ba662 tall clown arrives

clown comparison

Having given up trying to hand solder the SOT-363 bits on the OpenMusicLabs BA662 clone/clown, here’s a version with BCM847DS/BCM857DS transistors instead of the dust-like PMP4201/5201.

No idea if it’ll work. The specs look almost identical, apart from a tighter hfe tolerance on the smaller device which I don’t think will matter anyway.

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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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ua726 replacer quick test

My attempt last year to build the Doepfer A-110 heater circuit on a small daughterboard for my 702 VCOs went up in actual smoke. It took me a while to realise I’d used a bad CAN-10 footprint for the ua726 connection, so now we have this

ua726 replacer board

Is it gonna work? Is it gonna fill my room with smoke?

ua726 replacer board on 702VCO

…and nothing is on fire! it works!

It gets vaguely hot – putting my finger on it, it seems warm – and pointing my (newly acquired, slightly shit) laser thermometer at confirms this.

It was a drag to get it in the dodgy socket I’d fashioned out of bits of a standard DIP one. Next time I’ve resolved to build some sort of temporary jig (inevitably out of blu-tack) to make it easier. Getting the legs the same length would make it rather less difficult.

While breadboarding the circuit I tested both surface mount and through hole versions of the 3046, and I’ve gone for surface-mount – partly due because it’s being readily available from the usual places, but also because the SOIC version seemed to get up to temperature more quickly. I suspect this was because the through-hole version is bigger and takes longer to heat up anyway, but also perhaps because those massive legs conduct the heat away into the breadboard, though I can’t prove that.

ua726 replacer board on 702VCO

The components are a bit of a tight fit on the board (which measures 46 x 20mm), but I had to get it small enough to avoid covering over the existing trimpots. I like Dr Korg’s attempt at this but that’s beyond me at the moment.

Bunging my makeshift frequency counter on the output showed that it took longer than I expected to get up to a stable temperature – after about four minutes, it was still slowly dropping. When I returned after wrestling my son about ten minutes later, it had stabilised – a watched pot, and so on.

I’ll write up some numbers another time, and compare it to the original 726s inside my System 100. I’m interested to see if insulating the 3046 might help.

If you’re going to be replacing a ua726 in a circuit (I’ve got an original SH-2 oscillator board which needs a couple) you might need to change a resistor on the feedback of the CV input opamp to give the scaling trim pot enough leeway to tune the oscillator. I had to alter R9 on the 702 from 50k to 68k.

Here’s the schematic – should be familiar enough from the Doepfer version.

ua726replacer schematic

If you want to build it, I can’t guarantee it’s going to fit on your board, whatever it is, but I’ve shared it on oshpark, where it’s $7.20 for three. There’s a BOM and further instructions there too.

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