Wednesday, 15 December 2021

Synthesizer Build part-46: 808 KICK for EURORACK. (Juanito Moore circuit).

The kickdrum from the famous Roland 808 drummachine. With four controls and a print 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.

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 print straight to the back of the potmeters once I had the panel ready and this works just fine. I glued the topside of the print 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:

Print 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, especially in the filter section because of various reasons. If you want to include de-coupling caps then solder small ceramic 100nF caps over pins 4 and 5 and pins 10 and 11:

Here's an overview of the cuts and the wirebridges seen from the component side. As always; mark the cuts on the component side then stick a pin through the marked holes and mark them again on the copper side and then cut the marked holes. Do this and the wirebridges first and then solder in the rest of the components.

Here's the schematic I used to make the layouts. 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:

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. It shakes the windows in my attic and makes the dust fall from the beams LOL. It is really an exact replica of the original 808 kick drum sound. I have the Behringer version, the RD-8, in my little studio and it 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:

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. 

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 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.

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Tuesday, 7 December 2021

Synthersizer Build part-45: STEINER-PARKER DIODE FILTER for EURORACK.

This is the same Yusynth Steiner Parker diode multimode filter I posted in project 26 but with a new layout for Eurorack.

I'm busy setting up a Eurorack system dedicated to live performances, so I want to remake some of the modules I built earlier to make them fit the Eurorack 3U size. So here's the first one I converted, one of my absolute favourite filters, the Steiner Parker multimode diode filter with Lowpass, Highpass, Bandpass and Allpass. This is a Sallen-Key type filter with positive feedback so that you don't loose volume when you increase the resonance like you do with the Moog ladderfilter for instance.
I won't go further into how it works etc. You can go to the previous Steiner Parker article for more details.
I started out matching the diodes I needed by measuring the voltage drop but they all came from the same batch and the measurements were so close that I stopped matching and just put them in (and the filter works absolutely fine). The transistors however must be closely matched otherwise the filter won't be in balance. You can set the right balance with the 1K potmeter but that's only a fine control so make sure the transistors are matched. You can match them by simply measuring the HFE and look for two with the same values.
When you start out building, make the cuts in the copper strips first and then put in the wirebridges. Then you can put in the rest of the components.

Here are the layouts I made for this project. They are verified as always. I used these for my own build. I left out the second CV-IN and the second AUDIO-IN potmeters and jacks. You just copy the first input if you want two of them (which I strongly advise you to do especially for the CV). The stripboard is 24 by 41 holes. The switch to choose between Lowpass, Bandpass, Highpass and Allpass is a normal 2 pole 4 way rotary switch. 
Instead of using a reverse logarithmic 50K potmeter for Resonance I used a 100K linear type with a 100K resistor soldered onto it to get the reverse logarithmic characteristic. (See layout below. Two 100K resistors in parallel make for one 50K resistor). This is the recommended alternative in the original Yusynth article and it works really well. Of course, if you happen to have a reverse logarithmic 50K potmeter then use that instead of the 100K pot + 100K resistor solution. Should you have problems with resonance coming in too soon, put a 10K resistor in series with pin 3 of the resonance trimmer potmeter to get the throw of the resonance panel potmeter more to the clockwise side. I guess putting in a 20K trimmer pot instead of a 5K one should work too. Thanks to Nick in the comments below for the heads up on that one!  
For the level potmeters I used 10K linear ones because that's what I had. You can use any value from 10K up, it doesn't matter for level potmeters. Keep to the recommended value for the Cut-Off and Resonance though. I used a 100K for the Cut-Off frequency potmeter and I changed R26 to a 100K to make the voltage drop over the potmeter the right value. This works perfectly fine. You can of course use a 47K (50K) potmeter but then use a 47K resistor for R26. (R26 is the 100K resistor in strip A to the right).
For the 1,5nF filter capacitors I would recommend using good quality polystyrene, polyester or silver mica types. These form the heart of the filter so don't use ceramic caps for those.
Btw, I left out the two 10 Ohm resistors in the + and -12V strips because this filter was designed for 15V but running on 12V so I wanted to avoid any further voltage drops. I also left out the bypass capacitors but if you want to include those just put a 100nF capacitor from +12V to ground and one from -12V to ground right above the location of the chip. There's room enough left. (I did put them in later, just to be sure, but they are not on the layout or the bill of materials.)

Wiring Diagram:

Print only:

About trimmer T1: 
I changed trimmer T1 from a normal one to a multiturn trimmer which made it much easy to set. You need to set this trimmer so that the Cutoff frequency potmeter has the correct throw with full resonance at about 2/3 clockwise with the resonance potmeter set to almost self oscillation. I measured the resistance of T1 when I was done and it was about 640 Ohm.

This filter works best if it has a 1V/Oct CV permanently connected to it, although you can't play the self oscillation as you can with some other filters where you can use the filter as an oscillator. This filter's resonance is just too agressive for that.

Making the cuts accurately:
Here's a layout of all the cuts you must make and the wirebridges you need to solder in. This is viewed from the component side. Mark the cuts on the component side, with an Edding pen, and then stick a needle through the marked holes and mark them again on the copper side. Then you can cut them with a hand held 7mm dril bit. The cuts are all over the place so concentrate and be accurate otherwise the filter won't work. Don't forget the cut underneath the wirebridge at position S-19:

Bill of materials: Buy a batch of 100 BC547 transistors if you don't have any, so you have enough to choose from when looking for a matched pair. If you want to include de-coupling capacitors then order two extra 100nF caps because these are not included in the BOM. Order good quality polystyrene or silver mica or polyester types for the three 1,5nF filter caps.

Here's the schematic drawing by Yusynth:

Here are some pictures of the build proces and the finished product. Notice I had to put two capacitors in parallel to create a 680nF capacitor. I didn't have one in stock.

I soldered all the wires directly to the copper side of the print and mounted the print with the component side pointing backwards of course, otherwise you can't get at the trimmers. I put some Gaffa tape over the pins of the 4 way rotary switch to avoid accidental contact with the print or wiring.

A look at the finished panel. I managed to fit everything in nicely. I had this piece of powdercoated aluminium left over so that was perfect for this project. I made the 3mm mounting holes wider to give me some room to move the module sideways to fit the rest of the modules (which are yet to come ^___^)

Okay that's it for this one.  
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