Analog drums: The beginnings

 

 

Drums have always fascinated me as an instrument, but I have never owned a drum kit or properly learned to play them. But drum machines wrap drums and electronics in an intriguing package, so that’s the way to go. If not counting various software and synths, I own only one stand-alone drum machine, Yamaha RX-15, which is an early sample-based machine from the 80’s. So, I am clearly missing an analog drum machine.

Someone might now argue that this is against my “declaration of non-usefulness”, because drum machines are commercially available and people are willing to pay for them. But let’s face it, analog drum machines are not very usable today. Their sounds are available as samples, which are much more easy to use than the rudimentary sequencers contained in basic analog drum machines. So I’m counting this project as useless enough to be worth implementing.

Not surprisingly, YouTube contains loads of videos on various analog drum machines. After watching those for hours, I began to like more and more about Boss DR-55 sounds. Here is one video about it:



The kick, snare and hats sound nice, these could be used as a starting point. Some people like the rimshot sound also but I think its quite odd.

I dug up the schematic of the machine and noticed that the device is internally very simple. All sound generation uses only transistors, and not even many of them. I have no experience on analog drum machines, so this would be nice and simple to start with.

Next thing I noticed was me building up a copy of Boss bass drum circuit on a breadboard. It is only two transistors and a few passive components. I have no urge to build an exact copy of the original to achieve as authentic sound as possible. To me these circuits are just a starting point for my own experiments. So I did right away couple of modifications to the original circuit:

  • Transistors are 2SC-something, I don’t have those, let’s put in BC547
  • Original supply voltage is 6V, which is quite low, 12V will be more like it

After some experimenting, scoping and listening, the circuit was like this:

These are the additional modifications done to the original:

  • The original output resistor (150k) is quite high, lowered it generously to 12k
  • The output coupling capacitor (27n) seems quite low, raised it to 100n 
  • There is a quite sharp click in the beginning of the sound, added a 47n capacitor to ground to smooth it a bit
  • There are never too many possibilities to adjust things: replaced the original Q2 emitter resistor (820Ω) with 2k pot + a small series resistor

After all this, the result sounds like this. At around 0:20 you can hear the effect of adjusting the emitter pot. At around 0:40 I’m adjusting the trigger pulse width.

This is how the output waveform looks in the oscilloscope:

 

This was a promising start with analog sound generation, let's see where it leads.

 

Next: More sounds from Boss

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