The sound of a microphone preamplifier depends on how the microphone interacts with the type of preamp technology it is connected to. This interaction mainly affects the level and frequency response of the microphone.
Level
Professional microphones usually have low output impedances, which means that you can get more level by choosing the higher impedance settings on the mic preamp.
Frequency response
Microphones with specific presence peaks and customised frequency responses can be further improved by selecting lower impedance settings. Higher input impedance values will enhance the high-frequency response of the connected microphone, providing better ambient detail and clarity, even with average-performance microphones. Experiment with different microphone/preamp impedance combinations to achieve the desired colouration for the instrument or voice being recorded. For a creative approach to using impedance selection, refer to the section on how microphone output impedance and mic preamp input impedance interact.
Impedance setting – quick guide
In general, the following selections get the following results:
High mic preamp impedance settings:
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Generate more overall level
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Tend to make low- and mid-frequency responses of the microphone flatter
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Improve the high-frequency response of the microphone.
Low preamp impedance settings:
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Reduce the microphone output level
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Tend to emphasise the low- and mid-frequency presence peaks and resonant points of the microphone.
Dynamic moving coil and condenser microphones
Professional dynamic and condenser microphones usually have a low output impedance of 150Ω to 300Ω when measured at 1kHz. This low output impedance brings several advantages:
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They are less susceptible to noise pickup
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They can drive long cables without high-frequency roll-off due to cable capacitance
Having a low preamp impedance can affect the microphone's output level because it loads down the microphone's voltage and highlights any changes in impedance at different frequencies. Matching the preamp resistance to the microphone's impedance (for example, setting the preamp input impedance to 200Ω for a 200Ω microphone) reduces the microphone's output and signal-to-noise ratio by 6dB, which is not ideal.
Preamps are designed with an input impedance about ten times greater than the average microphone, typically ranging from 1.2kΩ to 2kΩ, to reduce microphone loading and improve signal-to-noise ratio. Higher input impedance settings, above 2kΩ, minimize frequency-related variations in microphone outputs compared to lower impedance settings. As a result, high input impedance settings provide a more balanced performance across low, mid, and high frequencies.
Ribbon microphones
The impedance of a ribbon microphone is worthy of special mention, as this type of microphone is affected enormously by preamp impedance.
A ribbon microphone has low impedance of about 0.2Ω. It needs an output transformer to increase the voltage level for the amplifier. The transformer has a ratio of 1:30 to boost the voltage. This ratio also raises the microphone's output impedance to around 200Ω at 1kHz.
The transformer impedance changes with frequency. It can increase a lot at certain frequencies (resonance point) and decrease at low and high frequencies. Just like dynamic and condenser microphones, the input impedance of the mic preamp affects the signal level and frequency response of the ribbon microphone's output transformer, and the microphone's sound quality. It's suggested that the mic preamp connected to a ribbon microphone should have an input impedance at least five times greater than the microphone's 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.