How to use mic preamps

Written by david james
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How to use mic preamps
Most professional microphones require a preamp. (Microphone image by denionline.de from Fotolia.com)

Aside from your microphone, a mic preamp is arguably the most important piece of equipment you'll buy for your recording studio. Mic preamps come in a variety of shapes and sizes from various manufacturers, but the functionality doesn't change. They are built to help you control the dynamics of the input signal from your microphone, before the signal is routed to your recording interface. Mic preamps typically have four basic functions: Gain, phantom power, compression and equalisation.

Skill level:
Moderate

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Things you need

  • Microphone
  • Microphone cables
  • Mic preamp
  • Recording interface

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Instructions

  1. 1

    Connect a microphone cable to your microphone, and connect the other end to the preamp's input.

  2. 2

    Connect a microphone cable to the preamp's output, and connect the other end to the input on your recording interface.

  3. 3

    Plug the preamp into a power outlet, and power it on.

  1. 1

    The knob labelled 'Input' controls the volume of the microphone's signal as it enters the preamp, before it is processed by the preamp.

  2. 2

    The knob labelled 'Output' or 'Gain' controls the volume of the signal after it is processed by the preamp.

  3. 3

    When the phantom power switch is on, your microphone is recieving 48 volts of power through the microphone cable. Some microphones do not need phantom power, and some microphones can be damaged by it. Check the specifications on your microphone before using it.

  1. 1

    The 'Threshold' knob controls at what input volume the compression will begin to attenuate the signal.

  2. 2

    The knob labelled 'Attack' controls how fast the compressor reacts to rises in the signal's volume. The smaller the value, the faster the attack.

  3. 3

    The 'Release' knob controls how fast the compressor reacts as the signal falls back below the threshold. The larger the number, the longer the signal is attenuated.

  4. 4

    The 'Ratio' control on your preamp regulates the amount of compression applied to the signal. If the ratio is 3:1, it means that when the signal reaches three decibels above the threshold, the signal is reduced to only one decibel above the threshold.

  5. 5

    The 'Knee' controls how the compression is applied to the signal as it reaches and passes the threshold. A 'Soft Knee' gradually will apply compression to the signal before it reaches the threshold, applying the full ratio only after it reaches the threshold. A 'Hard Knee' applies the full ratio of compression all at once and only after the signal has passed the threshold.

  6. 6

    Most compressors have a VU meter that allows you to monitor the amount of attenuation that is applied to the signal. Some VU meters show the level of the signal instead, and some will allow you to toggle between the two.

  1. 1

    The 'Gain' controls for the equaliser allow you to reduce or raise the volume of all the frequencies within the range defined by the corresponding 'Frequency' controls.

  2. 2

    Each 'Frequency' control allows you to select the range of frequencies that will be effected by the corresponding 'Gain' control. If the 'Low' frequency is set to 120, all frequencies under 120 hertz will be effected by the low gain control. Conversely, if the 'High' frequency is set to five, all frequencies above five kilohertz will be affected by the high gain control. If the 'Mid' frequency is set to 400, a range of frequencies defined by the 'Q' control, and centred at 400 hertz, will be effected by the mid-gain control.

  3. 3

    The 'Q' control allows you to adjust how broad of a bandwidth will be effected by the equaliser's gain controls. The smaller the Q's value, the wider the bandwidth. The selected bandwidth is centred at the selected frequency, as previously mentioned.

  1. 1

    Many preamps have a switch that is labelled with the Greek symbol Phi. When switched on, the audio signal's phase is inverted.

  2. 2

    If your preamp has a knob labelled 'Tube', it controls the amount of warmth that is added to the signal as it is compressed by your preamp's built in tube amplifier. At 100 per cent, the signal sounds distorted, much like an electric guitar. The typical setting for warm, clear sounding vocals is around 20 to 30 per cent.

  3. 3

    Mic preamps often have a low-cut switch. The low-cut switch removes the portion of the signal below the indicated frequency, which is usually 80 hertz. Some preamps have a low-cut at 40 hertz, and some allow you to set the frequency.

  4. 4

    The normal signal chain for a preamp is compression first, then equalisation. Many preamps have a switch that allows you to reverse the signal chain, that is, to run the signal through equalisation first then compression. The switch is usually marked EQ>COMP.

  5. 5

    Some preamps' equalisers allow you to toggle the high or low frequency to behave as a bandwidth filter like the mid-frequency. The switch is usually labelled 'Band' or 'Notch'.

Tips and warnings

  • You'll often find that small adjustments to the equaliser and compressor can make big differences in the signal. If you are trying for a natural-sounding recording, a 'less-is-more' approach may be more effective. Before you start making adjustments to compression, you should research and learn as much as you can about dynamics processing.
  • A signal can be recompressed, but compression cannot be removed from a signal. Over-compression can be detrimental to the sound of a mix, but under-compression can always be compensated.

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