[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.

https://pdf1.alldatasheet.com/datasheet-pdf/view/129657/HITACHI/HD44790.html

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.)


Best,

  René



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