[sdiy] Since it's 9/09, it's time for more 909 questions :)

[Colin Fraser]

> -----Original Message-----
> From: TooManySynths [mailto:p8051 at yahoo.com]
> Sent: 09 September 2001 21:30
> To: Colin Fraser; 'TooManySynths'; synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] Since it's 9/09, it's time for more 909
> questions :)
>
> > All 909 trigger pulses are 2ms.
> > In the bass drum, the trigger pulse charges C7.
> > While C7 is charged, Q3
> > turns on Q4. Q4 applies the accent CV to Q8.
>
> Do you mean C8?

Yes. Brain to finger communication error...

> Aha, now we're getting somewhere.  Have you looked at
> the envelope with a scope?

I've looked at the waveforms in Wavelab.

> My tests suggest that the
> cap will discharging as soon as voltage is applied,
> but, will discharge at a slower rate.

There is a slight initial decay, but not much.
The 'flat' part is very short.
Look at http://www.octavo.demon.co.uk/909compare.jpg
This is an image of the sound with a very long decay (modified over
standard) above a shorter sound.
The 'flat' part only lasts about 4 or 5 cycles before the C8 decay takes
over - about 20ms.
Calculate the time constant of C7 discharging thru R17 and R18, and you'll
see where this period comes from.

> Also, I just assumed that d8 and d9 are
> garden variety 1n914 or so, ok assumption?

I used 1N4148.

> Nevertheless, the kick is definately fatter when the
> gate pulse to the envelope is longer.

I've never experimented with this, but increasing the value of C7
significantly, and adding an extra pot in parallel to ground as a discharge
path should give you an 'initial hold' control to allow more 'compressed'
sounding envelope shapes.

> 1)What is the nature of the Accent CV. Is it positive,
> negative? 0 to X, -X to Y?

0 - 5v positive.

> 2) Do Q8 and Q9 only serve to extract a pulse for the
> Attack portion of the sound?

They make the click part of the sound.

> 3) Q3 appears to be NPN and Q8 appears to be PNP,
> correct?

Yes.

> 4) Q12 seems to be providing VCA functions, albiet, a
> dirt simple one.  Does that contribute signifactly to
> the "shape" of the sound?

It's quite a commonly used cheapo VCA circuit.
It'll certainly add some dirt to the sound.
I think there may be some discussion of this in the archives.
I'm not sure how linear it's control input will be.

Colin f