GK-2a/GR-300 Interface

One problem with the old Roland guitar synths is that you have to use a Roland synth guitar to play them. Here is a circuit which will allow you to play an old Roland guitar synth from your own guitar with a newer GK-2(a)-style pickup. In order to use this circuit you must first modify one old and one new Roland guitar synth cable as shown on the cable cut page.

What the interface does

Power supply conversion - Old Roland guitar synths output a +/- 15 volt power supply, but the GK-2(a) needs to run on +/- 7 volts. The circuit uses LM317 and LM337 voltage regulators to convert the voltages. The pots must be adjusted for +/-7 volts. ADJUST THESE BEFORE HOOKING UP THE GK-2(A) TO IT.

Amplification - The levels of the signals from the old Roland synth guitars are much hotter than the ones coming from a GK-2a pickup, so the signals must be amplified to get the old synth to respond.

Schematic

In this version I used basic 1458 op-amps for U1, U2, and U3.

At the time I didn't know Roland's pin-numbering scheme for the 13-pin cable. So this is my numbering system, when you are looking into the end of the 13-pin cable:


  1   2   3   4

  5   6   7   8
 
  9   10  11  12

        13

But I think this is the same as Roland's pin numbering system anyway.

Construction

Here's a variation of the same circuit on the wire-wrap board. Two ribbon cables from the DB-25 connectors plug into the DIP headers at the bottom.

This circuit has been incorporated into Wayne Jones' very cool RC-1324 Bus Converter.