Error in CV input processing circuits

[TomWhitwell]

From mbartkow
I'm sorry, if this has been asked before, but I couldn't find a discussion about this issue using the forum search function.

Anyway, I am wondering, why the CV inputs are implemented the way they are, i.e. going through R10+R9 to a follower, and then force clipped to 0-3.3V range through R1 and D1+D2. This same applies to R20+R21, and then R2+D3+D4.

First of all, I can't see any reasonable explanation of putting a 22k resistor in series with the input of an opamp whose impedance is many Mohms. Secondly, force limiting through a 47 Ohm resistor puts a significant stress on the opamp should the CV excess 3.3V, which is quite likely to happen in a modular environment, as the voltages are in -10..10V range.

I am suspecting that perhaps there is a mistake in the schematic, and R9+R10 should rather be employed in a voltage divider, that would reduce the 10V range to 10V*22k/(47k+22k)=3.18V range.

[mbartkow]

[mezzman]

This may be the root of a lot of the CV issues I have been experiencing with my builds. If I mod the PCB and wire the resistors at the CV inputs as mbartkow illustrates, will this have any effect on how the module interprets incoming CV with the current firmware?

[TomWhitwell]

@mezzman What issues are you having?

I agree with @mbartkow analysis and it's probably worth changing in future, but think that the CV input is fairly accurately reflecting incoming voltages - that part has been tested quite extensively, and I've had no user issues reported.
The firmware simply reads 0-3.3v voltages coming into the Teensy - so if you rewire the input, it should work in the same way without any firmware changes.

[mezzman]

Issues I have encountered were Reset triggering while sending >5v into the Start input, and high pitch digital noise coming through other modules when CV is connected to the RM.

[TomWhitwell]

Trying a PCB with this arrangement, but the resistances reversed (going through 22k, with 47k to ground) so that 5v is reduced to 3.4v - anything above that is clipped. I think the majority of use cases for this will be 0-5v - sequencer or LFO outputs. Easy enough to flip the values if you'd prefer 0-10v input.