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

Tom Wiltshire tom at electricdruid.net
Fri Sep 29 15:34:46 CEST 2023

You all seem to agree that it's down to the feeble output power of the processors ports, which makes PNP transistors a better bet, and that necessitates the negative-going pulses.

On a modern processor with much more powerful outputs we could presumably (I'll have to check the currents) drop the PNPs too and use positive triggers, saving even a few more parts.

Many thanks!

> On 29 Sep 2023, at 12:42, Roman Sowa <modular at go2.pl> wrote:
> The microcontroller data ports don't have enough oomph to quickly charge (via 220R resistor) the envelope/decay capacitor of every voice so it had to be boosted by PNP transistor, hence the need for active-low pulses. If there was enough juice in the micro pins, they would have used positive pulses and a diode instead.
> Roman
> W dniu 2023-09-29 o 12:31, Tom Wiltshire pisze:
>> 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?
>> 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.
>> Thanks,
>> Tom
>> Full schematic:
>> https://electricdruid.net/wp-content/uploads/2023/09/DR-110Schematic.png
>> Service notes:
>> https://electricdruid.net/wp-content/uploads/2023/09/Boss-DR110-drum-machine.pdf
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