Hello Sam,
no I haven’t swapped a 1k for R42 yet, the reason being that I was having trouble tuning/calibrating the VCO and I wanted to figure out what was going on before I changed anything.
With no external CV plugged in, I used the REF trimer to adjust the the octave switch’s reference voltage to exactly 4V and checked that the position of the switch gave very close to exactly 0V, 1V, 2V, 3V and 4V.
I then used the octave switch and the TRK trimmer to calibrate the oscillator over the 5 octaves.
I then plugged the output of my BeatStep Pro into the 1V/oct input and found that even though the BSP had a 1V/oct output, the oscillator did not track very well at all.
After some analysis, this is what I found out. (Sorry here comes another long post).
- The voltage provided by the octave switch is turned into a current by the R19 100k resistor.
- The external CV on the 1V/oct input is turned into a current by the R22 100k resistor.
- The voltage from the fine tune knob is turned into a current by the R24 100k resistor.
- The CV from the link interface (if used) is turned into a current by the R25 100k resistor.
- The voltage on the external CV input is attenuated and turned into a current by the R20 470k resistor.
The sum of all these currents is what determines the frequency of the oscillator.
The attenuated external CV does not need to be in tune so let’s ignore it.
The other voltages are all converted into currents by 100k resistors, but unfortunately they are not all exactly 100k.
Like most people probably do, I used 1% precision resistors.
Let’s look at a worst case example involving only R19 and R22, the summing resistors used for the octave switch and 1V/oct external input respectively, as nobody uses the link yet and the fine tuning knob is less critical.
At 1% precision, the octave switch summing resistor could be as high as 101k and the 1V/Oct input’s summing resistor could be as low as 99k.
When one calibrates the oscillator using the five voltages provided by the octave switch, the oscillator is adjusted to 1V/101k= 9.9µA per octave, but the external voltage provides 1V/99k= 10.1µA per octave, a difference of 2%. That doesn’t sound to bad, but unfortunately it adds up for each octave.
For middle C the BSP outputs 5V which pushes 5V/99k=50.5µA into the oscillator which is expecting 5x9.9µA = 49.5µA, the difference of 1µA corresponds to more than one tenth of an octave which is more than a semitone and it just gets worse for higher notes.
I see two solutions to this problem.
a) Use closely matched 100k summing resistors for the octave switch, the 1V/oct input, the link CV and the fine tune knob.
Or, if you don’t care about the Link CV and the fine tune exact range.
b) Calibrate the oscillator using the external 1V/oct input and TRK trimmer, and then use the REF trimmer to adjust the ref voltage (to not quite 4V) in such a way that the octave switch gives exact octave jumps (not exact 1V jumps).
That’s what I did, and combined with the HF trimmer, got good tracking from 10Hz to 10240Hz (eleven ten octaves).