[sdiy] Roland drum trigger spec's

[Colin Fraser]

> -----Original Message-----
> From: nss at hevanet.com [mailto:nss at hevanet.com]
> Sent: 14 March 2001 05:05
> To: Colin Fraser
> Cc: synth-diy at node12b53.a2000.nl
> Subject: Re: [sdiy] Roland drum trigger spec's
>
> So the pulse at the input of the tone circuit would be from
> 4-14 volts,
> that's after those two trans that add the trig and the acc pulses?

Yup.

> >Disable the twin-t oscillator by removing the trigger input
> cap to it (C86 in the mid tom) and see if you are getting no
> noise, or just very quiet noise.
>
> No noise, just a thump. I cranked up the PN input volume with an
> external amp, still no noise.

It might help for you to know how the vca that controls the pink noise level
works.
They are cheap to say the least...

If you look at the schema you'll see a transistor with no bias input
(remember a transistor needs 0.7v to start working).
Conected to the base of this transistor is an input from a cap charged by
the trigger pulse, and an audio input.

Basically, the trigger pulse charges the cap C85 so that the transistor is
biased enough to turn on.
Then the audio signal (in this case noise) that is added to this bias
voltage will be amplified too.

As the cap discharges, the amount of bias decreases, and the signal is
increasingly clipped, then shut off.
This design is very nasty and introduces loads of distortion, but is
exteremely cheap to implement, and sounds fine for noise signals.
In fact the use of this type of vca is characteristic of many of the 808
sounds.

If you're just hearing a thump, it seems likely that the cap discharge
voltage that biases the transistor is OK.
You need to check that you have a noise signal riding on the top of the
voltage.

A scope would be very handy at this point...
Try feeding a different audio signal in at the pink noise input - it'll be
easier to hear if it is being amplified by the vca.
Also check that the transistor pin-out is what you expected.

> I'm comparing it to the re-birth version, as I have never seen an 808.
> Is that an ok aproximation?

Need to have a listen to rebirth - I'll get back to you.

Colin f