3340 VCO Schematic for PCB design

Hi all!

So, I’ve built the core and square out versions of Sam’s 1V/Oct VCO on strip board and now I want to convert it to a PCB. I’ve been looking for a project to start learning EasyEDA and make custom PCB boards, and this seems like the perfect project!

Since this is my first foray into schematics and PCB design, would any of you mind putting your eyes on this schematic and letting me know if you spot any errors? It would be greatly appreciated!

This schematic is for the core version of the module with just a saw and triangle out. I’ve added male header banks, one for power and the other for the panel connections.

Thank you!


You’re missing one connection – the left side of R2 (5.6k) and R4 (10k trim) should to be connected to the negative 3340 supply (the wire that goes to pin 3, i.e. to the right side of R3 (620 ohm)).

Also, R9 (470 ohm) should probably be in series with a 10 nF capacitor (similar to R7), but it looks like all LMNC designs have it directly to GND and so does my own “LMNC-inspired” builds :thinking: so that doesn’t seem to cause any problems.

Some random suggestions:

  • Use a 0.2 or 0.3 inch footprint for the timing capacitor C2, so you can fit a polyester if you have one (putting a narrow component in a slightly wider footprint is easier than the other way around).

  • Consider adding 100 nF decoupling ceramics to the power pins on both ICs (from +12 V and −12 V to ground, close to the ICs). They’re not in the stripboard layout, and you don’t need to populate them if you don’t have any, but since you’re doing a PCB anyway…

  • Did you just pick random 16-pin and 8-pin chips/sockets? That’s fine (if possibly a bit confusing) if you’re just aiming to create a working layout, but if you want to make the schematic nicer, you can find the real things in the library (at least there’s a ton of TL072s in there). Make sure to double-check the footprints if you’re doing this, though.

I posted some more tips here earlier.


GOOD CATCH! Thank you for spotting that missing connection. I just added it.

And thank you for all the other suggestions as well! I will look in to adding those!

And yes, I just picked random 16/8-pin sockets…I just needed something to put there to get to the PCB stage. I hate that they’re different sizes. LOL. But, the PCB looks fine and I already have the sockets in stock from the previous strip board builds, so I’m just going to put those in.



I have the core one almost done in KiCad, few bits to check and revisit. I am waiting for my first two orders to turn up to verify quality and my design before I order the VCO… My design allows for one 5x200 Unit have two VCO’s.


Just curious, and it’s probably obvious once you tell me, but what’s a 5x200 unit? Kosmo is 200 mm, but 5×200 mm sounds like a rather extreme form factor :slight_smile:


I was always taught “Allong the corridor and up the stairs” for Co-Ordinates… :wink:

Yeah but 5 mm by 200 mm?

Kinda narrow even for Eurorack. :smile:


the real problem I think it is especially that you answer one of your posts, without answering the questions of others (here Fredrik), it happened on several subjects, notably with your problems of IN and OUT on a module, where after having spent some time to help you, make explanatory pictures, tried to go step by step with you … not a thank you and often no answers …
In my opinion by ignoring others you will discourage people from helping you again (which is a bit my case)


@analog… yes 50mm not 5mm…

If you understood what they meant, please enlighten us, 'cause I for one have no idea what you are talking about.

width x height . width = moving along a corridor right to left or left to right. height = up and down like stairs or top to bottom / bottom to ,top . at least thats my guess of what he was talking about …

Never understood this, its the same object and thats relative. Length can just as easily be width or height of another name. Could just be an idiot though.

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in some fields it dose make a difference, like carpentry . it could mean the difference in how you cut pieces out of a sheet of wood to keep the wood grain going in the right direction . so it is always width x height . with the depth or thickness last


I’m going to assume that 5×200 was in euro rack HP×U units and they’re using a rather narrow but very tall case, because why not.

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sure, but that only works because i assume there is a convention that says where the grain is oriented. to me, it just makes more sense to be direct and be 8ft “with the grain” so to speak.

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yeah I thought about it later and that wasn’t a good example . say you had to install a 40"x32" cabinet box [ no base , no doors ] and the plans call for a 40"x32" box , you would know that it was 40" wide by 32" tall so you didn’t turn it the wrong way. and once again we are getting a bit off subject . it was just a typo that started us down this path lol :crazy_face:

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When doing my layout i used the 3340 and 072 components… TBH I think that was a bit of a mistake in this instance as it made it more difficult to replicate the original layout from Sams post.

If i were to try and draw it out again then I would probably just use generic DIP sockets numbered sequentially…


Couple of potentiometer suggestions:

  • For the VR2 scale trimmer, the target resistance for the combination of 24k and pot is ~30k, so you may want to use a 10k there to put the target closer to the center of the pot. Also, the symbol is a bit strange, but I guess that’s a Kicad thing.

  • For the VR1 tuning pot, connecting it in series with the summing resistor results in a non-linear and somewhat limited tuning range. A more “correct” approach would be to connect it as a voltage divider, like it’s done in the performance VCO. See here for a discussion of some alternatives used in LMNC designs.

Another tip is to make sure the timing capacitor C2 footprint is large enough that you can fit a quality film capacitor there, and not just tiny ceramic (see here).



the 10K/100K must have been a late night copy error. Would not have spotted that…

Have had a look at the TunerVCO schematic seems I just need to therefor replace the CV input section with the POT as the voltage devider and add in the OP-AMP… Will do that this afternoon and tidy it up a bit.

There are probably other KiCad Symbols that could be used, the tracks could probably do with being moved to make it more clear…



While it could be drawn as a pot with the wiper connected to an end, what’s seen here is a fairly standard variable resistor symbol in my experience. http://www.resistorguide.com/variable-resistor/