The kickdrum from the famous Roland 808 drummachine. With four controls and a stripboard small enough for Eurorack (although a bit deep). Naturally you can just as well build it in a Kosmo size if that's the size you're using to build your modular synth.
Now that I've started to play the modular synth I built more and more, I felt the need for some percussive action so I started out with this famous kick drum sound using a schematic from Juanito Moore who is famous for building his modular system without using any circuitboards at all just 'dead bug' soldering and he's really good at it too. A real inspiration for DIY synth builders like me.
The layout I made worked flawlessly right from the get go. This is quite a straight forward build. It requires 4 panel potmeters of different values and you must keep to these values too. You need a 5K, 10K, 100K and 500K panel potmeter. (470K instead of 500K and 4K7 instead of 5K will be fine too.)
You can modify the T-filter by changing the 15nF caps. Smaller values will give you higher tones but it will disrupt the balance of the filter and cause the Decay function to stop working correctly. Also there's no CV control for this module because it's not practical to implement. Here's what Juanito himself had to say about that:
"The decay not working right with different cap values is due to the properties of a bridged-T filter that oscillates with a ping of voltage. I gave up on voltage-controlling an 808 kick because the decay, dictated by the laws of physics, changes with pitch. Also, if you use a fancy voltage-controlled resistor (LM13700 datasheet) when you change the CV, the kick will trigger. A Vactrol was the best I managed to get."
So you can get away with putting a Vactrol over the 'Pitch' potmeter but that's about it. I personally didn't bother with CV control.
LAYOUTS:
Here are the verified layouts I made for this module. I marked two screwholes on the layout but I didn't use them. I just hot-glued the board straight to the back of the potmeters once I had the panel ready and this works just fine. I glued the topside of the board with the eurorack power connector pointing downwards (see pictures below). Beware that this does make the overall depth of this panel 7.5cm which won't fit some eurorack cases!
Wiring Diagram:
Stripboard only.
Pay extra attention to the connection of the transistor in the upper left. The emitter leg skips one copper strip and is soldered directly to the ground strip of the power rails. Strips B,C and D are all ground and I connected them together on the print by putting extra solder under the power connector so it bridged the middle three ground pins, shorting them together. Make sure to use polystyrene, polyester or silver mica type capacitors for all but the de-coupling caps (if you choose to include de-coupling caps. They're not on this layout). It's important not to use ceramic caps in the filter section because of various reasons. If you want to include de-coupling caps then solder some small ceramic 100nF caps over pins 4 and 5 and pins 10 and 11 of the TL074:
Here's an overview of the cuts and the wirebridges seen from the component side. As always; mark the cuts on the component side with a sharpy marker pen, then stick a pin through the marked holes and mark them again on the copper side and then cut the copper at the marked holes. Do this and the wirebridges first and check the cuts and wirebridges by measuring with your multimeter for continuïty. Then solder in the rest of the components.
Cuts and Wirebridges seen from the component side:
Schematic:
Here's the schematic I used to make the layouts, drawn by Juanito Moore in his distinctive cool style. There are two versions of this schematic in circulation and one of them has the clipper section wrongly connected but this is the correct schematic:
Here's a Falstad simulation of this circuit which allows you to test different component values and see what the result is on the output. I put an audio output at the end that allows you to play the result and listen to it. It comes really close to the real thing.
The 'Clipper' switch increases the amplitude of the drum sound extra (when the switch is open) and provides a little bit of distortion which makes the sound more audible. The low frequency of this kick drum can be so low that you can hardly hear it but through a good PA system you will feel it in your stomach because of the extreme low frequencies. It shakes the windows in my attic and makes the dust fall from the beams LOL. It really is an exact replica of the original 808 kick drum sound. I have the Behringer version, the RD-8 drummachine in my little studio and the kick sounds just the same.
I soldered the 33µF electrolytic cap for the Decay straight to the potmeter to save space. There was one opamp left over as you can see in the schematic. I used that opamp to drive a little LED connected to the output so we have a visual reference of the output without pulling any current from the output to drive the LED. It's always handy to have a visual indicator to see if the circuit is triggered correctly. Plus a LED always looks cool in a module. The 1K current limiting resistor for that LED is soldered directly to one of the legs and reinforced with some heat-shrink tubing.
Here's the Bill of Materials. Make sure not to use ceramic caps, except for the 100nF de-coupling caps for the chip if you want to include those, but they are not included in the layout or this BOM. 100nF de-coupling caps can be soldered directly on the copper side from the plus pin of the TL074 to ground and from the minus pin of the TL074 to ground. Make sure the legs have some heat-shrink tubing on them so they don't cause short circuits:
Pictures:
Here are some pictures from the build proces and the finished panel:
Like I mentioned before, the depth of the module as you see it here is 75mm (7.5cm) so it might not fit in some Eurorack cases. Keep that in mind. The width of the module is 6hp. (3 cm). You could save some depth by rearranging the potmeters to be directly underneath eachother and then hot-glueing the print straight to the back of the panel instead of on top of the potmeters.
DEMO:
Instead of making my own demo I thought I'd embed Juanito's own video here. This video is over two hours long because he shows the complete build process but this link will start the video at the end where he demonstrates the functions. It should start at 2:23:36 If not, then just jump to that time manually.
If you can't see the video on your mobile device then CLICK HERE to view on YouTube directly.
Here's a link to Juanito's YouTube channel. Subscribe to his channel while you're there :)
Okay that's if for this one, with grateful thanks to Juanito Moore for his reactions and for just being awesome :).
If you have any questions or remarks please put them in the comments below of post them on the Eddy Bergman Facebook group where we have an awesome little community willing to help you with any problems you may encounter.