Wednesday 1 May 2024

Synthesizer Build part-59: BASS++ DRUMVOICE by THOMAS HENRY.

Finally a new drum related project and this is a really good one. This circuit creates all sorts of percussive sounds from Bass drum and Tom-Toms to Woodblock and a lot in between. 

Once again a Thomas Henry project. They're just so good and I wanted to build a drum related module for a long time to expand on the 808 Kickdrum from Juanito Moore. This is a relatively simple circuit to produce such a wide variety of drum sounds. 
Here's the schematic I used to make the stripboard layout:

Just two IC's and 4 transistors. The core of the circuit consists of a VCO built up from the first of the two OTA opamps in the LM13700. This is called the Shell VCO. The VCO's pitch is influenced by the little Envelope Generator consisting of the 2,2µF capacitor and the Decay potmeter fed from IC-2a. This opamp takes the Trigger pulse and with the Sensitivity control, which is an offset control really, turns it into a useable Envelope signal. The VCO also has it's own manual pitch control and there's an external CV input with a level control (Range) and this is where the real magic lays because if you input a VCO signal into this CV input all sorts of weird sounds result. The VCO output goes through the second OTA which functions as a VCA with a buffered output. There are DC blocking capacitors (2,2µF) to get rid of any offset voltages.
Then there's an Impact section which provides the beat, so to speak. It creates the initial hit of the stick on the drum which you can control in tone and intensity. This is fed from the same trigger pulse as the Envelope Generator.
I did not have any 2,2µF electrolytic caps in stock so I used 3,3µF or 4,7µF caps in those places where they are used as blocking capacitors. The value of those isn't that critical. However C11 is the capacitor that determins the Decay length and for that I used two 1µF caps in parallel. With the wide tolerances they have that came to 2,3µF when I measured them together so that was ideal. 
C2 is an other critical capacitor. It determins the frequency range of the VCO so keep to the indicated value for that one. You can make that cap switchable with other values to change the base frequency of the VCO. I leave that up to you.

If you want to know more or you want a more detailed explanation of the circuit then I urge you to read the original article on the Birth of a Synth website.

There are some modifications to this circuit out there some of which you can check out via this link to the electro music forum:
One thing you can do is add an extra socket so you can input external envelope signals. Have it normalled to the built in E.G. and then break that connection when a patch cable is inserted.  I think a good place to insert that signal would be between the output of the opamp (pin 1 of the TL072) and the junction that connects to all the diodes. 
An other thing is to add a built-in noise generator. This would be handy for snare like sounds.
You can also add a switch to choose between different values of timing cap (C2) to be able to change the frequency range of the VCO.
With the circuit being this simple it will be easy to add to it and make it your own. I will leave that up to the individual builder. This article just deals with the original version.

Below is the series of layouts I used. They are verified as always and this project was yet an other hole in one. It worked straight away, no trouble shooting required. I kept the layout small enough to fit behind a Eurorack panel. Just 24 strips with a width of 41 holes.
Wiring diagram:

Here's the stripboard only view:

The cuts and wirebridges:

And finally the cuts only view. Seen from the component side. 

Mark the cuts on the component side with a waterproof Sharpie or Edding pen and then stick a pin through the marked holes and mark them again on the copper side. Then cut the copper at the marked positions. Be very accurate here. There's not much room for errors.

Here's the Bill of Materials:

Here are some pictures from the build process:

The finished stripboard:

Because this circuit is a bit difficult to understand and therefore the function of the 9 different knobs is also not easy to understand I decided to make a faceplate with a sort of block diagram on it so I could show which knob controls which part of the circuit. This would make the faceplate a lot bigger but it would also make the user understand which part of the circuit he was influencing.
It was not easy to design a flowchart or block diagram was simple enough but also true to the functioning of the circuit and it took me about 4 drafts before I was happy with the design.

Here's the resulting faceplate:

Everything in place ready to be wired up:

And here's the final result. Each section of the circuit has its own colour knobs. The trigger pulse is visible through the yellow LED inside the triangle:

Here's a little video of me trying the module out for the first time. Don't expect too much from this. I was just fooling around with it. I had a pulse wave on the Trigger input and a sawtooth from my VCO (which was connected to the sequencer) going into the CV input.
I recorded the audio of this video separately with a Zoom H8 to get the best bass tones which my phone would never pick up.

This module can replicate that late 70's 'Peewww peeewww sound made famous by Kelly Marie in her song 'It feels like I'm in love' . Remember that one? (LOL)
It's capable of much more than can be seen in this demo video, if you go to the 'Birth of a Synth' website Bass++ article and listen to the sound sample posted there, you can hear what this module is really capable of.

Okay that's an other one done and again a Thomas Henry design. They are just so good. They always deliver.
If you have any questions or remarks about this project then please put them in the comments below or post on the special Facebook Group for this website.