hello, I have a question about the tune pot.
in all the VCOs that I did (and they work very well) there is nothing connected in 1 of the Tune pot
like here :
but in this schem the 1 is to connect to -12v
could someone explain to me why and what differences if it’s not an error … please and thanks
My guess would be that it’s related to the addition of sync and fine tune.
Any chance of a schematic I could have look at?
The addition of the -12V connection enables you to transpose down from the value driven by the CV input, so if your controller only has a narrow range e.g. centered around middle C at 5V but you want to play bass with it, you can add a couple of negative volts of offset to the CV.
As to why it is there in the second layout, my guess is that they needed the negative connection for the fine tune and decided to add it to the course tune too for consistancy and/or to add the aforementioned ability to transpose down.
thanks guys !!!
someone here has already built the 2nd one?
What’s the source of the second one? From a very quick look, it looks like a slightly expanded version of the last one on this page:
which is pretty similar to the schematics I’m using, so I guess I’ve built the circuit but not that exact stripboard 
Wow, those yellow back panels, what are they? They look like they have hinges?!
I just found the source of the 2nd schem, apparently it shit a little, but I have a little trouble understanding everything (i’m french) … but after tests the guys removed the -12V in question which does not work
here is the article:
They’re 3D-printed “polypanels” that I’m using as PCB carriers; see this thread and the LMNC video linked from it for a few more of them and how they fit together.
Ha, I remember the video and the mini machine. Good fun!
CEM3340 and AS3340 -6V max !!!
Are we talking about the tuning or the supply? The 3340s have a zener on the negative supply that caps the max voltage (7.2 V for CEM3340 and 7.4 V for the AS3340), so as long as you keep the current within bounds you can feed it with anything. That’s what the 620 ohm resistors are there for; they keep the max current at around 7.5 mA ((12−7.2) / 620 = 7.7).
it’s about the tuning : “-12v on the Tune pot” … not good apparently
Sorry, I’m still confused – is “tune pot” the 10k pot that’s connected to pin 1 on the 3340, or one of the external pots? The former is connected to the negative supply via the 620 ohm resistor (so held at around −7.5 V by the zener), the latter are voltage dividers connected to a summing node at pin 15 via 100k resistor.
As Antoine says, having a negative voltage there lets you tune both up and down relative to the other inputs, but you could connect that to GND instead if you don’t need that.
Not connecting it at all seems a bit odd; then you’d rely on a constant voltage weighted by the tuning pot and the mixing input resistor in series, which gives you a more limited (and iiuc more non-linear) tuning range. Guess it still kind of works, though.
Ok, looked at various LMNC schematics and the stripboard one does indeed leave it unconnected, the #1222 performance one uses proper voltage dividers (centre note is 0-12 V, fine tune is 0-1 V) and the Fartbox has a switch between GND and unconnected!? Hmm, need to go look at that PCB, I think.
EDIT: Turns out the switch is what’s called “sphnc tight/loose” on the Fartbox, or robot (
) vs. human (
) according to the video. The stripboard is a bit more sanitized, and calls it “low end on/off” which is pretty much what it does – as you move from unconnected to GND, and on to −12 V, the further down you can tune things with this control. Your choice 
thanks, but since my other VCO (with Sam’s schema) work well, and the guys in the 2nd schem article deleted this connection after a few tests I think I’ll leave it without connection and only add the 2 Sync in, the fine tune, and also FM.
I also found a schem for the Sine out but I have already prepared my panel so it will be for another VCO
here it is if it interests someone (no tested) :
Id love to see how you added the syncs and FM to Sam’s stripboard. I’m having a massive blind spot on figuring out the schematic. Also, can anyone show me how to add the octave selector to this board or would I have to go for a ref voltage? I’ve kept the sin wave separate, using the board you’ve shown (I think I got it from electronic music) but added a jack to the vco panel and a trim. Any and all info appreciated.
Simplest sync is 1 nF capacitors between input and the corresponding pin:
To add an octave selector, you need a precise voltage source (e.g. an LM4040 via a trimmer), then a precise voltage divider (e.g. rotary switch with precision or hand-matched resistors), then you inject the voltage by via a 100k resistor to pin 15, in the same way as the ones marked (A) on the picture here (CV in and tuning):
(the 3340 will sum up all voltages, and generate a frequency corresponding to the total voltage).
Many thanks @fredrik. Just what I needed to get started on a sync input. I’ll be adding a switch to select between hard and soft (can we do both? - what is the maximum)
As for the octave select I’ll start a fresh 3340 on a bread board and see if I can follow.
