Octave Instability?

[no one ever]

My OC-2 is going nuts, switching from one octave to two octaves to the fundamental frequency... could there be something wrong with a compressor chip?

what causes this?

[Mark Hammer]

The usual cause of such instability is an input signal that momentarily dips below the threshold for generating a division by the flip flop in the octave circuitry.  Since not many octave dividers come with onboard compressors to maintain the needed signal level (including the OC-2, unfortunately), the common recommendation is to make sure you have some sort of compressor or other pedal that fixes the signal level above detection threshold.

You may or may not have such a pedal so I"m going to make a suggestion for a simple fix on the board itself.

Near the wire side of the board, you'll see a dual op-amp (TL022) labelled IC1.  Between pin 1 of that chip and the edge of the board will be a 10k resistor (R7) mounted up on its end.  Beside it will be a 2k7 resistor (R7), also mounted on its end.  These two resistors set the gain level of a noninverting op-amp stage that feeds a lowpass filter that feeds the octave division circuit.  The gain is set by the ratio of (R6 + R7)/R7.  Currently, that equals 4.7.  If you replace the 10k resistor with 18k, you'll have a gain of 7.7, and a gain of 9.1 with 22k.  Neither of those are huge gains, but they represent a noticeable increment (in proportional terms, 7.7 is a 64% increase in gain).  This increase in gain will not assure the consistency of signal that a compressor will, but it will save you the misery of having to add a booster in front of the pedal, and should (in theory) provide more consistent detection of octaves and stop the skipping around.  The boost is applied only to the octave division process and is not present when you bypass.

In case such things matter to you, note that this voids any warranties on the pedal.

[no one ever]

I assume more gain is better... i've only got some 22k's, I'll try them out. Thank you, Mark!

oh, and did you mean that the 10k is r6?

[no one ever]

build report: substantial increase in octave quality around 12th fret w/ any pickup, but sub 8th fret tones are still lost. works for me!

[Mark Hammer]

Excellent!

Octave stability on lower frets is a problem with many analog designs, whether they include compression or not.  The primary reason is that the longer the string the more room for pitch error.  As the string gets shorter or tighter, pitch error decreases.

At the other end, octave stability gets buggered up on top notes because: A) the octave-divider has lots of lowpass filtering to keep from being  "tricked" by high-emplitude harmonics of lower notes, B) shorter strings have much shorter decay times, and insufficient amplitude to triggeroctave-detection.

And yes, you wisely saw through my typo.  R6 is normally 10k.