Saturday, 15 January 2022

Synthesizer Build part-47: DUAL LFO for EURORACK.

 A simple LFO with pulsewave (with variable pulse width) and a seamless transition between a Ramp wave, Triangle wave and Sawtooth wave using one potmeter. With LED rate indicators and Speed and Shape controls.

Well what more is there to say about this LFO. It's such a simple design that I thought I could easily fit two of these on a small piece of stripboard and still have it small enough to fit a normal Eurorack Skiff. The circuit is derived from the 'Utility LFO' by Ken Stone which is a larger version of this LFO. The depth of this module is 55mm. I made the panel 4CM wide, that's 8hp, and I put the potmeters to one side leaving enough room to glue the print straight to the back of the panel at a 90° angle using hot glue. All the output sockets fitted nicely next to eachother at the bottom.
Naturally you can just as easy build this module in the Kosmo size and run it on 15V. If you do, you need to keep to the resistor values in the schematic not the layout because as I mention further down, I changed the 1K output resistors to 1K8 to get a nice +/-5V output signal. If you power this with 3 more volts you probably don't have to do that. Do some testing first to make sure though.

Here's the schematic drawing of the circuit:


The module consists of two of these circuits on a single piece of stripboard. I placed the LEDs on a separate piece of stripboard with a dual opamp, the good old TL072, and I used bi-coloured 3mm LEDs in red and blue. I drilled two 3mm holes to the left and in the middle of the first two- and last two potmeters for the LEDs and glued them in place with hot glue so the little print sits over the potmeters. See pictures below for illustration. Btw, you can use any dual opamp chip for this circuit as long as the pinout is the same; like the TL082, NE5532, LM358 etc.

LAYOUT:
Here is the layout I made for this Dual LFO. As always, the layout is verified. I used it for my build. I placed the Eurorack powerconnector on the left side for better access. In my build it's on the other side and very near the panel. Not a good place for a power connector but you only find these things out when you start building it. See, I make the mistakes so you don't have to LOL! (I hot-glued the print to the back of the panel with the righthand side closest to the panel.)


Print(s):


After doing the first tests I found the output voltages a bit on the low side. They were just +/-3,24V so I decided to experiment with the 1K resistors between the outputs and ground. I tried several values and I ended up using 1K8 resistors. That brought the output voltages to a nice +/-4,8V. Almost 5V so that's perfect for eurorack. 
I wanted to make one of the LFO's a bit slower than the other to give me a wider overall range so I used a larger capacitor for LFO number one. I used a 147nF and that made it perfect for my needs, between 0,2Hz and 10Hz
 
TECHNICAL DATA:
Here are some measurement results for this Dual LFO:
Duty cycle of squarewave is 5% to 95% this varies a bit with the frequency but nor more then 2%.
Lowest frequency: LFO-1 = 0,219Hz  LFO-2 = 0,653Hz
Highest frequency: LFO-1 = 9,82Hz   LFO-2 = 34,2Hz
Output voltage is +4,8V or 9,6Vpeak-to-peak. That's after changing the 1K resistors in the schematic for 1,8K ones. Otherwise the voltage was just 3,2V and 6,4Vpp.
Current draw: positive: average 12mA max.: 18mA
                       negative: average -13mA max.: -18mA

Here's the Bill of Materials:


Here are some screenshots from the oscilloscope with some measuring data underneath the images. Some images may still show the lower output voltage but that's been fixed:




The following are screenshots from the oscilloscope showing two signals, one from each LFO, being combined in a simple passive multiple. A squarewave and a triangle wave each at different frequencies. The results are amazing:



In the top picture you see more of the waveform in the positive voltage region and very little below zero Volts. You can set that with the shape potmeters to your own liking or best sounding result. As you can see this makes the Dual LFO module much more versatile as a modulation source. Plenty to experiment with.


PICTURES:
Below are some pictures of the print. I took these before I changed the 1K resistors to 1K8 ones. In the top picture and the 3rd one you can see how I mounted the little print with the bi-colour LEDs. The print rests above the middle two potmeters and the LED's are bent backwards over the sides of the stripboard and straight into the holes in the panel and are secured with hot-glue. The little print itself is not mounted in any way. It just relies on the LEDs to keep it in place.





This time, instead of spray-painting the panel I decided to keep it blank aluminium and I used an engraving tool to put the text on. That didn't work too well and it doesn't look good but at least the module works perfectly and that's what matters right? ^___^  I'll design a nice panel in Photoshop later and print it out and apply it to the front of the panel but at the moment I can't really be bothered. ^__^
I don't care much for aesthetics as long as it works right. 

TRIGGER OUTPUT
A few days after completing this build I added a trigger output to this module. I connected it to the squarewave output of the second LFO (the faster one). I thought it might come in handy to have a trigger source. You can see in the picture below how I did that. It gives of both positive and negative trigger pulses of 5V and a length of about 4mSec.  If you're thinking of putting in a diode to only get positive pulses forget it. That won't work. It'll kill off the pulses completely. If you turn the Shape potmeter the positive and negative pulses will move further away or closer to eachother. Just like the rising and falling edges of the squarewave with different pulsewidths.


(The above drawing actually translates to a high pass filter with a cut-off frequency of 268Hz. So it filters out the actual square- or pulsewave and only lets through the initial overtones of that wave, creating this spike pulse response, but you can forget about that. This is not important.)

Here's a look at the final panel with trigger output. I just made some labels with text to put on the panel. Looks better than the engravings.



Okay, that's number 47 done! A very useful little module and I saved a few bob my building it myself instead of buying a dual LFO module. Okay it doesn't have any fancy extra's like synchronization but that's okay by me. I think I'll mostly be using this as a clock source and some random modulation. That's why I made both LFO's run at different frequency ranges.

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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. 

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. 
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 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:


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.:


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.


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 thinking about setting up a Eurorack system 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 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. The transistors however must be 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.

LAYOUTS:
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). 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.
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 filter where you can use the filter as an oscillator. This filter's resonance is just too agressive for that.

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 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 I couldn't cut it to exactly the right width I had wanted; 17hp. It's two millimeters wider. I made the mounting holes wider to give me some room to move the module sideways to fit the rest (which is yet to come ^___^)



Okay that's it for this one.  
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Sunday, 10 October 2021

Synthesizer Build part-44: AD/AR ENV. GENERATOR.

This is a simple AD/AR envelope generator by Ole Stavnshoej.  With the little alterations I made it's a very useful AD/AR for use with filters.

CHANGES I MADE:
After re-doing the layout for the 7555 AD/AR I decided to try an other design and I found this one online. It's a simple design and easy enough to build. I intend this particularly for use on the CV inputs of filters and after I built it I made a few changes to make it better suited for this role.
First of all I made the electrolytic capacitor smaller so that the circuit would react faster and be more accurate. Later I added a SPDT switch (ON-Center OFF-ON) with some other electrolytic caps soldered to the switch so you can choose how fast you want the envelope generator to be. I also changed the 1M resistor coming of the 200K trimpotmeter to a 820K to make that trimmer more effective. Later I included the attenuverter potmeter and I changed the trigger input capacitor from 10nF to 3,3nF to make the circuit react better to fast playing. That change made all the difference. It now works much better and is very responsive. This is now my go to AD/AR for use with filters.

The trimmer sets the zero volt line or offset voltage but the strange thing with this circuit is that if you turn the Release potmeter you also change the offset voltage. If you turn it all the way open the offset can be as high as +5V. So on long release times it never returns to zero. That's why this design is really only suitable for use to excite filters where you have very short envelope times. If you connected it to a VCA it would stay open all the time with long release times. The attenuverter mod however could help a bit in setting this straight but I haven't tested that.
However if you use a 1,5µF cap for C4 it doesn't give this problem. It starts with higher value caps that's why I put in the time range switch with different value caps. I talked about it with Ole and we both think it's due to residual voltage in the capacitor C4.

Instead of the TL084 you can also use a TL074 or an LM324 or really and quad opamp with the same pinout. Same goes for the TL082.

LAYOUTS:
Here is the layout I made. It's verified as always. The inverted output is grayed out because I didn't use it and with the attenuverter mod you get both at the same time. I sent the normal output to a second opamp buffer, which I originally put on there for testing, and connected that to a second output. Always useful to have more than one output I think.
Oh and this circuit runs equally well on +/-12V as on +/15V but the output envelope can reach almost the positive voltage rail so if you need lower amplitudes install the potmeter in the attenuverter mod. (Schematic + layout is further down the article)
Wiring Diagram:

Print only. (The green wirebridges indicate connections to ground):


Here's the schematic drawing. You will note that some values on the schematic are changed on the layout. I did this after testing so follow the values in the layout. Like I mentioned earlier I changed the 10nF trigger input capacitor to a 3,3nF one because the circuit was too slow if you play fast on the keyboard. 


Bill of materials for the version without the attenuverter:


ATTENUVERTER MOD:
Here's the version with the attenuverter modification, which I of course only discovered after completing this build. But it's easy enough to implement so I changed my module later and included this option. It only requires a potmeter on pins 8 and 12 of the TL084. It did mean that I had to find a spot on the panel to put the potmeter. I ended up putting it between the output sockets. A bit awkward but at least it works very well.


Here's the layout, adapted to include the attenuverter potmeter. The only other change is that resistor R19, the 47K from pin 12 to ground, is removed.


Print only:


The way the attenuverter is wired up in the layout, you will get the uninverted output if you turn the potmeter fully clockwise and inverted output fully counterclockwise. (On my panel I had it the other way around. ^^) And wow does this make a difference! If I send the inverted signal from this AD/AR into the CV IN of the Steiner Parker filter you get almost a flute like, very clear sound if the filter is set a certain way. I loved it.

CUTS AND WIREBRIDGES:
For extra clarity, here are the layouts showing just the cuts and the wirebridges, which you should do first before soldering on the components. These layouts are the same for the version with or without the attenuverter.

Cuts and wirebridges, component side:

Cuts only, COPPER SIDE!
 
Bill of materials for the version with the attenuverter. 



Here are some pictures of the print and panel. I didn't have the attenuverter connected yet in these pictures:



A look at the panel. You can see the Time Range switch I added to the left of the Trigger/ Gate switch.
I used a dual pole switch with a middle off position so I could have three ranges, short, medium and long. The Short setting (in the middle) uses just the 1,5µF cap on the print. The Medium setting adds to that a 2,2µF cap and the Long setting adds a 4,7µF cap to the one on the print. You can see the attenuverter at the bottom crammed in between the in and output sockets.



Here are some images from the oscilloscope:

Fast squarewave on input and then turning release up. 

Again opening Release on a fast pulse train with a little bit of attack. Note how the pulses go a bit below the zero volt line here. (Not a big deal for use with filters):

Fast Attack, tiny bit of Release:

Turning the attenuverter from negative to positive output. I'm running this module on +/-12V and you can see the maximum output is +/-11 Volt! Just beware that the output voltage can be quite high, especially if you decide to run this on +/-15V..

Just turning the Release knob without any input using a 4,7µF cap for C4. Note how the voltage on the output changes. This should be a flat zero Volt line. If you use the Time Range switch and set it to longer times and you turn up Release, it doesn't come down to zero volt anymore. Again, it doesn't do this with the 1,5µF cap only with higher values.

And finally a little demo video of how the attenuverter influences the filters while turning it from positive all the way to negative envelope output. Note the tuner on top of the case. It's connected to one of my Thomas Henry 555 VCO's and it's rock solid in tune!




Okay, that's an other article done. If you have any questions or remarks, please put them in the comments below or put your question to our awesome little community of DIY synth nerds on the EDDYBERGMAN Discussions Facebook Group.

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