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

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Fri Sep 29 12:43:24 CEST 2023

I'm guessing it's so that they can use those PNP Common Emitter stages 
to boost the current for charging the envelope capacitors.  If they used 
active-high pulses coming out of the micro instead they would have to 
have an NPN at the bottom and a PNP at the top to get sufficient current 
to charge the envelope capacitors.  So using active-low pulses out of 
the micro means they can do away with the common-emitter NPN at the 
bottom and drive the C-E PNP at the top directly, saving a few NPN 


On 2023-09-29 11:31, Tom Wiltshire wrote:
> 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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