# CEM3340 schematic questions
**URL:** https://lookmumnocomputer.discourse.group/t/cem3340-schematic-questions/8096
**Category:** DIY STUFF
**Created:** [February 28, 2026, 7:03pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-schematic-questions/8096 "2026-02-28T19:03:43Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@nigeld](https://lookmumnocomputer.discourse.group/u/nigeld)
#### Post date: [February 28, 2026, 7:03pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-schematic-questions/8096/1 "2026-02-28T19:03:43Z")
I found the attached schematic somewhere, I _think_ it was somewhere on the LMNC site, but as a newbie to circuit design I have a couple of questions about it.
First is the +5V power reference coming into the octave selector (red box). Why is it like it is? What is the advantage to using this design over a simple voltage divider or even just using the Eurorack +5V supply directly?
The other is the sine wave generator (green box). It has a couple of trim pots, are these something that would be tuned frequently like some of the other trim pots or are they more ‘_set and forget_’ type trims. If they are tuned often then I’d like to design something where they are accessible on a front panel, if not then I’d leave them as internal trims.
Thanks for any advice!
---
### Author:  [@Sandelinos](https://lookmumnocomputer.discourse.group/u/Sandelinos)
#### Post date: [February 28, 2026, 7:41pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-schematic-questions/8096/2 "2026-02-28T19:41:51Z")
> [@nigeld](#):
>
> First is the +5V power reference coming into the octave selector (red box). Why is it like it is? What is the advantage to using this design over a simple voltage divider or even just using the Eurorack +5V supply directly?
Precision and stability. In order to not sound out of tune, the 5V reference needs to be within 0.1% of 5V and never drift. The power supply voltage rails are nowhere near precise enough, so a precise voltage reference (LM336) is used instead. C1 is used for filtering the current going into the voltage reference and R5/C20 are filtering the voltage coming out (I have no idea what the purpose of R13 is) U1A then amplifies the 2.5V from that reference up to 5V which can be very precisely adjusted by the trimpot.
---
### Author:  [@nigeld](https://lookmumnocomputer.discourse.group/u/nigeld)
#### Post date: [February 28, 2026, 8:15pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-schematic-questions/8096/3 "2026-02-28T20:15:25Z")
Thx @Sandelinos that makes a lot of sense, so once this is set can it just be left or will this also need constant tuning to ensure it stays within range? I’m trying to figure out just how temp. sensitive some of these things are. I’d assumed R5 + R13 were functioning to halve the voltage getting to the OpAmp -’ve input thru C20. Yet you also have the input connected there too. I’m a bit hazy as to what this configuration is doing. I clearly have a lot more to learn about OpAmps!!
---
### Author:  [@clarionut](https://lookmumnocomputer.discourse.group/u/clarionut)
#### Post date: [March 5, 2026, 4:35pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-schematic-questions/8096/4 "2026-03-05T16:35:35Z")
R5, R13 and C20 are another low-pass filter to help remove any noise from the zener diode. R5 and R13 will not affect the voltage going into the non-inverting input (pin 3) of UA1. This is a very high impedance input, so R13 is not really needed. Both the potentiometer on U1A and the sine symmetry potentiometers are trimmer types as shown on the schematic (the wiper is shown as a little line, while panel potentiometers have an arrowhead as the wiper connection). You would generally expect to adjust these once while setting up the module and probably never have to adjust them again.
---
### Author:  [@nigeld](https://lookmumnocomputer.discourse.group/u/nigeld)
#### Post date: [March 5, 2026, 5:12pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-schematic-questions/8096/5 "2026-03-05T17:12:32Z")
Thanks @clarionut this is helping me make sense of the refrence power structure 🙂
As for the sine generator, I get that these are all multiple turn trim pots and I really appreciate you letting me know these are what I would term ‘_set and forget_’, rather than used for tuning.
I’m trying to decide which I leave on my main circuit board in the PCB design and which I migrate to the control PCB so they can be accessed from the front panel. Given the small size and tolerances involved for making access holes I’m aiming to minimise these!
I appreciate your advice !!