System 100M VCA clone

I’ve had a bunch of Curetronic 100M PCBs for ages and done nothing with them, so finally decided to start building them up as a test for Schaeffer UV-printed panels… only because I wanted to test out my version of the OpenMusicLabs clown/clone BA662, I built a single one up on stripboard.

System 100m 130 VCA stripboard photo

For stripboarding I’ve switched to Veecad, which is much less pretty than DIY Layout Creator but at least allows for verifying the layout using a netlist exported from Kicad. Here’s the Veecad layout using a BA662 as the OTA:

Roland System 100m 130VCA stripboard layout

In case you don’t have a BA662 clone or original to hand, here’s a stripboard adaptor for a BA6110. Without any other compensation it’ll be slightly quieter than the ‘662. I measured 2.68dB difference between the two, everything else being the same.

ba662 > ba6110 adaptor

If you build the VCA, don’t be tempted to socket the transistors – I ended up frying a small pile of my clone ‘662s due to dodgy connections.

Here’s my redrawing of the VCA schematic.

As for the clone (…of the clone) BA662 – the OTA part of the chip seems to work OK. The 100M VCA doesn’t use the onboard buffer, so I need to check that as well.

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121 testing testing

still not convinced tbh

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703e VCF with LM13700s

Someone asked on electro-music about subbing a LM13700 for a couple of CA3080s in a Roland-style filter, so I thought I’d have a go. I levered out the 3080s in my 703e clone and wired up three LM13700s (two for the four poles, one-half for the resonance control) and to no-one’s great surprise, it works.

Here is some quick fiddling with an out-of-tune clone 700 oscillator into the 703e VCF.

It sounds a bit overdriven to me – I was trying to avoid any red lights, but being in a hurry to get something down it’s possible I got something wrong along the way.

Now I think about it, it could be that I didn’t recalibrate the DC offset trimmers from when it was set up with the 3080s.

The wires strewn all over the board probably aren’t helping either, so I’m tempted to build up another 703e and make some LM13700-to-CA3080 adaptors to test this further.

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