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A 2-channel guitar preamplifier

Each channel has bass, mid and treble controls and there's an optional digital reverberation unit as well.

By John Clarke

Main Features
  • Level control.
  • Bass, Mid and Treble controls.
  • Master volume.
  • Effects return control.
  • Balanced & unbalanced line outputs.
  • Headphone output.
  • Line and effects return inputs.
  • Effects send output.
  • Optional Digital Reverberation in effects loop (to be described in a later issue).

As well as the mandatory tone controls, this versatile unit has several other desirable features. These include effects send and return, a line input socket and a headphone socket so that you can practice without disturbing others. There’s also an optional digital reverberation board (we’ll show you how to build that in a future issue).

While most people wanting a guitar amplifier would tend to purchase a commercial unit with an inbuilt speaker, they often have mediocre performance, with plenty of hum and buzz, and even pickup of radio stations and mobile radios. They often have quite a lot of distortion too, particularly with the unbaffled loudspeaker and modest power output. Even the headphone outputs are often noisy and distorted.

Click for larger image
Fig.1: block diagram of the Guitar Preamplifier. It has two identical channels (one optional) which are mixed together and then mixed again with an effects return signal (eg, from a reverberation unit). The resulting signal is then amplified and fed to the output sockets.

By building this SILICON CHIP preamplifier and teaming it with, say, our 175W plastic power module (described in April 1996), you can produce a very high quality guitar amplifier. Why would you bother with anything else?

This completely new design comprises two PC boards, with the larger one carrying one channel, the mixing for both channels and the regulated power supply. The second, smaller PC board carries two ICs and four pots, for the second channel.

Actually, while we are presenting this design as a 2-channel setup, there is no reason why you could not add more channels, just by building the required number of the smaller PC boards.

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