scrappy kobol-ish ads envelope

My dodgy Kobol-ish ADS envelope in a Eurorack module in my kitchen

Didn’t have any pot plants to hand, so apples from the tree in the garden had to do.

I’ve been meaning to add the RSF Kobol to my pile of started-but-unfinished projects for ages. It’d be much easier to buy the recent cheap clone, but that’s hardly any fun, and we have to do things the hard way.

Now a modern version of the SSM2040 is available, the only real sticking points are the envelopes, which are the now super-rare SSM2050 in the original. The more five volt-y 2056 is probably the closest, and that’s easier to find but they’re still really expensive.

The gods that are Alfa in Latvia have recreated the CEM3310 (as used in the Pro-One, a bunch of Oberheim polysynths, the PPG Wave, and a load of others) and that seems like a fair substitute. The circuit will need twiddling to fit it in, as the output is half of the 2050 at 5V and the inputs expect a range of negative voltages.

I needed a way of testing out the circuit changes for the 3310 before starting on drawing out the main Kobol board, and I couldn’t be bothered with sorting out a breadboard so I made a little test PCB (basically the Digisound 80-10 with the input voltage levels adjusted and the “decay” switch tacked on) with the thought that I could re-use it as a Eurorack envelope.

Here’s a sample ropey oscilloscope shot (as they all will be, sozza) of the output at minimum attack and decay.

Oscilloscope shot of my Kobol-ish ADS envelope at the fastest possible speed - 0.9 milliseconds

The minimum timing in the original is quoted at 1ms, so the 22nF timing capacitor I’ve used here is – and I can’t believe I’m saying this – really just a bit too fast, in that any waggling of the pot has no appreciable effect until about where the number 2 would be on the decay pot.

Also the maximum release time is a bit short at about 10 seconds, so I could probably do with bumping the timing capacitor size up a bit.

Obviously I’m zoomed in quite a lot here, but I was interested in that flat area at the top of the envelope after the attack finishes and before the decay starts, as it’s not like the usual Roland envelopes I’ve been looking at. Here’s a zoomed-in photo, I wanted to measure how long it lasts – 130 microseconds, ish.

AS3310 envelope flat-top

I wondered if this was down to my particular implementation of the AS3310, so while I had my Pro-One apart to try and fix the octave switching I had a poke around in there with my oscilloscope probe.

This is the Pro-One envelope at full-speed:

Sequential Circuits Pro-One envelope generated by original CEM3310, showing the fastest possible envelope shape

There’s that flat-top again, lasting for about 130 microseconds. It seems like Alfa have done a good job replicating the basic shape of the 3310 output.

While I’ve got it apart to fix something, here’s the CEM3310 in-place on the Pro-One main board.

CEM3310 envelope chips in-place inside a Sequential Circuits Pro-One

Just for a comparison, here’s the envelope shape from my 100M 140 envelope clone, set to fastest speed – ignore the timing differences, see how it hits a point and falls sharply from that:

Roland System 100M 140 envelope shape shown on oscilloscope

Alright that’s lovely, but does it make any difference to the sound of the thing being controlled?

No audio demo here this time, but I set my 100M 140 clone to the same timing as my 3310 envelope and had a listen and… they sounded about the same. Which should have been predictable really, it seemed like a tall order to be able to hear 130 microseconds.

I should mention the decay switch, as I wasn’t 100% sure what it was going to do, but it seems obvious now – it’d be better named “release”, as it enables/disables the release portion of the envelope. This sounds a bit rubbish generally, but maybe it’ll be useful in some contexts.

I thought was being Quite Clever using stripboard to bridge between the potentiometers attached to the front panel and the circuit board attached using a metal backing plate, with the standoffs glued to the metal plate and screwed into the stripboard, but it just meant I spent hours trying to work out where I’d bridged the stripboard tracks.

My Kobol-ish envelope generator resting on its side, trying to show the way the circuitboard is attached to the front panel

Next time round I’d use a chopped up pad board instead.

Either way it’s still neater than some of my previous shit builds; when Mr Beep borrowed my Polivoks filter a while back, he was a bit disconcerted to find the circuit board was attached with Blu-tack. For some reason he didn’t seem too keen to use it, I dunno.

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  1. 17th February 2024Rob Keeble says:

    Great work!
    Behringer used a 555 circuit for the envelope generators in their Kobol Expander and it works well. They also replicated the VCO exactly but used a transistor based circuit for the SSM2040. There are more details on my website about the replicas design.

    I have designed a Kobol Expander II, which I may make..

    Best regards

  2. 22nd February 2024ua726 says:

    Cheers Rob.

    Yep, it does seem like the Behringer clone is getting good notices; if I was less of a masochist then I’d just buy it, but (entirely self-imposed) rules are rules… Given how keen everyone is on the B clone Kobol envelope I’d imagined it would be analogue – not that it’s impossible to make a great digitally-generated envelope, it just seems like people tend to fluff it.

    (Rob’s post on the Behringer Kobol for anyone else following along:

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