Synthesizer Build part-65: YAMAHA CS FILTER w IG00156

 A LP, BP and HP filter in one chip. Not a filter you can just decide to build on a whim though. It uses Yamaha's own IG00156 VCF chip which is very rare and very expensive if you can find one. Like three figures expensive. This projects deals with a Eurorack version of this filter but of course it will work equally well for Kosmo sized synths.

PCB'S AVAILABLE ON REQUEST. Contact me on Facebook messenger. (2 in stock). They are 6 by 8 CM in size. The boards work fine but miss a connection you must make yourself but it couldn't be easier, just connect pins 1 and 9 of the IG00156 chip with a wirebridge.

A good friend of this website, a fellow Dutchman who happens to be an amazing psy-trance producer by the name of Jake Jakaan, signed to a top record label, who uses modules from this website gave me one of these chips. He managed to get hold of a few of them. He states this filter is great for filtering FM sounds from the TH VCO555. It's very low-mid heavy.

It was not easy to find a good schematic for this filter. In fact, all I had was the service manual for the CS-5 and a stripboard layout that someone put together which looked very dodgy and had some mistakes in it (although it did seem to work for my friend but I didn't use it).

I made a completely new schematic for this filter using the original circuit from the service manual and from that I made a new layout, small enough to fit a Eurorack system. The layout turned out to work faultlessly straightaway for which I was very grateful because the IG00156 is not a chip I'd like to blow up. The chip is actually quite robust, I found.

There are two things that are unique to this filter; it has a frequency dependent Q (resonance peak) and a gentle single pole lowpass effect. Resonance (Q) is achieved by damping rather than using a positive feedback loop and because damping will not go to zero the filter can not self oscillate.

This filter is used in all of the Yamaha CS range of synthesizers. Even in their flagship synthesizer, the CS-80. It's a two pole 12dB/Oct. state variable filter. In the CS-80 one chip is used for a lowpass filter and a second one for a highpass filter and in other CS synths like the CS-5, one chip is used for lowpass, bandpass and highpass. The filter I present here has all three functions under a switch although you could have each output go to its own socket but then you have to redesign the output stage or simply bypass the output opamp which I wouldn't advise. You should have opamps with a little bit of gain on the outputs.

Here is the schematic I made and on which I based the layouts below:

Here's a block diagram of the inside of the IG00156 chip:

Source: Yamaha IC Guidebook.

There's an in depth analysis of this filter on the Modwiggler forum: CLICK HERE to read that.

I added an audio level control on the input because this filter is rather sensitive to high volume levels. I also added a gain potmeter on the output that gives you the option to set the gain from 2 to 4 times with a 100K potmeter. That is more than enough, but if you want more gain, put in a 500K potmeter which will give you 12 times gain, or 1M which will provide 23 times gain. It will just clip. There's no use in doing that. I really wouldn't advise it.

The schematic says to use +/-15V but I tested it on a dual 12V powersupply and it works fine. The chip inputs for the cutoff and resonance functions are very sensitive and the complete cutoff range is controlled by a voltage that goes from 0 to 0,25V. Only 250 milliVolt for the full range. This is achieved by the voltage divider consisting of the 22K resistor in the cutoff line and the 470 Ω resistor between pins 7 and 8 (or pin 7 and ground really). The potmeters for Cut-off and Resonance need to be fed with +10 volts so I added 1K8 resistors to the pins where the power comes in to cut off roughly 2 volts. You can actually use other values for these potmeters because they're only used as voltage dividers but then you will have to re-calculate the values of those two resistors to make sure the pots receive +10 volts. For instance, if you use 100K potmeters you're need to use two 18K resistors.

I used this schematic to make my stripboard layout which wasn't that difficult because it's quite a simple filter. There aren't many components in it. It worked straight away although at first I couldn't get it working because I had not wired everything up yet. I thought I had connected all the knobs I needed for testing but I forgot the V/Oct input. That needs to be connected to ground if it is not in use and once I had done that the filter sprang to life. After testing I added a V/Oct section to the stripboard layout as explained in the next paragraph.

VOLT per OCTAVE INPUT:

At first I had the V/Oct socket grounded through the socket switch but then I realized that isn't needed because the V/Oct input is always connected to ground via the 470Ω resistor. 

I applied a voltage divider to the V/Oct input consisting of an 18K and a 470Ω resistor, I went with an octave range of 8 octaves, meaning that the input would get 8 Volts at maximum which would need to be reduced to the same range as the Cutoff input because they all enter the same summer inside the chip (see diagram above). 18K with a 470Ω would give 0 to 203milliVolt which works out perfectly.

Because the filter can not self-oscillate anyway, the filter can not be used as a sinewave oscillator with the resonance fully open. So accurate volt per octave tracking is not an issue and therefore none of these calculations need to be super accurate, it just needs to work so that it sounds good and the filter now tracks nicely up with the octaves.

LAYOUTS:

Here are the layouts I made for this filter. As always they are verified, I used them for my build. As you can see it's a really simple project. There's only 25 resistors, a few capacitors and wirebridges and the chip sockets to put in. The biggest job will be the wiring up of the potmeters and sockets and the making of the panel.

Wiring diagram. Note that the Resonance potmeter is connected the other way around from all the other potmeters, with ground at the clockwise lug. This is usually the case in VCF's.  

Stripboard only:

It's best to use bi-polar or none polarized capacitors for the 1µF caps in the filter outputs and on the audio input. This is because we're dealing with bi-polar signals that go through the zero Volt line. The caps don't need to be this specific value. You can use anything between 1 and 10µF without problems.

As you can see all three filter outputs have a 100K resistor to ground. Together with the 1µF capacitor this forms a highpass filter with a cutoff frequency of 1.6Hz so in effect it keeps DC voltage from passing and all the other frequencies get through. Make sure you use high quality capacitors for the two 1,5nF filter caps. I used Polystyrene ones which always sound the best.

Cuts and wirebridges. As always, mark the cuts with a Sharpie or Edding pen on the component side and then stick a pin through the marked holes and mark them again on the copper side. Then cut the strips at the marked positions with a sharp hand held 6- or 7mm dril bit. Make sure you work accurately!!

And here is the bill of materials. It won't be easy to find an IG00156 chip. They are long out of production so your best bet is websites that sell rare synthesizer components. They go on Reverb for $189,- but that is top dollar. They should go for between $70,- a $100,- 

I did not include any bypass caps or extra electrolytic caps for the voltage rails. If you want to include those you need to add them to the list (2 x 100nF and 2 x 10µF/25V). They're not on the layout or schematic either but there's room enough on the stripboard to put them in over the voltage rails.

The two trimpotmeters should really be the normal kind and not multiturn trimmer. There's really no use in having multiturn trimmers because there's no need for that kind of accuracy.

NOTE: There is a Hongkong based listing of the IG00156 chip floating around on the internet selling them for €10,- Don't fall for that, it's a scam!!

PICTURES:

Here are some pictures from the build proces:

All components soldered on:

Testing:

Drilling the panel using a copy of the panel design I made in Photoshop as a dril guide:

Waterslide design applied to the faceplate and now drying on the central heating. All the creases you see will be gone by the time this is dry.

Finished panel ready to receive the pots and sockets etc. After the waterslide design has dried we cut out the holes with a very sharp hobby knife and then apply two more layers of clear acryllic lacquer and let it dry overnight.

Finished module. The module is 14hp wide (7 CM) and 3.7 CM deep.

I only had a 4 way rotary switch, that's why the HP mode on the panel has 2 settings. Lugs 3 and 4 of the switch were connected together.

Side view:

Side and back view:

Here's an oscilloscope screenshot of a squarewave wave in LP mode with full resonance applied:

CALIBRATING THE FILTER:

There are two trimpots on this filter that need to be set.

The first one is the 100K trimpot. You use this to set the throw of the Cut-Off potmeter. Set it in such a way that you get the most resolution of the Cut-Off potmeter.

The other one, the 200K, is used to set the Resonance to maximum. Turn up resonance and turn the trimmer until you're at max resonance. This will probably be around the middle of the trimmer at about a 100K.

It's best to have normal trimpots not multiturn ones. There's no need for those and the normal trimpots are easier to use.

VIDEO DEMO:

Here's the first test I did after finishing the project. This filter literally makes the room shake. If you turn up the volume (with good speakers or headphones) you will hear stuff starting to rattle in the background. It's a very bass heavy filter which really sounds great!

Here's a short demo I made with the X4046 VCO hard synced by the 555 VCO going through the filter.

Here's an other video I found on YouTube dealing with the CS-5 Lowpass filter:

https://www.youtube.com/watch?v=5F8_RfVzO2A

Here's a Facebook video of my friend Jake Jakaan using 3 of these filters in bandpass mode to create a formant filter that makes sounds akin to human speech.

https://www.facebook.com/share/v/1A2Q6Ws1AE/

Okay that's it for this article. Not a filter anyone can build alas but this website is an archive of all the modules I built myself so it certainly belongs here. I also noticed that Yamaha filter schematics that specifically deal with the CS filter are almost non existent on the internet except for the service manuals. So I hope this article will provide at least one good schematic for those looking for it.

If you have any questions or remarks about this project then please put them in the comments below or post them on the special Facebook group for this website.

Synthesizer Build part 55: CROSSFADER with VACTROL CV CONTROL.

This circuit lets you fade between two inputs manually or with a Control Voltage by means of two Vactrols. Project fit for Eurorack or Kosmo modulars. 

This was an other project by request and since I haven't built anything like it yet, it seemed like an ideal project for the website, so I decided to look for a good crossfader schematic. 
I came across a schematic from "sfcs.neocities.org ©Astro / SYNTHFOX 2020" on which it said 'okay to redocument when properly credited', so I hope this is okay.

I built this project for Eurorack but of course it can just as easily be built for a Kosmo sized synthesizer. In fact with a Kosmo module you have more space to put in more stages so that would be even better.

This project works on both -12/+12V and on -15/+15V without needing any changes.

Here's the schematic I used:

HOW IT WORKS:

It's a very simple circuit. You have a DC voltage that can swing between the voltagerails coming into the inverting input of an opamp by means of the 'Init' potmeter (Initial Voltage) and a second connection in the shape of a Control Voltage input with a level control, summed to the same input on the opamp. The output of this opamp controls the lighting up of the LEDs in the Vactrols. One Vactrol does the negative phase and one does the positive phase, or one does input A and the other input B. Whichever you prefer. 

The LDR's (Light Dependent Resistors) of the Vactrols are each connected to one input source and the other sides are connected together and go into the second opamp which is simply an output buffer. That's all there is to it.

The CV input comes in via a lower resistance than the Initial Voltage, 22K instead of 100k. This will, in combination with the 100K feedback resistor R4, give the CV input an extra gain. We can calculate that gain with the formula: Gain = (-Rfeedback / Rin) which is -100/22=-4.45 The minus symbol simply means that the output voltage is inverted. 

This is done (I think) to give lower control voltages enough effect to influence the vactrols because a CV voltage can never be as high as the Initial Voltage which covers the full powerrails potential. So if your CV input is not effective enough you can try lowering R2 and so get even more gain, but don't overdo it. Measure first before you start altering things. Make sure this change is actually needed. (I left it as it was myself).

I built one Crossfader stage as seen on the schematic but it would be easy enough to build a few of these so you can mix together more inputs. The circuit, as you can see, only has a few parts so it doesn't take up much room.

With the Init potmeter (Initial Voltage) you can manually fade between the two inputs. This works as long as there's no CV voltage present. When you input a CV voltage that will then take over control of the crossfading the higher you set the CV Level and you can steer the output signal more to one or the other input with the Initial Voltage control potmeter.

I made the Vactrols myself, in fact I used Vactrols I had already made for the LowPass Gate project, so I don't know exactly which LDR's I used to make them with but I used red 5mm LEDs in them. It is preferable to use red LEDs because they have the lowest voltage drop (about 1,5V) but with the extra gain the CV input gets I don't think this is very important. LDR's do react well to red light. The LDR's I used have a resistance of about 300Ω when the Vactrol is switched fully on. I think it is best to use quick reacting LDRs in the Vactrols so it can handle a wide range of LFO frequencies on the CV input.

The test results I got after finishing building this project showed that, when using the CV input I still got some bleed through of one signal onto another. In other words the separation of inputs A and B was not perfect when the CV voltage was on full positive or negative voltage. With just the manual control (Initial Voltage pot) I did get a good separation between the signals when the potmeter was fully clockwise or counter-clockwise. Of course the total voltage of the CV input could be the problem here. If it is not as high as the voltage produced by the Init. potmeter then it won't have as much influence. That is why the CV stage has extra gain on it to boost the signal.

So please don't expect perfection from this analog Vactrol design. It's old school and that's what I like about it.

LAYOUTS:

Here are the layouts I made for this project. As ever they are verified. I used them for my build.

Stripboard only:

Cuts and wirebridges. As ever, mark the cuts with a Sharpie. Stick a pin through the marked holes and mark them again on the copper side. Then cut the strips where marked with a sharp hand held 6 or 7mm drill bit.

Here's the Bill of Materials. If you make your own Vactrols then refer back to the Lopass Gate project to get more info on the LDRs I used there. There's a complete paragraph discussing the Vactrols in that article. I did not include any bypass caps in the BOM so if you want them, order two 100nF ceramic caps and solder them from positive to ground and from ground to negative power at the top of the stripboard.

Here are some pictures of the finished module. Mine is 6hp wide (3CM) and about 5,5CM deep which is a lot but it will fit in a Nifty Case, no problem.

The two striped components on the top near the power header are just 100nF bypass caps. You can include them of leave them out. They're not really necessary and I left them out of the layouts and bill of materials. 

In front of the vactrol you can see a little pin header. There's also one on the other side that you can't see. I soldered those in for test purposes. I could connect my multimeter probes to it to measure the resistance of the LDRs in the vactrols.

SCOPE IMAGES:

Here are some screenshots from the oscilloscope. In the three images below the yellow line is the output signal, the blue is input A and the purple is input B.

The first image represents the crossfader with the Initial Voltage potmeter fully counter-clockwise so only the squarewave is let through to the output. There is no CV input present in any of these screenshots.

In the second image the Init potmeter is in the middle position and we get a mix of both inputs A and B.

As you can see here with the blue and purple traces some of the signal is fed back on to the input signals, changing their character a bit but this is easily explained. When both vactrols are fully active the output and both inputs are separated from eachother by only about 600Ω, the series resistance of the vactrol LDRs, so it's obvious some of the input A signal will find its way to input B and vice versa. This is not a problem however and it can not damage your VCO's. Exactly the same happens when you mix signals with a passive multiple for instance, so nothing to worry about.

And in this third image the Init. potmeter is set fully clockwise so only the Triangle wave is let through.

If you connect a sinewave LFO to the CV input and turn the level up, then the result will be a cycling through these three stages. By turning the Initial Voltage potmeter away from the middle position you can emphasize one of the input signals.

The signal that is connected to the CV input needs to be a bi-polar signal, so a signal that has a negative and a positive voltage phase otherwise it will only fade one of the inputs. If you want to use a uni-polar signal, you could try using a capacitor on the CV input socket to turn a uni-polar signal into a bi-polar signal but I haven't tried that myself.

Here is a little test video I made while I was testing the circuit just after finishing it. You hear me discussing a CV leakage problem, that when the CV Level is fully closed it still had some influence, but that turned out to be a grounding issue, as I thought it must be, and once I put the stripboard behind an aluminium faceplate and wired everything up that issue was no longer there. So problem solved.

Okay, that's it for this project. A simple one just like the previous last few projects because I didn't have time to do really big projects in the last few months. However I am working on some new and bigger projects so watch this space :)

If you have any questions about this project please comment below or post your question in the special Facebook Group for this website.

Synthesizer Build part-53: RINGMODULATOR with AD633 for Eurorack.

This is the MFOS Ring-Modulator based on the AD633 analog multiplier chip and it sounds awesome. It's very small and will easily fit a Eurorack system so naturally no problem for a Kosmo system either. This ring-modulator is perfect for creating bell sounds and all sorts of timbres.

This is the first article in which I did not actually build the project myself. I was asked by one of our Facebook members, Justin Andrews, to make a stripboard layout for a schematic that he found. It sounded like an ideal little project for the website so I set out to make the layout. Justin built it up and it worked like a charm first go. The AD633 is a small DIP 8 Analog Multiplier chip and they can be a bit pricey. They cost about €22,- each. The MFOS article states they are cheap but they seem to have gone up in price. Make sure you get them from a reputable source though, not from AliExpress for a few dollars. Those will be 100% fakes! I looked around and it seems they are no longer in production but there are still electronics webshops who have them in stock so they shouldn't be hard to find.

The other chip in this circuit is a single opamp, the LF411. I don't know why this type was chosen over the normal µA741 and I suppose you can use a 741 if you wish. The pinout is the same. Only the actual opamp connections of the chip are used not the offset controls. The 43K resistor is a bit of a strange value. You can get away with a 47K too I reckon. Not even the 2,2µF input caps need to be specifically that value. They are DC blockers and in that function you can use 1µF upto 4,7µF without any problems. Together with the 100K resistors these caps form a highpass filter with a cutoff frequency of 1.6Hz if you use 1µF for the caps. It's even lower with higher values so no influence on the sound what so ever whatever value cap you use.

Here is the link to the full article on the Music From Outer Space website.

SCHEMATIC.

Here's the schematic for this project. As you can see it can hardly be simpler and there are no trimmers to set so no calibration necessary. If you read the article linked above you'll see that right at the top Ray Wilson says not to build this project as it has been superseded by newer ones but that doesn't mean this design doesn't work of course. Far from it in fact. It works very well. It's just very basic in its setup.

Both inputs, the Carrier and the Modulation input, are AC inputs. They have an electrolytic capacitor in series with the inputs so this ring modulator is only for audio range signals, not for CV. In the MFOS article it is stated in the Bill of Materials that these should be ceramic capacitors so in fact bi-polar, but I would just put in electrolytic capacitors. That always seems to work just fine and they are used in many other projects for the same function without any problems.

The circuit has two settings: Modulate and Multiply. Justin's experience was that the Multiply mode had a more choppy sound with more artifacts and harmonics. You can see in the scope screenshots at the bottom of the article why that is.

The circuit needs signal at synthesizer levels to work well. The input is meant for 10Vpeak-to-peak signals so if you want to use it for lower level signal you are advised to amplify those first to at least a few Vpp before they enter the ring modulator. 

The circuit is designed to run on +/-12V but I don't see why it wouldn't run normally on +/-15V either.

There is an updated version of this ringmodulator called the Sonic Multiplier which is more difficult to build and has a quad opamp in it and uses an internal sinewave generator with an LM13700. I have not made layouts for it but here is the link to that project on the MFOS website:

---  CLICK HERE  ---

LAYOUTS:

Here are the layouts I made for this project and they have been verified. Beware the size of the stripboard is only 16 strips by 26 holes. I see I made one little oversight in the layout design. It would have been better if I had put the power connector at the bottom instead of at the same side where the faceplate is meant to go. But it still works fine of course :) 

Beware the negative 12 Volt is the top connection and the positive 12 Volt is bottom connection of the power header.

Wiring diagram:

Stripboard only view:

Below are the cuts and wirebridges as seen from the COMPONENT SIDE! As always, mark the cuts on the component side and then stick a pin through the marked holes and mark them again on the copper side. Then you can cut them with a sharp hand held 6 or 7mm drill bit.

Bill of Materials. I typed this bill of materials in Notepad so it's a bit small:

PICTURES.

Here are some pictures of Justin's work. He did a great job and made a really cool faceplate for it too, in Eurorack size. 

The finished module. Justin used waterslide decals for the faceplate artwork and sealed it in with a coat of clear lacquer:

Oscilloscope screenshots:

Here are some scope screenshot combining different waveforms in both Modulation and Multiplication Mode so you can see the difference in processing. I put multiple images together in one to save some space.

DEMO VIDEO.

And finally a demonstration of the ring modulator in action.

So that's all for this article. Not much of a write up but then again there's not really much more to say about this Ring Modulator. It does its job and it does it very well. If you have any questions or remarks you can put them in the comments below or post them in the special Facebook Group for this website.

Synthesizer Build part-50: UTILITY LFO for EURORACK.

This is the Ken Stone Utility LFO, the bigger version of project 47 with an extra feature that I added. I managed to make the stripboard even smaller than the 'Simple Dual LFO' so it fits even the 'Nifty Case' Eurorack skiff. The LFO has the following waveforms: Pulse, Square, Saw to Triangle to Rampwave and a Variable output which has a mix between Square and Triangle/Saw/Ramp waves. I later added a mini-mixer that adds the two variable outputs together to get even weirder waveforms.

An other LFO might not be the most exciting of projects but I think this LFO will be worth it because of the weird waveforms it can make and because I realized, now that I have my own Eurorack system, that modulation sources are important to have. To quote a popular YouTuber: Modulation is what makes Eurorack interesting. I even added a feature of my own later on. It's the ability to mix both variable outputs together to get a variable A+B output. That's at the bottom of this article.

The Dual LFO from project 47 is just the first two stages of the Utility LFO without the mixing stage. Because I actually use the Dual mixer in my Eurorack setup and I loved the idea of having the full Utility LFO available, I thought I'd build the whole LFO this time and try and make it as small as possible. It is 49mm deep and the panel is 9HP wide (4.5 centimeter). This is a bi-polar LFO meaning the waveforms go both positive and negative in voltage.

This LFO does not have a sync option but this is a really useful LFO for a Eurorack system because it has not only the normal waveforms you'd expect but the option to merge waveforms together which makes for some very weird modulation possibilities. And because this is a Dual LFO you could mix the 2 variable waveform outputs together in a Multiple and make even weirder shaped waveforms. That mixing needs to be done outside of this module though. I couldn't fit that inside this design. This LFO would pair really well with the Dual Voltage Processor for that reason.

I managed to get everything on an 18 by 24 hole wide piece of stripboard. Six potmeters and eight output sockets on the panel, and an extra little print for the two bi-colour rate indicator LEDs which I made separate from the main print just like on the Dual LFO and as mentioned before a second little print to mix the two variable outputs together. The panel I made for it is 9 HP wide (4.5 centimeter). You might be able to make it even smaller if you use smaller potmeters in the panel. Even though I included an L-Bracket in the layout, I didn't use one. The stripboard is actually hot-glued in place in between the two columns of potmeters straight to the back of the panel making sure no copper strips make contact with the metal of the panel. I did this to keep the overall depth of the module as low as possible.

I did not include a Eurorack power connector on the print although there is room enough left to put one in. Instead I made a powercord with a Eurorack connector at the end so it always stays connected to the stripboard. This is handier because eurorack ribbon cables take up space too and this is a smaller footprint solution with only three thin wires.

This is mainly a Eurorack project but naturally this circuit will work just as well in a Kosmo sized synthesizer. I optimized the circuit to run on +/-12V but it was originally intended to be run on +/-15V. You will get higher amplitude waveforms when you run this on a dual 15V powersupply so if that's a problem for you, you can change the 1K8 resistors I used on the waveform outputs back to the 1K's you see in the schematic. The original schematic uses +/-15V.

SCHEMATIC:

Below is the schematic I used for my layout. I put in bigger timing capacitors because I wanted one LFO running really slow and the other to about 20Hz. That is more useful for my modulation needs but you might want different values so I strongly advise you to do some testing with different capacitor values to get the LFO in the frequency range you desire.

The types of quad opamps and the one dual opamp used in this project are not critical. The schematic calls for TL074 and TL072 opamps but you can use anything with the same pinout. I used two LM324's and an NE5532 dual opamp. Btw, I did not include the transistor with the rate LED as seen on the schematic but I designed my own circuit for that so I could use bi-coloured LED's. It's just two opamp voltage followers (or buffers) feeding the LED's with the signal from the Pulse output. You can connect them to any output you wish but the pulse is the clearest for the LED's.

Here's the schematic drawing. Beware the opamp numbering does not follow the opamp order I used in the layout.

  

LAYOUTS:

Below are the layouts I made for this project. As ever they are verified. I used them for my build. I changed the order of the opamps used from the order on the schematic. I setup one LFO using all the opamps on the left hand side and the other LFO using all the opamps on the righthand side of the IC's on the stripboard. That made it more compact plus easier to keep the overview. Luckily I did not need to make any changes or do any troubleshooting. I built it and tested it and everything worked first go.

You need four 100K and two 500K linear panel potmeters to this project. Measure and test them before you use them. That can save you some troubleshooting later. If you plan on mounting the stripboard to the panel with hot-glue like I did then try to solder the hook-up wires of the righthand side (the side that's glued to the front panel) as far to the middle as possible so you can still get to them once the stripboard is glued in place. This won't be possible with all wires but try. You could also solder them straight to the back/copper side.

Wiring diagram:

Stripboard only view:

Here are the cuts and wirebridges as seen from the component side! As always, I advise to mark the cuts on the component side first with a black waterproof marker pen and then put a pin through the marked holes and mark them again on the copper side. Then you can make the cuts accurately and you can see where the cuts are when you're soldering in the components.

Bill of materials. 

The timing capacitors listed were chosen for my specific needs. I advise you to go by the schematic (47nF) or else test and determine the best values for your needs. You can use the Falstad simulation linked at the bottom of the article to test different values before building.

No bypass caps are included in this BOM because I didn't use any. They are on the schematic and you can put 100nF caps over the power connections on the IC's to ground if you want to. To do all IC's you'd need six 100nF ceramic caps (you don't have to do the LED driver IC).

This BOM does not include the components for the little mixer I added on further down the article. For that you will need 4 x 100K resistors, a 1K resistor and a 200K trimpot and a TL072 dual opamp.

PICTURES:

Here are some pictures of the build proces and the panel I made for it. I had some fill-in panels left from when I bought my 'Nifty Case' Eurorack case and I used one of them to make my front panel. It was already cut to the right height so ideal for this project ^___^  All I had to do was cut off a piece that was 4.5cm wide (9HP) and spray paint it. I labeled everything with an Edding 400 marker pen.

Wirebridges and cuts. I made some minor changes to the layout after I built this so there are a few discrepancies between this picture and the current layout.

Stripboard ready for testing side A:

 

Panel, not yet labelled:

The finished module with the little 10 by 6 hole stripboard for the rate indicator LED's glued to the back of one of the potmeters. If you look closely at the sockets you can see I soldered all ground connections together with one copperwire going round them all. A wire goes from there to the ground connection on the stripboard.

The finished product:

Here's a little demo video I made showing the LFO in action in a little Eurorack setup. You can hear how the 'Pico Voice' VCO changes in sound as I connect the LFO to it. But then I get distracted by the Pico DSP effects module and the 2hp Freez, LOL. Oh well. You can hear the difference it makes anyway :)

(As you may have noticed, I really suck at making demo videos, LOL)

WAVEFORMS:

Here are some screenshots from the oscilloscope showing some of the waveforms. The one below shows the sharpest a sawtooth wave you can get. Pretty fast rise!

Cursor readings at LFO-B squarewave output:

Here is a collection of variable output readings from Variable output B:

To give you an idea of what can be achieved by mixing the two variable outputs in a simple passive multiple, here are eight images I put together to give you an impression. This should sound awesome with very slow running LFO's which is why I soldered a few more caps in parallel over the timing cap of LFO-B.

You can imagine that some of these waveforms, when put through a quantizer, would generate awesome melody- or bass-lines that can be easily manipulated with the LFO parameters. I tried to capture a little of that in the demo video I posted below.

Here's some technical data from the LFO:

LFO A: freq. range: 270mHz to 24Hz. 

LFO B: freq. range.: 41mHz (one full cycle every 24 seconds using a 650nF cap) to 2Hz.

Duty cycle of the pulsewave output goes between 2% and 98%. I made a mistake and used a 100K potmeter for the Shape but it needs a 500K potmeter. I only had one 500K pot so I put it in LFO B and the screenschots above are from that LFO. You can use a 100K for shape but that will significantly lower the range of the duty cycle (20 to 80%) also the slope of the Saw-/Rampwaves will be slower rising. It will also speed up the overall frequency of the LFO.

Maximum current draw = 20mA for both the positive and negative side.

The maximum amplitude of the waveforms is about +/-5V except for the Variable waveform output. When that potmeter is set to the mid point the output amplitude drops to about +/-2.5V because the potmeter acts as a voltage divider so in the mid position the amplitude is half of what it is when it is fully clockwise or counter clockwise. (We effectively have two 50K resistors on either side of the wiper of the 100K potmeter.) These may seem like low voltages but keep in mind that an LFO is usually attenuated anyway because if the changes are too big it just doesn't sound good on a VCO or in most patches. If you really need higher voltages you can make the 1K8 resistors on the outputs even bigger.

Here's the link to the Falstad simulation of this circuit. This is the Dual version with one speed potmeter at 100K and the other at 500K as it is in my LFO. You can change these variables by right clicking on them and choosing 'Edit':

--- CLICK HERE --- 

EXTRA FEATURE:

Okay as I write this it is four days since I posted this article and the cool looking results from the mixed variable outputs kept going through my mind. I really wanted to incorporate that into this design and now I found a way. I used an other small piece of stripboard on which I soldered a dual opamp and wired it up like the mixer in article 17 with 2 inputs. I knew there was no way to put an extra output socket on my panel so I sacrificed the squarewave output of LFO-B. That was the least useful of the outputs because I can get a squarewave anyway from the pulse output if I need one and I still have the squarewave of LFO-A. So I carefully soldered in the little mixer print which was just as big as the little LED driver and wired it all up and I re-labelled Square output B into 'Vari A+B'. I used a 200K trimmer to go over the inverting input of the second opamp and the output so I could adjust the gain and make it a little higher than the Variable outputs. I placed the little print above the sockets and used hot-glue to stick it in place. It is actually glued to the little LED driver board. This works perfectly! Now I have an output with two variable waveforms mixed together coming out of it.

Here is a picture showing how I added the mixer. I used two rigid copper wires to tap the signals from the sockets and lead them into the mixer. The non-inverting inputs of the opamps are connected to ground on the copper side:

Here's the layout of this little mixer. Mine is even smaller than this layout, but this will work fine. All resistors are 100K:

Here's the schematic for this little mixer:

DEMO VIDEO:

Finally I want to show you a little experiment I did using the new output. I put the Variable A+B signal through a quantizer (the 2hp Tune) and from there into the Pico Voice Wavetable Oscillator. The audio then went through the Pico DSP for some added reverb. You can really generate the weirdest melodies with this although this setup would benefit from the Voltage Processor because the negative phase of the signal does not produce any notes so it needs a positive offset voltage and the signal can do with some attenuation to get the notes closer together but I think you get the idea watching this short demo:

Okay, that's it for now. Article 50, wow I can hardly believe it. This journey started for me in October 2019 and I knew nothing about synthesizers then, but I was determined to get to grips with it and learn as much as possible. And what better way to learn then to build your own modular. So here we are more then 3 years and 50 projects later with a cool collection of builds helping hundreds of people to do the same. I'm really proud of what, not only I created but also of all the people who helped so much along the way with comments and directions. I'm not gonna name names but you know who you are, all of you. Thank you!! So many people told me they find the site a great help in their hobby and that's the biggest reward I could wish for. I am however going to wind down the DIY aspect of the hobby because having now built my own system and also haven gotten into Eurorack, I need to spend more time actually using it and figuring out how to use it all together. But I will remain available on Facebook and here to answer questions and help as much as I can to ensure you have a good experience using this website and get as much enjoyment out of it as I did.

PCB Version.

I recently made some Eurorack friendly PCB's for this LFO including the Variable output mixer I added on.

Here's the KiCad schematic I made for this LFO:

To get your PCB, goto the Menu and click on the top option called 'PCB Service' to find these circuitboards.

If you have any questions please put them in the comments below or post them on the special Facebook Group for this website where there are some awesome people willing to answer your questions.