I built a homemade bass pedal controller for my Rogue in 1995. It worked well with one exception: because I drove the pedalboard resistor chain with a voltage rather than a current, if I pressed a button adjacent to the one I was pressing for the note I wanted, a weird “in between” note would be presented to the CV bus. I noticed this right away but didn’t know how to fix it back then.
Flash forward to today.
Using good advice on the Dusty Schematics blog (http://dustyschematics.blogspot.com/2009_02_01_archive.html), I decided to make a current driver for my pedals. I used a variant of the Rogue keyboard circuit, which uses +/-12V and an LM353 as its constant current source, as follows:
- The pedalboard already had +/-5V available, so I scaled the current resistors down to work with the decreased voltage. (If you’re familiar with the Rogue keyboard driver, that means I used 5.5K 5% instead of 14.3K 1% as the main current source and 47K 5% instead of 130K 5% as the current tweaker.)
- I didn’t have an LM353 handy, so I used a TLO82 for the current source.
- The pedalboard is 12 100 ohm 1% resistors, C to C
I wanted the pedalboard to start on low C instead of F (like the Rogue). I wanted to be able to still use the Rogue keyboard if I wanted, so I didn’t want to align the Rogue for this difference. Since ground (low F) was the reference at the non-inverting pin of the op amp, I wondered if I could reference low C by giving it a different reference, i.e., -.417V. That seemed to work!
So, everything works just like I’d hoped, with one exception. Low C on the pedalboard is low C on the Rogue and the pedalboard is low note priority UNLESS I hold down high C and low C at the same time, in which case I get a weird “in between” note. While this is unlikely, since I won’t be playing the pedalboard with two feet, I wonder what’s wrong? Both Cs playing at the same time would mean that the CV bus is effectively shorting out the 12 100 ohm keyboard resistors. Shouldn’t this make the -.417V low C voltage appear at the output, kind of like a unity gain op amp (I know I am discounting the current drive still present at the inverting input)?
Thanks ahead of time for your help!
Mike
