[sdiy] Negative-going triggers in the Boss DR-110 drum machine - Why?

René Schmitz synth at schmitzbits.de
Fri Sep 29 13:09:10 CEST 2023

Hi Tom and all,

Am 29.09.2023 um 12:31 schrieb Tom Wiltshire:
> Hi all,
> I've been reading old drum machine schematics, and I've got a query about the Boss DR-110. It uses negative-going trigger pulses from the processor, and I wondered if there is any substantial reason why?

Some old logic families could sink better than source current. N-MOS, TTL.

But here I think the feeble drive strength of the processor would 
neccesitate some sort of buffering.

So it also makes sense from a standpoint of what happens after the 
transistors that invert:

To fill the envelope capacitors (which are returned to GND) you could 
either use an emitter follower, and lose a Vbe, or invert the logic, use 
a PNP, and only lose the Vce(sat).

> For some of the sound-generating circuits, this makes little odds, but some of them have a transistor to invert the pulse before they can use it. Given how stripped-down this thing is (one-transistor "VCAs" all over the place, and even some of those are re-used by multiple voices) this seems a bit odd, like they wouldn't have used negative-going pulses unless they had to.
> I wondered if it might be to do with the processor itself, which is a 4-bit Hitachi HD44790A-44, but I don't know anything about this chip. As far as processors of that era go, I know the Z80 and the 6502 and that's about it!
> Anyway, any thoughts or pointers appreciated.


Says its CMOS.

This is micropower stuff as was used in Thermometers, Calculators and 
the like, so not much muscle in the output stages. (Driving LCD, and a 
few buttons typically.)



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