# CEM3340/AS3340 - POT question **URL:** \ **Category:** DIY STUFF\ **Created:** [April 12, 2020, 4:54pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602 "2020-04-12T16:54:00Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [April 12, 2020, 4:54pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/1 "2020-04-12T16:54:00Z")
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 : ![alfa4-vco](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/6/6f5037b176e498656d8c4aa6ddc053dee471bed1.png) but in this schem the 1 is to connect to -12v ![ovgggpa09qx21](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/d/d89f69d0454298300d94316b65541e968a7daa97.png) could someone explain to me why and what differences if it’s not an error … please and thanks ---
**Author:** ![ChristianBloch](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/christianbloch/32/56_2.png) [@ChristianBloch](https://lookmumnocomputer.discourse.group/u/ChristianBloch)\ **Post date:** [April 12, 2020, 5:51pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/2 "2020-04-12T17:51:02Z")
My guess would be that it’s related to the addition of sync and fine tune. ---
**Author:** ![Jos](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/jos/32/151_2.png) [@Jos](https://lookmumnocomputer.discourse.group/u/Jos)\ **Post date:** [April 12, 2020, 7:18pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/3 "2020-04-12T19:18:01Z")
Any chance of a schematic I could have look at? ---
**Author:** ![antoine.pasde2](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/antoine.pasde2/32/326_2.png) [@antoine.pasde2](https://lookmumnocomputer.discourse.group/u/antoine.pasde2)\ **Post date:** [April 12, 2020, 7:30pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/4 "2020-04-12T19:30:23Z")
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. ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [April 12, 2020, 9:09pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/5 "2020-04-12T21:09:39Z")
thanks guys !!! someone here has already built the 2nd one? ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [April 12, 2020, 9:36pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/6 "2020-04-12T21:36:07Z")
> [@Dud](#): > > 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: > **[STUFF - LOOK MUM NO COMPUTER](https://www.lookmumnocomputer.com/projects#/cem-3340-diy-simple)** > > Click on the images for more info on the projects, this is a work in progress and will build on the pages as i go. which is pretty similar to the schematics I’m using, so I guess I’ve built the circuit but not that exact stripboard 🙂 ![IMG_20191028](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/5/5a257db1cde22094c9d28338f1d62d9a80109dbb.jpeg) ---
**Author:** ![Jos](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/jos/32/151_2.png) [@Jos](https://lookmumnocomputer.discourse.group/u/Jos)\ **Post date:** [April 12, 2020, 9:39pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/7 "2020-04-12T21:39:54Z")
Wow, those yellow back panels, what are they? They look like they have hinges?! ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [April 12, 2020, 9:42pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/8 "2020-04-12T21:42:27Z")
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: > **[r/synthdiy - LMNC CEM3340 Reduced w/ Sync and Fine Tune](https://www.reddit.com/r/synthdiy/comments/bnm742/lmnc_cem3340_reduced_w_sync_and_fine_tune/)** > > 67 votes and 56 comments so far on Reddit ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [April 12, 2020, 9:56pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/9 "2020-04-12T21:56:27Z")
They’re 3D-printed “polypanels” that I’m using as PCB carriers; see [this thread](https://lookmumnocomputer.discourse.group/t/gameboy-minimachine-aka-mini-megamachine-build-notes/339) and the LMNC video linked from it for a few more of them and how they fit together. ---
**Author:** ![Jos](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/jos/32/151_2.png) [@Jos](https://lookmumnocomputer.discourse.group/u/Jos)\ **Post date:** [April 12, 2020, 10:01pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/10 "2020-04-12T22:01:27Z")
Ha, I remember the video and the mini machine. Good fun! ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [April 12, 2020, 10:06pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/11 "2020-04-12T22:06:17Z")
CEM3340 and AS3340 -6V max !!! ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [April 12, 2020, 10:18pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/12 "2020-04-12T22:18:35Z")
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). ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [April 12, 2020, 10:26pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/13 "2020-04-12T22:26:59Z")
it’s about the tuning : “-12v on the Tune pot” … not good apparently ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [April 12, 2020, 11:16pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/14 "2020-04-12T23:16:10Z")
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. ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [April 12, 2020, 11:24pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/15 "2020-04-12T23:24:52Z")
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 😃 ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [April 13, 2020, 8:00am UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/16 "2020-04-13T08:00:00Z")
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) : ![3340_sine_output_135](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/7/72379b96b16f041f256c965822fd5a859505b290.png) ---
**Author:** ![Farabide](https://avatars.discourse-cdn.com/v4/letter/f/ecc23a/32.png) [@Farabide](https://lookmumnocomputer.discourse.group/u/Farabide)\ **Post date:** [June 9, 2020, 7:06pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/18 "2020-06-09T19:06:36Z")
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. ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [June 9, 2020, 7:31pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/19 "2020-06-09T19:31:22Z")
Simplest sync is 1 nF capacitors between input and the corresponding pin: > [@AS3340 VCO /w hard/soft sync, FM modulation](https://lookmumnocomputer.discourse.group/t/as3340-vco-w-hard-soft-sync-fm-modulation/249/2): > > The AS3340 example circuit uses a higher pulse out pulldown, and the onboard zener is a bit different so you can use a slightly lower Vee series resistor, but it works just fine with the CEM3340 values. The sync inputs expect brief spikes, and the example schematics just use 1 nF capacitors on the way in, to turn pulses into spikes: EDIT: The 1222 performance VCO follows the example schematics, and routes the sync input via a 1 nF capacitor (C4). EDIT: I just noticed that [the CEM334…](http://www.bustedgear.com/images/datasheets/CEM3340-3345.pdf) 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): > [@Oscillation problem on my VCO](https://lookmumnocomputer.discourse.group/t/oscillation-problem-on-my-vco/894/6): > > So that looks like Sam’s simple layout with a few minor tweaks. I’ve marked the cv source voltages below. Measure them either at the resistors (A) or near the wires (B) relative to 0V (eg the rail marked below): (the 3340 will sum up all voltages, and generate a frequency corresponding to [the total voltage](https://lookmumnocomputer.discourse.group/t/need-some-help-with-the-kosmo-vco-w-tuner-please-i-ve-lost-weeks/691/50)). ---
**Author:** ![Farabide](https://avatars.discourse-cdn.com/v4/letter/f/ecc23a/32.png) [@Farabide](https://lookmumnocomputer.discourse.group/u/Farabide)\ **Post date:** [June 10, 2020, 12:02am UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/20 "2020-06-10T00:02:09Z")
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. ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [June 10, 2020, 9:31am UTC](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602/21 "2020-06-10T09:31:18Z")
for the sync, you either do as you say with a switch, or an individual output for each the FM input is also very simple to add (R1M + 100nf cap) in pin 13 ![AS3340 circuit VCO](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/7/720450507eb3e14ebe4ca8990321d4e4be587028.jpeg) [Next page](https://lookmumnocomputer.discourse.group/t/cem3340-as3340-pot-question/602.md?page=2)