Preamps 101

Here’s some info about preamps. Both for electric guitars/bass guitars, and for microphones.

Preamps (or pre-amplifiers) are necessary if you want to do some signal processing on your live music. Microphones and guitars simply don’t product loud enough signals. We can’t work on them as-is. We need to amplify them just enough to allow analogue-to-digital conversion. Most ADCs work on signals in the range of +/- 3V or +/- 5V, while microphones and guitars produce signals in the range of millivolts.

Why preamps and not amps? Because amps are much heavier. They’re designed for driving speakers, producing much louder signals. We don’t need that. Proper amplifiers have a much higher power consumption, much bigger footprints, and require different electrical components (e.g. chips with heat sinks).

I compiled a list of preamps I like. I didn’t invent any of them, but I did build them all so I got to test them myself. I also managed to insert some minor modifications to these preamps which seem to better suit my needs. I even designed PCBs for some of them. Check out my OshPark shared projects for that.

Guitar/Bass preamps:

Tonemender by RunOffGroove. I built this for my bass and it sounds really good. The three tone knobs work well for bass/mid/treble, and the gain is fine.

Why I like it:

  • Fine gain, clean boost
  • Bass/mid/treble controls
  • Pretty clean, no distortion

My modifications:

  • Replaced the power circuit with +VCC and -VCC. Scrapped the voltage-divider resistors.
  • Replaced the suggested op-amp with OP284/OPA1642 to enable power supply of up to +/-18V.
  • Changed the resistors/capacitors in the tone stack to fit a FET op-amp and suit bass guitars as well.

Note: if you build this with all the pots and stuff, you’d better build a good ground shield, or it’s going to be noisy. Avoid using long wires and PCBs with stripes, or print a PCB with ground planes.

My suggested schematics are here.

Simpler guitar/bass preamps can be found in the following links:

Microphone Preamps:

  • TI’s INA217Works perfectly with dynamic microphones. High-end quality.
  • A simpler one. This one, with minor modifications, can be built with my PCB designed for a guitar/bass preamp (schematics are here). Just change the caps/resistor values.

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