More sounds from Boss DR-55

 



Noise generator

After building a not-so-exact copy of Boss DR-55 bass drum, it was time to try out other circuits from the same machine: snare and hi-hat. They are not that complicated either, but they will require some additional circuitry: a noise generator. Boss, of course contains one, implemented with two transistors (in Boss, almost everything is implemented with two transistors, except those parts that are implemented with one).

Schematic of the Boss original noise generator looks a bit strange. Does this kind of circuit really generate noise? That must be tested. I built the circuit on breadboard and quickly found out that it does generate strange noises, but not that nice white noise which is needed for snare and hats. After some component changes and other adjustments I came to the conclusion that this circuit is not suitable for my use. Maybe it is very critical for transistor characteristics or something, I don’t know. This is not the only noise generator circuit in the world, no use in trying to get it working.

Internet contains several noise generator circuits which could be used. But during my YouTube marathon searching analog drum machines, I fell also for Korg KR-55 (more about that in an upcoming post). And it contains a simple and straightforward noise generator which looks like it could work out of the box:


 

This looks more like a noise generator should: a reverse biased transistor BE-junction generating the actual noise and an op amp amplifying it. It was a quick build on a breadboard. Substituted op amp with TL072, which I happen to have plenty of. It worked right away. I tested a couple of transistors, old ones are better suitable here than the modern ones because they are more noisy. Old and trusty BC108 sounded best here.

 

Hi-hat

Hi-hat circuit is again quite simple: just an envelope generator and a VCA/gate-like construction, using two transistors. When triggered, it generates a decaying envelope which lets the decaying noise burst out. The output is filtered with a simple passive band-pass LC-filter. Here is my modified circuit:


Some of the component values have been changed after testing on the breadboard. The transistors are BC547. As I have a chronic knob mania, I of course added some adjustability: the original fixed decay time, which is determined by R55, was replaced with adjustable one, adding the 2M pot.

The LC-filter coil was a bit problematic: 45 mH is not that easily available inductance value. Rummaging my junk bin, I noticed couple of unknown coils which I had sometimes bought from a local electronics store’s surplus stand for a very low price. It’s specifications in the store were “thin wire, plenty of turns”. My component tester showed inductance of 52 mH, might be useful. Next I measured its resonance using signal generator and scope, with a 6.8n capacitor in parallel; got about 8.5 kHz center frequency. Calculating theoretical center frequency for the component values in Boss schema give 9 kHz. Close enough.

The resulting sound is like this. In the middle of the sample I’m turning the 2M pot from end to end. The decay time is changing accordingly.

Here is the output waveform on the scope:


 

Snare

Snare is the most complicated of the circuits. It consists of three parts: a decaying sine wave oscillator, a decaying noise and a mixer for mixing the two. One can think of it as a combination of bass drum and hi-hat triggered simultaneously and mixed together. Of course frequencies, decay times and other parameters are different.

Again, Boss implementation of this is very simple, it uses only two transistors. Damped sine wave oscillator is implemented with one transistor and is similar to bass drum circuit, with different component values. The envelope generator for the noise is a bit simpler than the hi-hat circuit. The outputs of the two generators are mixed passively.

I consider the passive mixing a bit doubtful. The balance between mixing sources is hard to control and the sources are very easily influencing on each other. Because I’m not building an exact copy of anything, I decided to add a basic active mixer using one op amp. It makes many things more straightforward.

Otherwise I did not do many modifications to the original Boss circuit. Of course, the obligatory controllable parameter was added: the frequency of the sine wave part can be controlled with a pot, making it possible to tune the snare.

I don’t have the schematic of the breadboard version any more, but here is my final implementation of the snare:


Sounds like this.

The picture shows the snare waveform. The blue channel is the trigger pulse. You can see the noise riding on the damping sine wave. Noise is decaying a bit slower than the sine wave.



Accent circuit

Boss drum machine includes also an accent function, where certain beats can be accented. At the circuit level, this is just a decaying envelope generator which controls a VCA. All sound generator outputs are mixed together and brought to the VCA input. When the accent is triggered, it rises the volume of all sound generators for a short period in the beginning of the beat.

Boss implementation is again very simple: envelope generator is fully passive and the VCA consists of one JFET. I thought this kind of functionality might be useful in a drum machine, so I built this also. After some modifications, here is my circuit:



Breadboarding

The photo in the beginning of this post shows circuits described in this post + the bass drum circuit built on breadboards. The blue breadboard contains a simple one op amp mixer with level controls. This mixes the three sound generators + accenter output together.

Arduino is used for generating the four trigger pulses. I used a modified version of Jan Östmans O2_minipops software, dropping the actual sound generation away and using only the sequencer parts.

Here is a small sample of all generators together.


In the next post we will explore another classic drum machine,  Korg KR-55.





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