To: "'Analogue Heaven'" analogue@hyperreal.org,
From: "punkdISCO" forums@punkdisco.co.uk,
cc:
Subject: RE: [AH] KORG KPR-77 modification of drum sounds
Date: Thu, 5 Oct 2006 21:52:01 +0100

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Hi Tomasz

I too have a KPR that I want to modify. Unfortunately the only documented
mod in AH is this:

From: (Plutonique9) plutonique9@symbiocom.com,

Finally got some time yesterday to invest in refining the KPR-77 Bass Drum
circuit. The KPR-77 has to take the cake at "harbouring" the worst sounding
Kick of any programmable analogue drum machine, IMHO of course. The circuit
is just your boring ol' single Twin-T drum osc. Two elements (or lack of)
affect the performance of the Kick.

- It lacks any form of a pitch sweep envelope, which is essential for an
initial punch and "roundness", which i certainly like in a Kick.

- Secondly, the key to a nice sounding Twin-T drum osc. begins with the
trigger pulse itself, here where lookin' for a nice sharp pulse, longer
pulses result in a more "muffled" resonation. The Kpr-77's Bass Drum pulse
is set to 4ms long, by dropping it down to 1ms, it sounds way more tight.

The mods here are extremely easy to perform, yet the result is quite
gratifyin'.

Ok, here we go::

------------------------------------------------------------------
Parts list;

- 1nf capacitor (.001uf) (eg. Mylar film or Polystyrene) (x1)
- Germanium Signal Diode (eg. 1n34a) (x2)
- Delsoldering braid (for removing parts from the circuit board)
-------------------------------------------------------------------

Shortening the trigger pulse::

Inside the Kpr-77 you'll find a small square circuit board in the bottom
left hand corner. Locate on this board capacitor "C14" (8.2nf). Desolder (or
cut) this capacitor from the circuit board and replace it with a "1nf"
capacitor. Boom, your done.

-------------------------------------------------------------------

Adding in a Pitch sweep envelope::

Sounds tricky, but it's a dead simple procedure. By soldering two germanium
diodes "back-2-front" with one another (in parallel), you've contructed a
highly effective pitch sweep envelope for any twin-t drum circuit. I ripped
this idea from the 808 toms/congas. Without going into too much detail, the
GE diode pair will sweep the pitch of the Drum Osc directly in relation to
the amplitude of the Osc itself.

So once you've soldered your two Ge diodes "back-to-front" with one other,
take this diode pair and solder it across (in parallel) with Resistor "R37"
(1.5k). This Resistor is located on the circuit board (next to the master
volme knob) on the top half of the Kpr-77. Thats it basically.
------------------------------------------------------------------

You may have to seek out the Germanium diodes, there gettin' alil' hard to
find these days, you may be able to still order the 1n34a's through
radioshack, or try visiting a "real" electronics store for better luck. They
have to Be Germanium, silicon diodes just don't work right for what we want.

Paul
London
www.punkdisco.co.uk
www.myspace.com/punkdisco - Zero content, more pictures