Log Converter

This circuit to change the GR-300's period control voltages into 1 volt/octave voltages was derived from Bernie Hutchins Electronotes, Application Note #114 Page 2. It was described as part of a guitar synthesizer whose operation was very similar to that of the GR-300. The original circuit used a tempco resistor for temperature stability, but I opted to use a temperature regulator circuit to avoid unusual parts. The temperature regulator is based on the one from the Moog Filter Rick Jansen designed for his Formant modular synth. Rick's temp regulator is in turn based on the one from National Semiconductor Application Note #299. A similar circuit was also used in the Moog Prodigy.Another good reference on log converter circuits is National Semiconductor Application Note #30.

The GR-300's period detectors are calibrated to output 10 volts for an open string and 5 volts for a note at the 12th fret for every string when the guitar is in standard tuning. This means that level shifting is necessary on the output of the log converter circuit to make the output voltage correct for that string. In my circuit there is a voltage divider that outputs a Transposing Voltage for each string. For the resistors above and below the tuning pot (labelled R1 and R2 on the schematic), I used different values for each string to get the control voltages into the right range:

String    R1      R2
Hi E     100K    470K 
B        69.8K   499K 
G        52.3K   523K 
D        34.8K   536K 
A        17.4K   604K
Lo E     Wire    536K

Schematic:

Here's what one log converter looks like on the wire wrap board. The board contains six copies of the circuit, one per string.