Sounds from Korg KR-55

 

While searching suitable analog drum machines from YouTube, another interesting sounding machine catching my attention was Korg KR-55. It is not a programmable one, but it does not matter because I’m now after the sound generators only. Especially I liked its snare sound. It contains also other interesting sounds that could be used. Here is one of the presentation videos:


 

By looking at Korg schematic, we can see right away that this is much more complex machine than Boss. For starters, the schematic consists of 8 pages. Sound generation is based mainly on op amps, assisted with transistors. It contains two noise generators: analog and digital. Analog noise is used only for snare, all cymbals and cowbell use digital noise. The analog noise generator I already built and used with Boss snare/hi-hat.

The digital noise generator circuit seems too complex for my purposes, and I don’t actually like its sound either, so I will leave it for now. This also means that I won’t be able to use the hi-hat or cowbell from Korg.

But even without hi-hat and cowbell, this machine contains many interesting sound generators, so pull out the breadboards again and start testing!

 

Bass drum

Korg bass drum is quite simple one op amp T-bridge oscillator, which produces a damped sine burst when triggered. I used TL072 as op amp, and made the T-bridge adjustable, so that the drum can be tuned. Not many other modifications. Here is the modified schematic:


And here is a sound sample. Tuning pot is adjusted during the sample.

Waveform looks like this. The yellow channel in the scope is the trigger pulse, Korg uses active-low triggers.


Snare

Snare contains the same basic blocks as Boss (and all analog snares): a decaying sine wave oscillator, a decaying noise and a mixer for mixing them together. The sine wave is produced by an op amp T-bridge oscillator, a transistor is used for controlling the noise and mixer uses another op amp. I changed the T-bridge adjustable, so the snare can be tuned. Some component value changes here and there but basically the original circuit is used. Analog noise comes from the same Korg’s analog noise generator which I already used with Boss circuits.

I don’t include the schematic here, because the modifications are not that big. I will publish the full schematics in later posts, when the project gets that far. The sound sample is here.

Waveform can be seen below. The circuit seems to sum the noise only on the positive half cycles of the sine wave.


Actually, this looks amazingly similar to the audio sample files available from internet for this machine. Or, maybe it should not be a big surprise, I did not modify the circuit that much. But at least op amps and transistor are different, also some of the passives. So this proves, that even if the target were to duplicate the original sound as faithfully as possible, the fiddling with intricate details (using exactly the original transistor types etc.) is not that important anyway. And my target is not to duplicate anything, just to get out sounds which I like. (Remember non-usefulness: no requirements.)

 

Rimshot and Claves

There are at least two more sound generators in the Korg that are worth breadboarding: the rimshot and claves. I somehow like this rimshot more than the Boss one.

Rimshot is a highly damped T-bridge oscillator with one op amp, the signal is further amplified with another. I made some minor modifications to it, biggest one being to make the T-bridge adjustable with a pot. 

Claves circuit I built almost unmodified, except the obligatory adding of tuning adjustment. 

Sound samples: rimshot, claves.

 

Breadboarding

The photo in the beginning of this post shows the bass + snare drum circuit built on breadboards. Also Korg noise generator and Boss accenter are on the board. There is another hi-hat circuit in the picture, I will talk more about in the next post.

The blue breadboard contains a simple one op amp mixer with level controls. This mixes the three sound generators + accenter ouput together.

Here is a sample of all four Korg-generators working together (bass, snare, rimshot, claves). The hi-hat is from Boss.

 

In the next post, even more sound generators from Andromeda Space Rockers.

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