Guitar synth\F-CV converters

Mark Smart smart at
Wed Feb 21 00:26:59 CET 1996

> I am a guitarist and not a keyboardist and would prefer remaining a
> guitarist,not that i have anything against keyboards, but i want to build a
> guitar synth, i plan on using the GK-2a to control at least one vco per
> string (none of those silly scanning circuits) but i dont want to use midi, i
> want to build an analog interface from the gk to produce the cv for the
> vco's,i was wondering if anyone has any expierience with this or could help
> me with the frequency to cv conversion.
> Thanks

I would suggest getting a service manual for the Roland GR-300 guitar synth
and using its pitch-to-voltage conversion circuitry. I have a GR-300 and a
service manual. Unfortunately the copy is so bad that it is unreadable.
>From it I was able to figure out the basics of how it works, but actually
reading the schem would require a better copy than I've got.

I had a GR-50 guitar synth for several years, and got a GR-300 about a year
and a half ago. I was totally amazed at how good the tracking was on it. It
completely smokes mt GR-50 in spite of the fact that it is 15 years older
and analog (maybe BECAUSE of the fact that it's older).

It has an elaborate filter setup on the input of each of the six channels.
The filter not only follows the pitch of the note you are playing, but also
CHANGES SHAPE as you play up and down the fretboard. It's specifically
designed to prevent the synth from jumping to the second harmonic of the
string when you're playing near the bottom of the neck. If you've played
guitar synths before, you already know that almost all of them do this
occasionally. The GR-300 NEVER does it. 

The GR-300 also has envelope followers on each string which make the synth
follow the dynamics of the guitar. This makes it feel very natural to play.

Possible disadvantages: The GR-300 actually does period-to-voltage
conversion rather that pitch-to-voltage. If you wanted to drive an existing
oscillator with it some kind of converter would be required. It
would half to perform a 1/x function on the voltage, then chance it from
linear frequency to logarithmic pitch.

This circuitry can also be used with the GK-2 pickup, rather than with the
Roland guitar that with the GR-300. I figured out a way to do this so I
could play it from my good guitar with a GK-2. It requires amplification
(the output of the GK-2 is wimpy compared to the output of the older Roland
guitar) and power supply voltage regulation (the GK-2 runs on a +/- 7V power
supply, the GR-300 puts out +/- 12 or 15V, I forgot). 

In any case, I can't recommend their pitch-to-voltage circuit highly enough.
The GR-300 is the only guitar synth I've ever played that I didn't occasionally
get frustrated and want to destroy!! You can hear it on a lot of Pat Metheny

*     Mark Smart                               *
*     Network Technician                       *
*     University Communications Inc. (UCI)     *
*     smart at                      *

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