, the input impedance setting of .kΩ(ISA 0) is recommended.
Impedance Setting Quick Guide
In general, the following selections will yield these results:
High mic pre-amp impedance settings
will generate more overall level
will tend to make the low- and mid-frequency response of the microphone atter
will improve the 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
•
•
•
•
•
•
•