[sdiy] The Mighty Korg KR-55

[Stewart Pye]

Hi,

I only just noticed the conga/tom circuits. They actually have variable 
tuning as I described for the bass and snare drums, and also a switch to 
vary decay for conga or tom. So the mods mentioned should work fine if 
you get the values correct. Take a look at the conga/tom circuit for 
reference. For low conga VR7 varies the tuning and IC5 can bypass a 1M 
resistor to increase decay time.

For the bass drum a 500k or 1M for decay pot should be OK. Hopefully you 
get enough range out of a 1M pot for the snare drum.

I'm not quite sure how to do the decay for the hi hats yet.

Cheers,
Stew.

Stewart Pye wrote:
> 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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