Line isolator

Here is a cheap solution to get rid of ground loop and phase issues when routing a guitar signal into multiple amplifiers.

drawing

Note that this is not a DI box but rather a tool to help you conveniently “fix” you signal chain on stage.

Schematic

Line isolator schematic (Black and white version available here.)

It uses a cheap signal transformer which costs about 5€ (see TY-250P datasheet). You should be able to replace it with any symetrical signal transformer with a flat 20-20k Hz frequency response.

The input buffer is necessary because guitar pickups are high impedance (maybe around 5k to 15k) while the transformer has a low input impedance (in this case 1k).

I didn’t choose the TL071 opamp for any particular reason. It’s just what I had in stock, and it works great!

Features:

  • Buffer: takes a high impedance input signal and outputs a low impedance one.
  • Ground lift: isolates your input signal from the output, effectively removing ground loop noise.
  • Phase flip: flip the polarity/phase of your signal.
  • Balanced signal: converts an unbalanced signal to a balanced signal.

I/O

  • Active input: Takes a high impedance signal (i.e. guitar). Requires 9V power.
  • Passive input: Takes a low impedance signal (i.e. from another pedal’s buffered output) and bypasses the buffer circuit entirely. Doesn’t need any power.
  • Output: A low impedance, balanced (TRS) or unbalanced (TS) signal at the same level as the input.