[sdiy] The Mighty Korg KR-55

[Stewart Pye]

Hi Rafe,

Looking at the circuits, it is similar to a TR606 in many ways. I've 
never seen a KR55 so haven't tried this but I think the following would 
be a good starting point....

BASS DRUM

Tune: Reduce the value of R7 and put a pot in series with it. You'll 
have to play with the values, but maybe a 5k pot and 470 ohm resistor 
would be ok.

Decay: Since the oscillator is set to the point of not quite oscillating 
until it gets a trigger pulse this one's a bit tricky. Play with R6. 
Again probably reducing it and putting a pot in series with it will do 
the trick. As Tom said there may be some interaction between tuning and 
decay.

SNARE DRUM

Tune: R20 - Adjust as for bass drum.

Drum Decay: As for Bass Drum, R17 would be the one to adjust.

Snare Decay: The trigger pulse discharges C7, which allows Q1 to conduct 
and pass through the white noise. The circled 18 goes to somewhere in 
the sequencing circuit. I'm not sure why?? You should be able to alter 
the decay time by increasing C7 and putting a (pot/resistor in series) 
in parallel with C7. You could start by increasing C7 to maybe .22 and 
see how long it is and go from there. Keep in mind that the larger you 
make C7, the longer the attack time will be. This may or may not be 
noticeable.

Snare Level: Put a pot set up as a voltage divider (normal volume 
control) between C14 and R23. You might want to increase the value of 
R24 to enable a higher level.
Another option is to adjust R24. Decrease R24 and put a pot in series 
with it.

As I said, you will have to play with the values and the mods mentioned 
above are not guaranteed to work. Actually some of them won't work at 
all if the values are wrong.
If you need more explanation on anything feel free to ask. Good to see 
another Brisbanite here :)

Cheers,
Stew.









Dj Sinistral wrote:
> Hi everybody!
>
> I'm pretty new to this list, and I've got a challenge for some clever
> circuit gurus!
>
> The Korg KR-55, reportedly used by Jean Michelle Jarre and others, has
> some great analog drum circuits, but no sound shaping controls like 
> attack,
> decay and tune.
> So can it be done??
>
> I've uploaded the schematics here: http://rafe.com.au/KR55schems/
>
> The circuits look pretty basic with clearly separated sections for each
> instrument. Controls for the kick, snare and hats would be plenty. I 
> imagine
> a few well placed pots will do it.
>
> Thanks heaps!!!
>
> Rafe
>
> Brisbane, Australia
>
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