VCO/LFO PCB for CEM3340 chips

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heliosphere
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VCO/LFO PCB for CEM3340 chips

Post by heliosphere » Sat Aug 01, 2015 3:34 pm

I have a couple of CEM3340 chips I wanted to make into LFO/oscillator modules. I originally wanted to use designs from the old Digisound 80 systems but getting hold of the PCBs doesn't seem possible anymore, are there any DIY builds that use CEM3340's? Eurorack format would be preferred

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bennelong.bicyclist
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Re: VCO/LFO PCB for CEM3340 chips

Post by bennelong.bicyclist » Sat Aug 01, 2015 5:46 pm

heliosphere wrote:I have a couple of CEM3340 chips I wanted to make into LFO/oscillator modules. I originally wanted to use designs from the old Digisound 80 systems but getting hold of the PCBs doesn't seem possible anymore, are there any DIY builds that use CEM3340's? Eurorack format would be preferred
The Digisound-80 VCO and VCLFO modules are fantastic (I've got 5 of them in my Digisound) - excellent stability, linear and exponential FM, and soft sync, plus positive and negative hard sync, as well as both bipolar and unipolar waveforms, and PWM of course.

The Schematics are available, and eurorack-compatible PCBs could be played out using Eagle or similar. The hardest part would be adjusting the circuit for +/-12V operation, rather than +/-15V.

But may there is an existing eurorack VCO design that can use the CEM chips. I think Curtis ceased manufacture of the chips before or only at the dawn of Eurorack, so there may not have been much overlap.

Or just sell the chips. They are worth a lot...

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Post by nigel » Sat Aug 01, 2015 8:25 pm

I have a 10HP build that I designed, with single-sided component and panel PCBs (no wiring!) suitable for home etching. I thought I had posted it to the DIY builds section, but I can't find it, so here's some pics. It's a deliberately minimal design, just brings out the triangle and saw waves, but it has a dual op-amp which adjusts them both to +/- 5V. I was planning to use it as the core of a dual VCO and add external modulation and waveshaping, but it makes a nice module on its own.

ImageImageImage

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heliosphere
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Post by heliosphere » Sun Aug 02, 2015 8:18 am

Thanks for the replies.

@bennelong.bicyclist i've only done kit DIY and the odd normalling / utility stuff, but maybe it's time to have a go.

@nigel how about the sine out? i know it's not a pure sine from the CEM chips, but nice to have that when used as LFO. can you post the files etc?

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Post by CLee » Sun Aug 02, 2015 12:33 pm

The 3340 doesn't have a Sine out, you need to waveshape the Triangle. I use the waveshaper that's on the Thomas Henry VCO-1

http://www.birthofasynth.com/Thomas_Hen ... schem2.pdf

part count is kind of high but it works really well when properly trimmed.

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Post by heliosphere » Sun Aug 02, 2015 3:41 pm

I was thinking along the lines of the digisound circuit for getting a sine. but thanks! looks interesting

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Post by nigel » Sun Aug 02, 2015 9:36 pm

Yeah, I was going to add sine waveshaping on another board, along with one or more comparators to do PWM pulse outputs. I ran out of space to get to the square output on the chip (you can only do so much on a single-sided PCB).
I'll see if I can dig out the design and draw up some simple build docs.
I suspect there are few DIY projects for the 3340 simply because it is so rare. I built mine because I had several Digisound boards in storage from decades ago, and wanted to use them in Euro.

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Post by forbin » Sun Aug 02, 2015 11:43 pm

If you are going to use these chips then may i suggest that you make sure to buffer the outputs to protect the CEM3340. I was a very sad day for me when something went wrong and i think i drove something back into one of the outputs and it all stopped... It was a long time ago but I told that story to someone else who had a similar experience. I'm not sure how robust the drives are on all the pins. Some of the designs that are drifting around have outputs straight off the pins and it would be troubling to wave $50+ goodbye... better safe than sorry!

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Post by bennelong.bicyclist » Mon Aug 03, 2015 12:48 am

forbin wrote:If you are going to use these chips then may i suggest that you make sure to buffer the outputs to protect the CEM3340. I was a very sad day for me when something went wrong and i think i drove something back into one of the outputs and it all stopped... It was a long time ago but I told that story to someone else who had a similar experience. I'm not sure how robust the drives are on all the pins. Some of the designs that are drifting around have outputs straight off the pins and it would be troubling to wave $50+ goodbye... better safe than sorry!
More like $100 per chip these days...

Here's the Digisound-80 schematic, and no, not all the outputs are buffered...

http://www.digisound80.co.uk/digisound/ ... s/80-2.pdf

My plan is, if/when each CEM-based module in my Digisound-80 dies, I'll just build a Ken Stone CGS or Aries or similar +/1 15V replacement module to put behind the Digisound panel. I've already done that for a Digisound dual EG module and a VCF low-pass module - the Curtis chips are just too expensive to make it worthwhile replacing them (no, I'm not a purist when it comes to Digisound-80s...).

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Post by heliosphere » Tue Aug 04, 2015 3:30 am

thanks for all the info, a fellow wiggler is helping me do a schematic for it, exciting times :)

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Post by Rob Kam » Sat Apr 02, 2016 5:35 am

What's the optimum way to buffer the outputs?

TL082 op amp buffers?

Tony Allgood of Oakley Sound suggests all outputs should be put through a 1K resistor and a J201 FET follower

Wouldn't Schottky diode clamping do good enough?

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