Kassutronics VCO 3340 (eurorack)

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kassu
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Kassutronics VCO 3340 (eurorack)

Post by kassu » Wed May 02, 2018 12:46 pm

How to order:
PCBs and panels can be ordered directly from my blog: Order page

Full kits including all components are now available from Infinite Machinery.

Documentation:
The complete documentation can be found on my blog: https://kassu2000.blogspot.com/2018/06/vco-3340.html

Some pictures of the final product:

Completed module:
Image

Close-up of plywood panel (newer version with white background):
Image
Note: The panels are shipped with protective tape on both sides. Peel off the tape to reveal the actual printed surface!

============= Original post ===============
Hi,

I recently designed a basic VCO around the AS3340, which is a clone of the CEM3340 VCO chip. Locally (in Finland) there was some interest in the design, so I decided to make a second revision and do a small run of boards. I offer it here also in case people are interested. The goal of the design was to have an easy to build, affordable VCO with good tracking and all the traditional waveforms. The resulting module follows the datasheet circuit fairly closely, plus output scaling/offsetting and a JFET triangle-to-sine converter. Everything is throughhole, but still fits on a single PCB.

Image

Features:
- Pulse, Saw, Triange and Sine outputs
- Big frequency knob + fine-tune knob.
- PWM knob, serves as attenuator for PWM input or as pulse width control when no PWM is connected
- Exponential (V/Oct) and linear FM
- Switchable hard and soft sync (as they are implemented in the chip, different from the traditional hard sync)
- 9mm pots, thonkiconn jacks and all THT components* on a single PCB
- 8hp eurorack, current draw +12V 15mA, -12V 15mA typical

Short demo video: https://www.youtube.com/watch?v=5rmUaaMWEd8

* For the J201 JFET there are both THT and SMD pads, since the THT version has become rather expensive.

I will also buy a batch of AS3340 chips straight for the factory, and can offer them for a reasonable price if you buy the PCB.

Panels: For my own system I use 2mm plywood panels, which laser cut at the local library. You can see how they look in the Youtube video above. If there is reasonable interest (at least 15 panels) I can get a batch of plywood panels made. They would be printed, black text on white background, rather than engraved as in my own system. The text should be a bit clearer, but this is the first time I am trying the printed option so no hard guarantee. They will be made in Helsinki from Finnish birch plywood. In my experience the panels are pretty sturdy, and once the module is assembled the whole thing is rigid. (I know wood panels are not everyone's favorite, but I like them a lot which is why I prefer to offer this over PCB panels). Of course the panel design will be also available as PDF for those who prefer DIY.
Last edited by kassu on Tue May 28, 2019 1:13 am, edited 10 times in total.
Synth DIY blog and circuits: http://kassu2000.blogspot.fi/

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Swyndger
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Post by Swyndger » Wed May 02, 2018 1:05 pm

Yes, I would be interested in one, maybe two sets (panel, pcb and chip) ... you can never have to much VCOs ;)

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masterofstuff124
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Post by masterofstuff124 » Wed May 02, 2018 2:03 pm

this looks awesome. Im in for one complete set.

thanks for sharing!

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Post by kreiff » Wed May 02, 2018 2:33 pm

I'm in for one set!

csaban
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Post by csaban » Wed May 02, 2018 3:12 pm

I'm in for a complete set!

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Post by mtts » Wed May 02, 2018 3:24 pm

I'm in for one set!

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Post by Jarno » Thu May 03, 2018 2:48 am

Looks great, I am working on a 3340 VCO as well, but I am actually surprised how many parts it still needs for a complete VCO, and I am having great trouble fitting it on a parallel mount PCB (think I am on 10 or 12HP/TE now).

Nice job! Pretty compact and everything TH.

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kassu
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Post by kassu » Thu May 03, 2018 3:26 am

Thanks! Indeed, it was not too easy to fit, but that made it a nice design challenge :). If you look closely there are even resistors between the jacks. Also I use the smaller 1/8W resistors with 0.2 inch pitch footprints (usual for 1/4W resistors is 0.3 inch), otherwise it would not work out.

A lot of space goes just to scaling and offsetting the outputs. I think these chips were really designed for making a complete synth, not modulars. If there is the typical fixed routing from VCO -> mixer -> VCF, you can just do the scaling with a different resistor value for each mixer input, and of course you could do the offsetting just with AC coupling. In the modular world we want DC coupled outputs and standard output levels which needs the extra opamps and a fair number of resistors.
Synth DIY blog and circuits: http://kassu2000.blogspot.fi/

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Silesius
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Post by Silesius » Thu May 03, 2018 3:33 am

This is exactly what I need for my 3 vco monosynth! I had started to build the vco's in stripboard because I needed them compact. And all the other 3340 projects come with several pcb's, or are smd, or you can't avoid buying a front panel... I can adapt these perfectly for my project (that's why I like Barton projects as well).
So I'm in for 4 pcb's (one extra just in case) :)

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SmartBits
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Post by SmartBits » Thu May 03, 2018 3:52 am

Nice job, I'm in for 3x PCB + 3x AS3340!
Can 1/4W resistors also be used, standing? How good is the tracking?

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Post by euromorcego » Thu May 03, 2018 4:06 am

i am certainly interested! 2 sets or so (best with panel).

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Post by cikl » Thu May 03, 2018 6:21 am

This looks great! I'm in for one complete set.

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kassu
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Post by kassu » Thu May 03, 2018 6:22 am

SmartBits wrote:Nice job, I'm in for 3x PCB + 3x AS3340!
Can 1/4W resistors also be used, standing? How good is the tracking?
Yes, 1/4W resitors can be used standing.

I have not measured the tracking in absolute numbers yet. When tuning (V/Oct tuning and high frequency track) I found I can get it to track well over the full piano keyboard range, so 7 octaves or so. I think the largest error was somewhere around 5-10 cents (a few tenth of a percent) at most. At frequencies above 5kHz the HF track started to overcompensate. Maybe I will do more precise measurements at some point.
Synth DIY blog and circuits: http://kassu2000.blogspot.fi/

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Post by autodafe » Thu May 03, 2018 6:24 am

I still have a couple of orignal CEM 3340 lying around. Can these be used as well? or is it better suited with the AS3340 clone?

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Post by Cablebasher » Thu May 03, 2018 6:33 am

Hey KaSSU

Great build. I think this is the smallest eurorack CEM VCO so far (thats through hole)

And it sounds great.

A quick question:

Do all the waveforms shift in pitch slightly when the change the pulse width?

I (and others) have noticed this on a few of the other DIY CEM VCOs and was wondering if its just a feature of the chips.

Sorry for going slightly off topic.

cheers

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kassu
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Post by kassu » Thu May 03, 2018 7:21 am

autodafe wrote:I still have a couple of orignal CEM 3340 lying around. Can these be used as well? or is it better suited with the AS3340 clone?
I have only tested the AS clone, but it should work just fine with real CEMs, old or new reissue.
Synth DIY blog and circuits: http://kassu2000.blogspot.fi/

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Post by kassu » Thu May 03, 2018 7:28 am

Cablebasher wrote:Hey KaSSU

Great build. I think this is the smallest eurorack CEM VCO so far (thats through hole)

And it sounds great.

A quick question:

Do all the waveforms shift in pitch slightly when the change the pulse width?

I (and others) have noticed this on a few of the other DIY CEM VCOs and was wondering if its just a feature of the chips.

Sorry for going slightly off topic.

cheers
No problem :). I have not noticed this issue, but I will have a listen tonight.
Synth DIY blog and circuits: http://kassu2000.blogspot.fi/

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Post by kassu » Thu May 03, 2018 9:25 am

I tested it out and indeed there is a very slight effect of the PWM on the frequency. I can only hear it when I beat two oscillators together, the difference is maybe 1Hz. How strong of an effect are you seeing? I guess it is a feature of the chip.
Synth DIY blog and circuits: http://kassu2000.blogspot.fi/

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sixty_n
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Post by sixty_n » Thu May 03, 2018 12:34 pm

looks nice, in for one perhaps two (pcb, panel, chip)

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Post by donpachi » Thu May 03, 2018 3:05 pm

Three full sets for me, please.
praise the carp

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Post by autodafe » Thu May 03, 2018 3:30 pm

count me in for 3xPCB + 3xAS3340
(sorry I don't like wooden panels that much...)

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ntrm
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Post by ntrm » Fri May 04, 2018 2:24 am

1 PCB and chip, please.

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Post by Cablebasher » Fri May 04, 2018 12:30 pm

Hey Kassu

Thanks for looking into the pulsewidth issue.

On My VCO maximus build, the pulse width knob changes the frequency by half a semitone so very noticeable.

It is also mentioned as an issue on the TIMO pcb build as well.

I guess the issue is with a Cem3340 but each implementation has a different level of effect.

Good luck with the module.

Cheers .

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Silesius
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Post by Silesius » Sat May 05, 2018 2:59 am

I'm having second thoughts about this after all the comments regarding the pw issue (in all 3340 builds). My plan was to build a 3 vco keyboard monosynth and one of my primary concerns is pitch stability. That was the reason I chose a 3340 design, because I thought they were known for stability and good tracking.

So, how bad is this? For example, what happens if I want to have two (or three) tuned vco's with slow lfo's modulating their pulse widths? Would they stay in tune? What happens if I have a carefully tuned two vco bassline and I accidentally touch the pw knob? Would they go out of tune?

Things like half semitone are quite unacceptable if you want to play in tune. It's difficult enough to have three analog vco's playing nicely together. Having to worry about this pw thing could be too much for me...

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Post by Jarno » Sat May 05, 2018 3:38 am

If you are dead set on having PWM, you could do a little add-on board which creates a PWM pulse from the triangle output, that way you separate the PWM from the 3340.
The VCO from the Thomas Henry book "a synth for the 21st century" has a circuit like that, two opamp stages and a handful of resistors.

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