Shrunken down CEM3340 VCO Stripboard Layout

Hey guys,

After i accepted my fate and gave up working on my Hagiwo FM Module, I decided to build a second CEM3340, or in my case AS3340 VCO (I find analogue modules just so much easier to understand). But this time, I tried to edit Sam’s Stripboard Layout so that it fits better in my Eurorack style Synth case.
But I thought, I’d better first post a pic of the layout in here and when I come back from my vacation, I’ve had a wonderful and productive discussion with this loveley community <3
By the way, I also found another awesomely edited version of this Layout, that inspired me:) This is the second one, I wrote my name on my version.

Greetings from Germany,


Please read:


Thanks for reminding! I’ll fix that.


Is this verified? Im rebuilding my kosmo to eurorack, this looks like a nice size

Yes, it is. Works like a charm.

Looks like a nice one, I’ve been wanting to build another 3340 to add a bit more beef to my sound.
Gonna build this layout and if I like it, do the same to the first one I made =D


I‘m a little Bit confused about the 470 k resistor from as3340 pin 5 to pin 7 on the Opamp. Sometimes it is 470 ohms and somtimes it is 470k. Wich one is correct?

In the image above which comes from the LMNC website it’s 470k. I don’t know where anyone has shown 470R. But… I don’t think it matters, and I think it could just be a wire. It’s a voltage output (from the op amp) into a high impedance input (on the AS3340), essentially no current flows and I don’t believe the resistor actually does anything.

Just to confuse the issue more, in the 1222 Performance VCO these two pins are connected by a wire, but there’s a 4.7k resistor between them and ground, for no reason I can think of.

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Hey! Wondering if you have a parts list available for this build, no worries if you don’t I would rather just ask before doing the work though.

A warning - there is a fundamental problem with the AS3340. Changing the pulse width causes shifts in output frequency.

AFAIK this is not a problem with the CEM3340 Rev G, nor the V3340, unless anyone knows better. I haven’t tried the V3340.

The simple workaround is to take the AS3340 triangle output and slice it at different voltage levels with a comparator. That gives you a variable width pulse. For example, Frequency Central’s CEMOsc uses this technique, using a spare op-amp. The AS3340’s own PW input and pulse output are unused.

If you are using a quad op-amp like the TL074 to buffer the 3 outputs (or in Sam’s case, two TL072’s), the spare op amp can be used as a comparator.

I see that Sam is using one half of a TL072 as a ‘CV Adder’ non-inverting voltage follower to control the PW input to the 3340. If you instead produce the pulse externally as described above, you could use that op-amp as the comparator instead, with a little juggling.

Alfa Rpar’s ‘solution’ is to produce a hybrid device , the AS3340-HYB, which contains an AS3340 derivative die and a comparator die, but it’s expensive and uses more power - you may as well use a CEM3340.

There are other minor differences between the CEM3340, AS3340 and V3340. I recommend Electric Druid’s website, he sells all 3 versions and the spec sheets are there. He also has an interesting article about historic designs using the CEM3340.

If building Sam’s VCO, I recommend sticking with the CEM3340.

A couple of comments on Sam’s design:

First, the ‘square’ output of Sam’s design is more accurately described as a pulse output, as the pulse width is of course variable.

Secondly, it is good practice to put capacitors across the supply lines near ICs to reduce noise and interference, say 100nF between ground and each supply rail. Also a couple of fatter electrolytics, say 10uF/25V, between ground and the supply rails would improve performance. These components cost pennies.

My understanding is that the CEM3340 has the same PWM pitch shift problem, I could be mistaken about that though. (The one CEM3340 I ever bought… first time I tried to use it I got it backwards and fried it. Don’t do that.)

There are several other approaches people have used to fix it. Sam has tried using a high value resistor (something like 1M, I think) between the CV summing node and the PWM CV. Alfa has circulated a design for a compensating circuit.

In my own breadboarding I found the problem was eliminated or at least minimized by using a regulated -5 V at pin 3 instead of -12 V and resistor. Alfa recommends doing this anyway for better stability.

Another problem is high frequency ringing on the pulse wave falling edges. I found a 1M resistor between pins 4 and 5 eliminates that.


I have an AS3340 and a @sonosus breakout board is on its way, so I am taking notes!

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