Buffered multiple, part 2: Abusing the system

Here’s another fun fact about the horstronic. Buffmult, or specifically about the build guide for the Buffmult:

There isn’t one.

None that I could turn up, anyway. I’d look on the horstronic. website but, well.

There isn’t one.

They have a Facebook page and an Instagram feed, but that’s not quite the same. And they post to the online forums, and there’s a thread about this module, but no actual build guide. I guess you’re supposed to know what you’re doing. Well, after building four Eurorack modules (and other things over the years) I figured I kind of do. Nothing looked particularly complicated or treacherous about the build. What could go wrong, he said confidently?

One thing that took me by surprise was the close spacing of the holes for the resistors. Checking the photo confirmed that you’re supposed to mount through-hole resistors vertically. (And the Synthcube kit confirmed you can use SMT resistors instead.) I’ve mounted the occasional resistor that way — one in the Even VCO is supposed to be vertical, and both of the ones in the MiniAtt — but never this many of them. No problem really, but if you build low to high you normally start with the resistors; if the resistors are vertical, though, they’re not low, so… well, don’t start with them, then, right?

I didn’t. I started with the diodes — which are mounted horizontally. I put the IC sockets on next, then the ceramic capacitors, though arguably the other way around would’ve been better. After that the resistors, electrolytic capacitors, power header. Then the trimmer pots.

I mounted the top- and bottommost jacks and used them to line up the front panel while I figured out where to bend the LEDs’ legs to get them in the right position. Then I placed the remainder of the jacks, secured the front panel, and soldered the jacks.

Then the moments of truth: Any power shorts? No. Any smoke or heat when powered up? No. I put an LFO into the top input and, yes! the LEDs blinked between red and green. This was particularly gratifying since the BOM said only “3mm LED, 2, 2-pin 2-color”, no more details than that, and I was not at all sure the parts I’d selected would work. They did. Or just one did if I plugged into the second input, which was correct. (With nothing in the second input it’s normalled to the first, so both LEDs blink.) I ran the outputs one at a time to the VCO MOD input on the Mother-32 and, yes, all the outputs worked.

I proceeded to calibrate, following advice from the forums: I measured the difference between voltage in and out, for voltages from 0 to 5V, and adjusted the trimmers to make that difference independent of the input voltage. (There’s an offset of about 5 mV on each channel.)

Then the true moment of truth. After all, the main reason I bought and built this module is because it was supposed to fix the problem with the M-32 keyboard control voltage, which is 1 V/Oct when not plugged in but sags to around 0.98 V/Oct when plugged into the Befaco VCO. And when plugged into the Buffmult? I tried it.

Same thing. Same sag.

Which was disappointing, but not wholly unexpected. I’d figured there was a chance of it. Either the advice I’d read was wrong (something wrong on the Internet! Alert the media!) or it depends on which buffered multiple you use.

But the kind of good news is, this particular buffered multiple can be calibrated. Unlike, as I said before, the Barton Eurobuffer, which is designed for fixed unity gain. Part of why I chose the horstronic. was I like having sufficient control to impose unity gain. And part was I suspected I might want to impose nonunity gain.

And I did. I intentionally miscalibrated one of the Buffmult outputs for slightly above unity gain, just enough to compensate for the M-32’s KB CV sag. Now, running the KB CV into the Buffmult and out the miscalibrated output, the Befaco VCO tracks the keyboard within a couple cents over seven octaves. It’s a kludge, yes, but it works.

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