909 voice circuits question

[Trevor Page]

Hi,

I am thinking about building a 909 clone. With thanks to Colin F I now have
the EPROM samples and the schematic diagrams for all of the voice (drum)
circuits. I shall probably go ahead and get all the drum circuits built on
PCB soon.

This is my question: If at all possible, I would like to be able to vary the
level (velocity) of each drum via the MIDI velocity or expression
controllers. I would want to be able to vary the output level of each drum
from complete attenuation to the maximum level, i.e. so that I can do snare
rolls etc.

I see that each drum circuit has an 'accent' input; could these be used as
they are for this purpose? That is, like a VCA? I am not quite sure how much
the output level can be varied using this input. I don't know what kind of
voltage range this input expects to see either. I've never used a TR909 but
from using Rebirth on the PC, it seems that you can set an additional
'accent' on each step which seems to just raise the level of the drum(s).
Unfortunately, I don't have the schems of the digital section. I am
wondering if these inputs are pulsed with a logic signal or something in
order to add accent to the drum sound.

My idea is to design a MIDI / CV which provides a gate and velocity output
for each drum circuit

*If* these inputs are not suitable for what I am trying to do, can anyone
suggest a suitable VCA circuit for this purpose?

Also (ahem, asking too many questions here :-) ) I would like to know what
voltages the + / - rails are. Also, what voltage should the trigger be (TTL
level?). There's a few more problems, but I'll leave those 'til next time
:-) !

Many thanks!

Best regards, Trev


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