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Balanced Mic Preamp For PCs & MP3 Players

Want to connect a professional microphone with balanced outputs to the line input of your PC's soundcard or an MP3 player, for high-quality voice recordings? This microphone preamp circuit lets you do just that. It features a balanced input, has a clipping indicator LED and can be powered from a USB port or from an external DC source.

By John Clarke

Main Features

  • Balanced input
  • Stereo output
  • USB or plugpack powered
  • Level control
  • Peak indicator
  • Line level output
  • Although most PCs have a microphone input for recording, these inputs are for basic electret microphone types only. Electret microphones are typically used in headsets and are generally of low quality.

    Similarly, some MP3 players include an internal electret microphone for recording but again the quality is limited. These players often also include a line input, to accept external audio signals.

    Using an electret microphone will generally be satisfactory for recording brief announcements and reminders. However, if you want really good sound quality, a professional microphone will be required. This type of microphone will also be necessary when the microphone needs to be more than just a metre or two away from the computer or MP3 player.

    Why are they better

    Click for larger image
    Fig.1: this diagram shows how the Balanced Microphone Preamplifier is connected to a computer. Power can come either from a USB port or from an external DC plugpack supply (see text).

    So why do the professional microphones give better sound than low-cost electret microphones? There are several reasons.

    First, professional microphones use a high-quality microphone element that has a smooth frequency response plus low noise and low distortion. Typical low-cost electrets do not have a smooth frequency response but a response that peaks around the mid audio frequencies. And while low-cost electret microphones readily detect handling and breathing noises, professional microphones are designed to minimise this problem.

    Professional microphones also have a tailored pickup response that is more sensitive towards the front of the microphone than to the rear. This lack of sensitivity towards the rear helps prevents unwanted noise pickup.

    Another advantage of professional microphones, particularly for voice recordings, is that they give more depth to the sound. That's because the bass response is more pronounced when the microphone is brought close to the mouth. A headset electret, on the other hand, usually has a poor bass response.

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