Synthesizer Build part-56: VCF-1 STATE VARIABLE FILTER by THOMAS HENRY.

 A very easy to build and awesome sounding state variable filter by Thomas Henry. For Eurorack or Kosmo systems.

I have covered many different types of filters on my website but I had not yet built a State Variable filter. What does that even mean I hear you ask. Well state variable means that it can simultaneously provide two or more types of filtering. In this case the filter has Lowpass, Highpass and Bandpass outputs and rather than having to switch the filter into these different modes, you have them available together, so each filtertype has its own output.

I found this particular schematic on "Birthofasynth.com", a website that has all of Thomas Henry's projects on it.

In the article about this filter he calls it a 'barebones' filter. Very much taken straight from the CA3080 datasheet. That may be so but it's still a great sounding filter. The way it sounds reminded me of the Steiner-Parker filter but I think it may even be better. It has a bit more edge to it I think.

It's a really old school filter with a 12dB/Oct. cutoff slope (2 pole filter) and it sounds like a 70's synthesizer filter should sound. It's pure sounding, phat in the low end (especially using squarewaves in Lowpass mode) with a beautiful but little bit agressive resonance. I own a Behringer Odyssey and that synth has a choise of three filters and one of them is a 12dB/Oct. filter and that is my favourite type of VCF. I always have the Odyssey set to the two pole filter. Anyway I urge you to read the article I linked to above if you want to know more about what TH said about this filter and its development.

THIS FILTER WILL RUN FINE ON BOTH A DUAL 12V OR A DUAL 15V POWERSUPPLY!

THERE ARE NOW PCBs AVAILABLE FOR THIS FILTER. Please click on 'PCB sevice' in the menu and scroll down to find them.

SCHEMATIC:

Below is the schematic I used for this filter. It says the filter is to be used with a dual 15V powersupply but I did all my testing running it on a dual 12V powersupply because many of you will be building this for Eurorack and it works just fine. I also used a Eurorack friendly size of stripboard which is 24 by 41 holes. That will fit behind a Eurorack panel.

As you can see the filter uses two OTA chips, the AS3080. These are the modern version of the original CA3080 chips and I believe these are less noisy than the originals too but they have that original sound. I got mine from Electric Druid, They are also available at Thonk and other online retailers. 

The LM13700 OTA chip also has two CA3080 chips inside and can be used in this circuit but then you'd have to design your own layout because that is a DIP16 IC. For this particular project you'll need the AS3080 chips. 

The numbering of the opamp pins has been changed to fit the layouts below, because I used the opamps in a different order to the original schematic.

The schematic shows a two transistor exponential converter with a PTC as temperature compensation. which we already know from the Thomas Henry 555-VCO. The temperature compensation is only useful if you intend to use the self oscillation of the filter as an extra oscillator. I never used a filter in this way and I can't imagine any of you will ever use the filter for that purpose so you can leave out the PTC and just use a 2K resistor. That's what I did eventhough I have these PTC's in my stock. In fact Thomas Henry himself used a 2K resistor as he mentions in the article linked above.

You do have to match the two PNP transistors though. I matched them as I always do just by measuring the Hfe on my multimeter and picking two transistor that measure the same value.

You might use the filter in full self resonance mode if you're looking for a special sound effect. I tested it and it will track with the keyboard because it has a Volt per Octave input. I have not tested how accurate the tracking is but I do think you can make it track over a few octaves if you want. Beware that the self oscillation is about twice as loud as the normal audio you get from this filter!!

You can leave out the Frequency Fine Tune potmeter too because that's only there to tune the self resonance for tracking. As a Cut-Off Frequency potmeter it is pretty much useless. One thing you can do is change the 3M3 resistor to a 100K and add a socket to that potmeter so it turns into an extra CV input with level control. Wire it up like the Envelope input. That's what I did myself. I find two CV inputs a necessity for a filter.

LAYOUTS:

Below are the layouts I made for this build. As always they are verified. I used them to build my filter. I was very thorough with checking this stripboard layout for faults before I printed it out and used it to start building my filter. I'm glad I checked it over a few times because I did manage to catch some mistakes in the design fase which saved me some hours troubleshooting I think.

Anyway the build went fine and apart from one transistor being faulty which needed changing out the filter worked straightaway.

Here's the wiring diagram:

All potmeters are seen from the BACK SIDE!

The Cutoff Frequency potmeter is wired up in such a way that the filter opens up when you turn it clockwise. The Resonance potmeter is wired up so that it gives more resonance when you turn it clockwise going into self oscillation when turned fully clockwise. It looks asif the wiring of the Resonance potmeter is the wrong way around but that's not the case. Resonance increases when the wiper moves clockwise towards the connection to ground. Resonance potmeters are usually wired up like this.

A little remark about the Frequency Fine Control. Only include this potmeter if you intend using the filter as an oscillator with the resonance at self-oscillation. As I mentioned earlier, the Frequency Fine Control is meant to tune the self oscillation of the filter so that it tracks with the keyboard. In itself it has very little influence on the CutOff Frequency so if you intend to use this filter just as a VCF and the tracking accuracy of the self oscillation isn't important to you than leave out the fine control potmeter. It will save some space on the faceplate too. I myself left out the Fine control potmeter too. I changed the 3M3 resistor for a 100K one and connected a second CV input to that point, complete with level control potmeter like the envelope input. Here's a detail of what that looks like:

Below is the stripboard only view. A little tip, when soldering in the trimmer potmeters put the wipers in the middle position. That way you won't have to do much tuning when you're testing the filter. 

Here's an overview of the cuts and wirebridges. Start soldering these in first before you solder in any components. There are 35 wirebridges to solder in. Make sure you're very very accurate here. It's easy to make a mistake and one wirebridge in the wrong position and the filter won't work.

And finally the cuts only as seen from the component side. As always, mark the cuts on the component side first with a waterproof Sharpie or Edding 3000 and then stick a pin through the marked holes and mark them again on the copper side. Then you can cut the strips at the marked places with a sharp hand held 6 or 7mm drill bit. This way you have the least chance of making mistakes.

Once again you need to be very accurate here because the component placement leaves no room for errors.

And here's the Bill Of Materials:

If you're going to build this module for Eurorack then I urge you to order miniature potmeters. You're going to need all the space you can get. There are 5 potmeters to accomodate and 7 sockets.

Adding extra's (like an LED and 2nd CV input):

As you can see there's only one audio input but it is easy enough to add more inputs. You can simply connect them through a 100K resistor to pin 9 of the TL074 and they will be summed together. You can also put in more CV inputs if you wish by connecting them through a 100K resistor to pin 2 of the TL074, which is what I did.

   The trimpots for the Offset control could actually be left out. They are not really necessary. They are part of the design because the early versions of the CA3080 chip (which was originally used in this filter) had quite a bit of variation in their offset voltages. But the latest generations don't have that problem anymore. I left the trimmers in because I stayed true to the schematic but it's up to you. The filter should work fine without them. Should you take them out then you can remove the trimmers and the 100K resistors in series with the wipers. Leave the 22 Ohm resistors to ground in place.

   Finally we have an opamp left unused! We have to do something with that right? :-)

I always like to include a LED if I can, so I altered the layout a little and made a jumpwire from the envelope input to pin 3 of the TL072. The green wirebridge from pin 10 of the TL074 needs to be lengthened and soldered straight to the bottom strip (X). Now we have configured the opamp as a voltage follower or buffer so we can connect a Bi-coloured LED to it without drawing any current from the envelope input. I used a big 4K7 resistor as current limiter so the LED will only be the brightest with the highest voltage. Below is the layout to show this alteration. 

The components for this change are not listed in the Bill of Materials because this was done as an after thought, but it's only a LED and a resistor. I used a red/blue bi-coloured LED.

ABOUT THE AC/DC SWITCH:

You may have noticed the audio input of the filter has an AC/DC switch in it. This is provided for instances where the full audio bandwidth of the signal is desired. DC coupling allows very low notes to pass through uninhibited by the input capacitor. This is actually the only filter on this website (apart from the ARP2600 LPF) that doesn't have a capacitor in the audio path, when switched to DC that is.

A DC signal can pass straight through the filter without ever encountering a capacitor so the filter can actually process a control signal! This opens up an entirely different can of worms. (In a positive way.)

For example you could patch the CV output of a sequencer through the Lowpass mode of this filter before it goes into a VCO. If you then modulate the cutoff frequency with an LFO you can create some really spacey effects.

CALIBRATION:

This is the procedure for the V/Oct. tracking of the filter in full self oscillation:

Connect the lowpass output to your VCA so you can hear the signal. Be careful, the self oscillation signal is quite loud! (Very loud in fact)

Connect a keyboard or other V/Oct. source to the 1V/Oct. input of the filter.

Put the filter in self oscillation by turning the resonance full clockwise and adjust the V/Oct trimmer for as close to a one Volt per Octave interval as possible. Go between C2 and C3 on the keyboard for instance and turn the trimmer to get the best result. Use the Frequency Fine control to help you tune the oscillator to the right notes, if you kept this potmeter.

About the trimmers.... what I actually did was put them in the middle position and just leave it at that. There really is nothing to trim because the modern AS3080 chips don't have erratic offset voltages in their output like the old CA3080's used to have. That's why this option is built into this filter, to trim away the offset from the CA3080's but with the new AS3080 there is no offset voltage.

So I didn't do any trimming and the filter works fine but I'll give you the procedure anyway, but as far as I'm concerned, you can ignore it.

Here is the procedure as mentioned in the official article: 

Put the trimmers in the middle position. With multiturn trimmers you start turning them untill they start clicking. Then turn back and count the number of turns until the wiper is at the other side and you hear it clicking again. Now turn the wiper half the number of turns you counted. Then it's in the middle position. You could also just measure the voltage coming from the wiper and turn until it's at zero Volts. Then it's in the middle too.

Connect a square/pulse wave to the filter input and monitor the Lowpass output with an oscilloscope. Now check for DC deflection as you turn the Frequency Cutoff potmeter through its range. Adjust the trimmer to get the least DC deflection at the output. 

The trimmers are interactive (they influence eachother) so you may have to go back and forth between them a few times.

There should really be very little to trim. As I mentioned earlier, the filter will actually work fine even without these offset trimmers and with the AS3080 there shouldn't be any offset voltage to speak of.

PICTURES:

Here are some pictures I took during the build proces:

As long as I was waiting for the new powder coated aluminium to come in the mail, I thought I'd make a template for the panel out of cardboard, so I can mount all the components in it and see if all the wiring is long enough. As you can see I changed my mind about the switch placement and I needed to lengthen several wires but it all fits nicely behind a Kosmo sized panel of 20 x 7.5 CM. I want to install this filter in my DIY synthesizer, not my Eurorack system. I think, if you want to fit it behind a eurorack panel, you have to make it a bit wider still.

Here's the finished module all ready to go mounted in my DIY synthesizer: I did all my testing running this filter on +/-12V and it was absolutely fine. But in my synthesizer I have two powersupplies, one for +/-12V and one for +/-15V, so I decided to connect it to the +/-15V supply for permanent use. 

(Boy, does this thing sound good. I love it!)

I wrote the labels with a white acryllic pen. I got new pens and this one is not as scratch resistant as the old pens I used to have so after I finished labeling everything I sprayed the panel with a layer of clear lacquer.

The stripboard is mounted behind the panel with a piece of plexiglass that I bent at both ends so it grips the stripboard like the fingers of a hand. I glued two little pieces of plexiglass at the ends so the stripboard can't slide out and I secured it with hot glue. Then I drilled a 3mm hole through one end and mounted it to the panel with an M3 bolt. I also used superglue to secure it and keep it from rotating should the bolt get loose.

When I first tested the filter I found that the Frequency control wasn't working. The Resonance was fine and I could see on the oscilloscope that the filter was doing it's thing but no Frequency control. I took my scope probe and tested the legs of the transistor pair and sure enough. Transistor Q2 was not working. So I put in a new matched pair of transistors and now everything was fine. It all worked as it should. Strangely enough the transistor was not faulty. It must have been a bad connection.

Here is a video of me testing the filter and the different outputs. It's a video I also uploaded to my YouTube channel.

I found with testing that the Lowpass sounds best with a squarewave on the input. The High- and Bandpass filters sound the best when you use a Sawtooth wave on the input.

Here's a little test video with a demonstration of the extra CV input I installed (with level potmeter). I have a sinewave connected to the second CV input. The rest is like the previous video:

Okay, that's it for now. Enjoy building this filter. It's a really good one!

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

If you enjoy this content and would like to support these projects and the upkeep of the website then you can buy me a coffee. There's a button for that underneath the main menu if you're on a PC or Mac. Otherwise you can use this Paypal Me link and cut out the middle man. Donations on Ko-Fi give you access to my blog there where I occasionally post about projects I'm working on.

Thank you!!

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 Extra's No.4: QUADRUPLE OFFSET BOARD for 3340 VCO.

 A simple little expansion board that you can use to turn the 0 to +10Vpp signals from the Digisound 80 VCO (Project 18) into -5/+5Vpp signals to make it more compatible with the rest of the modules on this website.

The one thing that always bothered me about the Digisound 80 VCO is that the outputs signals are unipolar. They're 0 to +10 Volt peak-to-peak and that is not very compatible with the rest of the builds on this website. Because this Digisound 80 VCO is the most popular project on this website, I thought I would design a little stripboard that gives you 4 offset options to turn all the signals of that VCO into more useful bi-polar signals at -5/+5Vpp.

There are more elegant ways of doing this perhaps but this project is meant more for people who are beginning in the DIY synth hobby and who are building the Digisound-80 VCO as their first VCO and they want bi-polar signals from that VCO. If you're one of those people you can build this project to solve that problem. It's a very easy to build and cheap project.

Btw, the Digisound 80.6 Lowpass filter works well with the Digisound VCO because it has a 1µF capacitor on the input that shaves off the offset voltage But I don't recommend capacitors on the VCO outputs because they can also act as filters.

OPTIONS:

You don't have to build this module into the 3340 VCO (project 18) if you don't have room. You can put this stripboard behind a small panel with just 4 input sockets and 4 output sockets and attach that next to the VCO. That way you have a choise of either using the outputs straight from the VCO at 0 to +10V or to patch them through this offset board and get -5/+5V output signals. That way you can also use the offset module for other things like LFO's if you want to. 

You can even 'normal' the VCO outputs to the socket-switch lugs of the offset input sockets and save yourself the trouble of having to use patch cables.

You can even add bi-polar LEDs on the outputs so you have a visual indication of the Voltage they output. As you see, if you want, you can really go mad with this project but I leave that up to you. 

Anyway, we also have the Dual Voltage Processor project to cover that functionality and it has extra options too so maybe it's better to keep this project simple.

I put in four offset stages eventhough three will be enough for the 3340 VCO so you can use the other for something else.  

SCHEMATIC:

It's a very simple design. Just 4 dual opamps, in this case TL072's (but you can use other ones if you wish as long as the pinouts are the same) each with an offset trimmer that allows you to give a negative 5V offset to the signals coming from the VCO and so turn them into bi-polar signals.  I choose to give every stage its own offset trimmer so you can set them all differently should you need to, but in principal you could feed all four opamps with the voltage coming from one trimmer and so have them all produce the same offset. That's simpler but not preferable I think so I went with four trimmers.

The schematic below shows two of the four offset circuits that are on the stripboard but they are all the same.

If the output voltages of the VCO waveshapes are 8.2V instead of 10V and you want to crank it up to produce 10Vpp (-5/+5V) waveforms then change the 100K feedback resistor over pins 6 and 7 to a 130K. That will give just enough gain to get the right output voltage. 

LAYOUT:

Below is the layout for this project. 

(Layout has been updated on 9th of March 2024. Previous version had 2 little mistakes in it.)

Below is the layout with just the cuts and wirebridges, seen from the component side. As ever, mark the cuts on the component side with a waterproof Sharpie. Then put a pin through the marked holes and mark them again on the copper side. Then cut the strips with a sharp hand held 6 or 7mm drill bit.

If you decide to build this into the Digisound VCO module, then it may be better to do away with the eurorack connector and simply use wire connections for the power. That way you can make the stripboard more compact too. The lower 4 copper strips are not used either so you can cut those off too or use the space to house the standoffs to connect the stripboard to the rest of the VCO.

I did not actually build this project myself but I know it should absolutely work the way it's presented here so that should not be a problem. I built so many offset circuits while I've been doing this hobby that I can dream them.

Here's the Bill of Materials for this project. Also order four 100nF ceramic bypass capacitors. I forgot to put them in this list.

TUNING THE CIRCUIT:

You need an oscilloscope to set the offset trimmers to the right value. Make sure you set the oscilloscope to DC when measuring otherwise the scope won't show offset voltage. Remember offset voltage is a DC voltage.

Connect the signal(s) from the VCO output to the input(s) of this stripboard and then connect the scope probe to the output(s) and set the offset voltage so that the signal displays the same amplitude on the positive side as the negative side of the zero Volt line. In other words, set it so the zero Volt line cuts nicely through the middle of the signal. That's it. 

Okay, that's all I have to say on this little extra project. I thought it might come in handy because the Digisound-80 VCO is really a very cool VCO and now you can make the signals more compatible with the rest of the projects on this website.

I hope it comes in handy.

If you have any questions about this project feel free to comment below or on the special Facebook Group for this website.

Synthesizer Build part-54: JOYSTICK CV Controller (Eurorack).

 An easy to build joystick module that outputs 2 CV voltages to control anything from pitch to filter cutoff and anything else that can be changed with a control voltage.

The finished Joystick module installed in a Nifty Case.

Before I started building my modular synthesizer I had a brief try at flying FPV drones. I bought all the gear and some cool drones but the damn things were way too fast for me to control. This was before the stabilized DJI FPV drones were on the market. Anyway.... the gear landed in the coupboard for a few years. I have now successfully taken the hobby up again and now I can fly FPV drones but my transmitter/controller was now outdated so I used one of its gimbals for this project. I found a good schematic on the Mod Wiggler forum.

Close-up of the circuit:

The image below shows the circuitry for one axis. You need two of these circuits to work both axis of the joystick, left and right [X-axis] and up and down [Y-axis].

HOW THE CIRCUIT WORKS:

It's a very simple circuit. Each of the 2 axis of the joystick is assigned two opamps. The voltage coming of the joystick potmeter goes into the inverting input of an opamp and added to that is voltage from the Zero Point trimmer to make sure the voltage is at zero when the joystick is in the rest position (middle). The gain of the opamp is adjustable with the 1M potmeter marked Range. This determines the maximum voltage you get when you push the joystick fully to one position. This goes from 0 to 10V max. when used with a 12V powersupply.

The CV voltage then goes into a second opamp which has an offset potmeter so we can turn the signal into a unipolar one if we want (all positive or all negative voltage) or just give it some offset or even just to make sure the voltage is zero when the joystick is in the middle position. 

This module is meant for Eurorack (dual 12V powersupply) but it will run just as well on a dual 15V powersupply and if you build it for a Kosmo or 5U synthesizer you have more space on the faceplate to accomodate some extra features.

The joystick I used came out of a Taranis QX7 RC controller/transmitter and it has the following resistance values:

When in the middle position (rest) the resistance is 1,31kΩ. Fully right is 2,15 kΩ and fully left is 550 Ω. Same for the up-down potmeter.

The circuit will take a wide range of joystick resistance values so practically any joystick can be used.

I left the springs installed so the joystick always returns to the middle position when let loose.

Ideas for extra features:

The circuit is very bare bones but you can extend it with, for instance, a momentary switch that cuts the CV voltage if you push it, or one that makes contact if you push it and so outputs an extra gate signal.

An other idea that was suggested to me is to have two input sockets with the voltage connected to the socket switches (normalized) but then you can input an audio signal that cuts the voltage and then the joystick controls the amplitude of the audio thus creating a Manually Controlled Amplifier (MCA).

I'll leave all that up to your imaginations. I didn't have room for extra functions on my panel so I left it as presented here.

I did put in two bi-coloured LEDs to give a visual representation of the voltages on the outputs. It glows red for positive and blue for negative voltages. I connected them straight to the output sockets but with a big 10K current limiting resistor so they only glow at their brightest with the full voltage applied and don't pull down the CV outputs. Also to keep the number of components to a minimum. It works like a charm and looks very cool. Their brightness is a good indicator for the amount of voltage present at the output sockets.  They start glowing at around 2V and then get brighter with higher voltages. I mounted the LEDs above the joystick so they are in full view.

You can use any type of quad opamp for this circuit. I used one of my fake LM324 chips from China and because there are no high frequencies involved it works just fine. You can use a TL074, TL084 etc. They all work fine as long as the pin-outs are the same. It's a good idea to use miniature potmeters for the offset and range controls to save some space on the faceplate. The offset potmeters don't have to be 10K, I used 100K potmeters myself. The range potmeters do need to be 1M otherwise the range of the range will be different ^___^

The trimmers can also be different values. I used 200K trimmers. Afterall they are just voltage dividers in this circuit, so the value is not that important.

CALIBRATING:

The zero point is the point at with the joystick is at rest, right in the middle and in this position the CV outputs must be at zero Volts. You set the zero point with the two multiturn-trimmers.  

The best way to set the zero points for both axis is to have both the Offset and the Range potmeters at the 12 o'clock positions and then connect the CV output to an oscilloscope or volt meter and turn until the voltage is zero. 

Then set the scope or meter to a more sensitive setting and again correct until it reads zero Volts. Try to get it as accurate as you can. After you're done calibrating both channels you don't have to touch the trimmers again. 

Here is the schematic I used for the layouts:

I made a Falstad simulation of the circuit which you can see by clicking here.

Here's one observation I made about this circuit. The voltages from the wipers of the joystick potmeters go through a 51K resistor into an opamp, the gain of which is determined by the 1M potmeter (Range). I noticed that the Range potmeter reaches its maximum at about 1/3rd before the full clockwise position is reached. I think this is due to the 51K resistor. I think it will be better to put in a 91K or even a 100K to get the gain in step with the throw of the potmeter. 

The gain of this stage is determined by the formula: Av = (-Rfeedback/Rin) = (-1M/51K) = -19,6 (the minus simply means the output is inverted). This is too much and that's why the potmeter reaches full gain way before it's turned fully clockwise. With a 100K the gain would be -10 and that would result in the full throw of the potmeter being used. To play it save and make sure you get all the gain you can before you reach the fully clockwise position of the Range potmeter I would suggest using a 91K resistor instead of the 51K on the layout. I've changed the Bill of Materials to include two 91K resistors. However I have not made this change in my own module because I can't access those resistors easily anymore, so I can not guarantee it will fully solve the potmeter throw issue but I can't see why it wouldn't work because the mathematics says it will.

The Falstad simulation doesn't really show this discrepancy so do not rely on it for component values. 

LAYOUTS:

Here is the layout I made for this circuit. It is verified, I used it to build my project. It is small enough to fit flat behind a 14hp Eurorack panel. Beware there are two copper strips underneath the IC that are not cut. They connect the grounded pins together. Pins 3 and 12 and pins 5 and 10. 

There are three 100nF caps visible in the layout but I also put a 100nF cap over pins 6 and 7 of the IC. This is to suppress any voltage spikes or noise. This cap is not visible on the layout and because I had no room for it on the component side I soldered it straight to the pins on the copper side. So there are 4 caps in the Bill of Materials. (I didn't use any bypass caps myself but they are in the layout and B.O.M.).

Here is the stripboard only view. 

Here is the layout for just the cuts and wirebridges. 

As ever mark the cuts at 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 dril bit.

And finally here's the bill of materials. It's quite a cheap project if you already have a joystick in stock and anyway, joysticks aren't that expensive if you know where to look. The resistance value of the joystick potmeters isn't that critical. The circuit just uses them as voltage dividers so any value will work. They usually don't go down to zero Ohms. The one I used goes from 550 Ω to 1K3 to 2K15 in the lowest, middle and highest positions.

You can find joysticks on AliExpress for under $20,- for a pair. Just Google: "Radio Rocker Joystick 5K." Those should work just fine.

How to determin which wire is for up and which for down, left or right with a joystick.

Connect an Ohm meter to the middle wire and one of the outer wires of one of the potmeters on the joystick. Say for instance we're looking at the potmeter for the Y-axis (up and down). Now we measure the resistance while moving the joystick up. If the resistance goes down you have the correct wire for the up position. If the resistance goes up that wire should go to the down position on the stripboard, for the Y axis. So if you have the correct wire for a specific direction the resistance between the middle wire and that wire should go down when moving the joystick in that direction, because the wiper of the potmeter moves closer to it. I hope that makes sense.

Here's a screenshot from my oscilloscope. Yellow = X-axis, Blue = Y-axis. In this picture I moved the stick to the outer most positions and you can see both voltages land on exactly 10V maximum with Range turned fully clockwise and no offset applied.

PICTURES:

Here are some pictures I took during the building process:

This is the faceplate I made. Notice the two square holes. I tried fitting two push switches for extra Gate outputs but I came back on that idea because I didn't have enough room to accomodate that.

I made the big round hole with a hand held jig saw.

The finished face-plate with everything installed but without the stripboard. As you can see the knobs are very close together which isn't ideal so when you design your own faceplate for this module take some time to find out the best places to put these potmeters. If you use miniature potmeters you have more room to move them about to find the best placement.

Below is the stripboard with all components mounted except the power connector. The bottom two strips I later cut away go have some more space for the gimbal to move because when I tried to mount the board behind the panel I needed a bit more space. The bottom two copper strips are not used so I could just cut them off.

Here's how I mounted the stripboard behind the panel. I used some plastic tube as a stand-off. If you do the same, drill a few small holes in the sides very near both ends so the glue can run into those and provide a good grip. Then I hot glued that to the back of the panel, making sure the glue flowed around some of the mounting screws for the joystick, for extra grip. Then I hot-glued the stripboard to that stand-off after the wiring up was all done. I had to be careful not to disrupt the movement of the joystick gimbal, keep that in mind when mounting the stripboard behind the panel. There's almost no place to drill a hole through the stripboard for a normal M3 threaded stand-off so this seemed like the best solution. Works fine.

And here's the finished product. Front view:

Back side. The depth of the module is just under 4 centimeters. It's 14hp wide (7CM):

Finally a little demo video of the module in action in my 'Nifty Case'. This is just a simple patch I put together in 5 minutes. The X-axis CV is controlling the cutoff of the filter in the Doepfer A-111-6 synthesizer voice and the Y-axis CV is controlling the reverb amount from the FX-Aid.

Okay, that's it for this one. Quite a simple build. The only thing I did wrong was that I forgot that the wipers of the offset potmeters connect to the inverting inputs of the opamps so I had the offset potmeters wired the wrong way around. An easy fix. This is a very easy to build module and, I think, a very useful one especially for live performing. It's in fact the equivalent of a synthesizers modulation- and pitch-bend wheels all in one.

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

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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.

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