Synthesizer Build part-29: VCA (Yusynth design).

A good working and easy to build VCA for use as extra VCA or as the endstage and line out for your modular set-up. The VCA as presented in this article is specifically meant for audio, not CV however if you leave out the input electrolytic caps you can use it for Control Voltages.

This module is a great solution for all your VCA needs. I needed a new VCA module for the second stage of my synthesizer and I wanted to upgrade from the first one I built, although it functions fine I hasten to add. This VCA is a more luxurious version of the first one you could say. And this one doesn't invert the signal like the first one did.

The function of a VCA:

Before I get ahead of myself, let me explain what a VCA does for all the people who are new to DIY synth building. A VCA or Voltage Controlled Amplifier, is used to stop your synthesizer from continuously making sound and to only produce sound when you press a key on your keyboard. So it's like a volume knob that is only opened up if it receives a signal from the Envelope Generator. Now the higher the voltage from the envelope generator (or ADSR)  is, the louder the output of the VCA will be. VCA's can of course be used for other things, like in drum machines, as a sort of Gate, to let through short pulses of noise to create percussion sounds but this is not what we're going to be dealing with here. This VCA is primary used as the last stage in your synthesizer from where the audio goes to the HiFi amplifier and the speakers. But the output must be further attenuated to make it suitable for the line in of a HiFi amplifier. In fact the term 'Amplifier' in VCA is a bit misleading. It should be Voltage Controlled Attenuator because when the VCA is fully open the output signal will have the same strength as the input signal. It has not been amplified.

So to sum up: The VCA lets through audio when it receives a signal from the Envelope Generator and it stops audio from passing through when there's no envelope signal present on the ADSR input of the VCA. Opening up the 'Gain' control will enable you to bypass the Envelope Generator Input and output sound/audio without pressing any key on the keyboard and therefor without any Envelope Signal being present but normally Gain is set to zero.

I hope that's clear but you can always ask me things in the comments if you need more info.

The VCA is capable of handling input signals of + and - 10V and outputs them at the same level if the audio and ADSR potmeters are fully opened up and if you use an Envelope Generator that outputs an envelope of  +10Vmax. This VCA is perfect therefore to pair with the Yusynth 7555 ADSR from article 24.
If you need to output the signal to a HiFi audio amplifier or mixing desk then you must use extra attenuation. You can easily do this with a resistor voltage divider that divides the amplitude of the audio to one tenth of the input amplitude. If you then turn the audio and ADSR potmeter back, the signal will be low enough to go into the Line-In of a HiFi amplifier or mixer. There's an extra additional layout below to show you how you can do that.
As to running this VCA on a dual 12 Volt power supply; I don't think there will be any problems with that. After all the opamps and transistors all work fine on 12V so I think it's just a matter adjusting the trimmers. You could have an issue where one of the trimmers is turned all the way to its limit because of the lower voltage but I have no data on this so I can't be sure. The first VCA I describe in chapter 10 is purpose built for +/-12V so you can always build that one if you're not sure. I've build 3 of those so far and they work very well. As you can see on the layout below, this design has one extra potmeter compared to the earlier VCA I built, and that's the audio level control potmeter on the input, which is always a good thing to have.

The two BC547 transistors Q1 and Q2 must be a matched pair. It's enough to just stick them in the Hfe meter of your multimeter and select a pair that have the same Hfe value. When you measure them let the transistors cool a little after you touched them because their values will change with temperature.

Here's the verified stripboard layout. Wiring diagram:

(Last revised: 16-Aug-2021: corrected a mistake at the input opamp.)

Print only. Btw, the green wire-bridges indicate connections to ground:

This is the schematic from which I made the layout. In my build I have the 10µF electrolytic capacitors on the input as is shown in the schematic below. That means this VCA is an AC version meaning it can not handle signals from a Low Frequency Oscillator (LFO). If you need a VCA capable of handling very low frequency signals (a DC version) then leave out the 10µF caps on the input but for normal audio use LEAVE THEM IN! In my own build I also added a 10µF cap on the output (+ towards the opamp) as an extra failsafe against DC offset voltages on the output but you don't have to replicate that. (It's not in the schematic but it's in the layout as extra option.)

Bill of Materials. The logarithmic potmeters are noted as linear types in the BOM. It hardly makes any difference it's just that a logarithmic taper sounds more natural to the ear but you can use whichever you prefer.

LINE OUT:

If you want to use the VCA to feed a HiFi amplifier then you can use a little voltage divider network to further attenuate the audio output level to make it suitable for line out levels which are usually around 100mV to maximum 1V. I made a little extra layout to show you how you can add such a voltage divider to this stripboard. I used a 1M resistor and a 100K trimmer potmeter to divide the output voltage by at least a factor of 10. You can set the initial output level with the trimmer and then further adjust the level with the ADSR and Audio Level potmeters on the panel.

These extra components are not in the Bill of Materials!

Calibrating the VCA:

Before we start, do all the measurements on the 'Normal Audio Output', not the AC one. I added the AC output as an afterthought to block any DC offset voltages from coming through but normally you can use the the Normal Output. So connect your probe there for calibrating the circuit.

With trimmer A2 you set the initial bias voltage on the base of transistor Q3. This influences the working of the Gain potmeter and you should set it in such a way that with the Gain potmeter all the way closed the signal is just muted. If you turn Gain up the last played note will then become audible without pressing any keys on the keyboard. So Gain is normally closed.
I found that the audio signal initially starts at 10V and then drops to about 6V. To counteract this you need to open up the Gain potmeter a little and then trim again so the signal is muted with Gain slightly open, This will stop the voltage drop of the signal and keep it at full power all the time. You will see this soon enough if you start testing it and connect this circuit to a scope. It's easy to counteract and it is in itself not a real problem because you hardly hear the voltage drop but you know, I strive for perfection ^___^
Trimmer A1 is the Balance trimmer. You set it so that the part of the signal that is above the zero Volt line has the same level as the part below the zero Volt line. In other words, you set it so that the signal has the same amplitude in both the positive and the negative part of the wave. This is best done using a triangle or sinewave on the input, together with an oscilloscope.
For trimmer A3 I advise to use a multi turn one. With this trimmer you trim away any DC Offset voltage on the audio output. Again you absolutely need a scope to do this but a cheap 20 dollar one will do nicely here.
The circuit has a LED that indicates the presence of an audio signal on the input. It's a sort of one LED VU meter and the brightness varies with the strength of the audio signal. If you turn the audio input level potmeter up, the LED becomes brighter.

Here are some pictures of the finished product. If you look carefully you can see the electrolytic capacitor on the audio output. This is the AC audio output option to prevent any DC offset voltage on the audio output signal. If you are going to use this VCA with very low frequency signals, like from an LFO, you need to use the DC output option: (If you're a beginner then don't worry about that, use the AC output.)

Normally the AC input is used for all audio signals and the DC input is used for Control Voltages.

You can see that I put a little white stripe alongside the counter clockwise first 20° of the throw of the Gain potmeter, to indicate to where you can turn the potmeter with the audio staying muted. If you turn it past the white, the signal becomes audible and normally the Gain should be slightly open but with in the white area.

The day after finishing this module I built two extra VCA's using the old layout and they work fine too. I wanted a few extra VCA's available but there was no need for the luxurious model. I'm going to use that as the end stage VCA in my second stage, with the output going to a HiFi mixer (at Line Out level) and use the other two VCA's for use in different patches.

Here's some pictures of the double model. I also posted these in the first VCA article:

Finally I want to leave you with this very informative video by Moritz Klein about the ins and outs of differential transistor VCA's. He explains why the transistors need to be matched and goes into the technical details of how a circuit like this functions:

TIP: Instead of using a CV signal to open and close the VCA why not try an other audio signal? That way the VCA will act as a (sort of) ring modulator. It's actually AM or Amplitude Modulation. Change the frequency to the lower/bass areas and experiment with modulating the VCO. You can get some weird sounds going that way.

So that's an other one done. One more and I'll have published 30 synthesizer related projects.

Okay, if you have any comments or questions please put them in the comments below or post them in the Facebook Group for this website. 

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Synthesizer Build part-28: WAVEFOLDER (Yusynth design).

Here's an alternative way to create awesome synthesizer sounds! It's a pretty easy project to build too.

After first experimenting with a wavefolder design I found on YouTube I decided to build the Yusynth version because I wanted this option in my synth. This circuit produces some awesome and very divers sounds and waveforms.

This is a really good sounding wavefolder with a VCA on the input so there will be no changes in audio level when you turn the 'Shape' knob. It has a different approach to wavefolding than the Triple Wavefolder and the Metalizer circuits which you can also find on this website. The addition of the diode ladder and the clamping diodes on the output opamp give this wavefolder a very distinctive sound which I personally really love. See the demo video further down the article.
The build was pretty straight-forward and it doesn't need to take up much room in your modular set-up either. This is a beginner to medium difficulty project but you do need to have a scope available to calibrate the circuit. A 20 dollar cheap one will do nicely. Make sure you use multi-turn trimmer potmeters for this. That will make it easier to calibrate later on. You can use normal trimmer pots if you have no multi-turn ones but it won't be as accurate. The schematic says to used matched pairs of diodes in the diode ladder but when I measured them they were almost all the same voltage drop. So as long as they come from the same batch I think you needn't bother with matching. The two transistors Q1 and Q2 need to be matched also but you can use the transistor tester in your multimeter. I recently built an adapter to change the pinout of my power supply bus system so I can test modules on dual 12 Volt and I'm happy to report the wavefolder works just as well on 12V as it does on 15V without any changes.

Here's the schematic I used from the Yusynth website:

Here's the (verified) layout I made for it and which I used for this project. These layouts have been updated on June 12th 2023. I made some cosmetic changes to them and colour-coded the wirebridges and removed the coloured bands from the resistors so the values are easier to read.

Wiring diagram:

Stripboard only. Beware that some stripboards are sold with 56 instead of 55 holes horizontally. The layout is 55 holes wide:

Cuts and Wirebridges seen from the COMPONENT SIDE! As always, mark the cuts on the component side with a waterproof Sharpie and then stick a pin through the marked holes and mark them again on the copper side where the pin sticks through. Then cut the marked holes with a sharp 6 or 7mm handheld drill bit.

Bill of Materials:

This wavefolder works best if you feed it a triangle- or sine-wave from an analog VCO or waves from a Wavetable Oscillator. This is convenient because an analog filter works best with square-waves and ramp- or sawtooth-waves, they have the most harmonic content and triangle- or sine-waves don't work that well in a filter. So with the wavefolder we have a use for those triangle and sinewaves. Ramp- or sawtooth-waves will sound good too through the wavefolder but square-waves pass through the wavefolder unchanged.
Here's a video demonstration of the type of sounds I'm getting from this module using a triangle-wave only. There's also a very low frequency LFO ramp signal going into the CV input.:

CALIBRATING:
To calibrate this Wavefolder you will first need to set the two trimmers in their middle position. Turn the Shape potmeter all the way counter clockwise and Range all the way open (fully clockwise). Connect a sinewave from your VCO to the input and a scope to the output and turn the 1K trimmer (above the two transistors) to get the best looking sinewave possible.  It will probably not be a perfect sinewave but try to get the top and bottom part to look the same. Once you set this, turn the Range potmeter all the way counter clockwise and turn the 50K trimmer so as to just mute the signal, so the level is 0 when the Range potmeter is turned all the way left (counter-clockwise).
That's the calibration done.

Here are some pictures of the finished product:

This is what your soldering should look like:

Finally, here's a video I found on YouTube demonstrating the Wavefolder:

Okay, that's an other one done. A very worth while addition to the synthesizer. I can really recommend it and all you need to build it are some very common parts.
As always, please put any questions or remarks in the comments below.
Thanks for checking it out. See you on the next one!

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Synthesizer Build part-27: QUADRATURE LFO Bergman edition.

A very quirky sinewave LFO with 4 outputs with a 90° phase shift to eachother. With extra waveform and an input for Frequency Modulation.

The quadrature LFO is something you won't find in many modular set-ups from the early days but it's a real little gem of a module to have in your set-up. It produces 4 sinewaves that are shifted in phase by 90° each. This can be used to create the 'Barber Pole' or 'Shepard Tone.' effect. "What's that?" I hear you ask. In short, it's an effect whereby it seems a tone is continously rising (or falling) without actually seeming to get any higher (or lower). See this Wikipedia Article for more on this effect. So that's one use. You could also feed four Voltage Controlled Panners with this LFO and have the sound go three-dimensionally around the room. Your imagination is the limit with this module which is why I was keen to experiment with this.

Eurorack 12V vs 15V.

I tested this circuit on a dual 12V powersupply and it works the same as on 15V. The maximum amplitude of the sinewaves stays the same too (+/- 7,5 Vpp). I think this is due to the zener diodes. They keep the amplitude constant. The maximum amplitude of the Trapezoïdal wave however will be lower at +/-10Vpp maximum. You can of course turn the amplitude of the waves down with the Output Level knob to, for instance, the Eurorack standard of +/- 5Vpp. No problem. So this is, in the way it works, a very Eurorack friendly module. For Eurorack you can cut the stripboard in half along the cuts in the copper strips, in the middle and fold it over. Then connect the top 3, strip J and the bottom ground copper strip together again with wire and you have a print small enough to fit behind a Eurorack panel.

How I came across the Trapezoïdal function:

I found this module with the schematic on the Yusynth website and set out to make a layout. Once again it turned out I made a faultless layout but it was with building it up that I made a mistake that turned out be an asset later on. This design uses two 5V Zener Diodes and somehow I had managed to stick one of them in the wrong hole on the stripboard so it was not connected correctly. This resulted in the output being Trapezoïdal instead of Sinusoïdal. Not knowing I had made this mistake, I posted about this on the Synth DIY Facebook page and people allerted me that it had to be the Zener Diodes that were at fault and they were right. After correcting the mistake everything worked fine but I then got remarks that I should include this waveform option in the final design of the module. So I did. I adapted the stripboard layout and added a switch to go between the two waveforms. If you want to build it as originally intended, without the switch, then just don't cut the copper between the two connection points for the switch on the stripboard, so the zener diode is connected again.

One more very important thing to note: YOU MUST USE POLYSTYRENE OR POLYESTER CAPS FOR THE TWO 10nF CAPACITORS!!  I've had several people build this module and then it didn't work because ceramic caps were used. They need to be high quality non-ceramic ones or it simply won't work.

Bill of Materials:

LAYOUTS:

Here are the new layouts. I have deleted the old ones I made because I don't want anyone using them. As always the layouts are verified. I added more cuts to the right of the stripboard to make the copper strips a bit shorter and to prevent short circuits with anything touching the side of the board. 

Wiring diagram:

The current limiting resistor for LED D4 (bottom right) is a 12K and not a 1K2 like the other three. I did this because otherwise the white LED would be much brighter than the other ones. So it's not a mistake, it should be 12K for any white LED you use.

Stripboard only:

Red wirebridges are connections to +15V, Pink ones are connections to -15V, Green ones are connections to ground and the blue ones are internal connections between components.

Cuts and wirebridges as seen from the Component Side!!

As always, mark the cuts with a Sharpie or Edding pen and then put a pin through the marked holes and mark them again on the copper side. Then cut the strips at the marked holes with a 6 or 7mm sharp hand-held drill bit.

THINGS TO KNOW AND SCHEMATIC:

The LFO needs a few seconds to start up, when you first switch it on. If the 'Rate' potmeter is turned all the way counter clockwise it won't start up at all. With Rate set to 1Hz it takes a good 20 seconds to start-up and before the LED's light up. So make sure it's not on minimum when you switch on, or you could have a long wait on your hands. Turn Rate all the way up and it'll start up almost immediately.
The normal sinewave output goes from -7.2V to +7.2V with a frequency of about one cycle per 30 seconds to about 140 Hz. You can set the lowest frequency you want available with the trimmer on the stripboard (T1). I had a warning from someone on YouTube not to set the trim pot all the way down because his went up in smoke. I had no such problems when testing this print but I just thought I'd mention it here. Anyway, he must have done something wrong because that shouldn't even be possible.
The Trapezoïdal wave has a higher amplitude and lower frequency so beware of that when you switch between them. It's maximum amplitude is -14,4 to +14,4 Volt and maximum frequency is 52Hz compared to the Sinewave's 140Hz. So you can roughly say the Trapezoïdal wave is double the amplitude and half the frequency of the Sinewave. That means both waves go high enough in frequency to be in the audio range so you can hear them. That opens up a wide range of uses for this LFO. One of my favourites is to have the CV control the Cut-Off frequencies of multiple filters and then feed the Quad LFO FM input with a ramp wave from an other LFO so you get a sweep. Sounds very cool!
The amplitude of the waveform(s) can be set by the dual gang 100K potmeter so you can set it at any level between zero and the maximum I just mentioned. The direction of the phase shifting between the 4 outputs can be turned around by the DPDT switch. You can see the direction it has by four 3mm LEDs on the panel. I didn't have an Orange LED so I used Red, Yellow, Green and White LEDs and the white one is quite a bit brighter than the rest so that's why it has a 12K resistor as current limiter and all the others have 1K2 resistors. This keeps the brightness perfectly in balance with the other three LEDs. One little drawback is that the LEDs only come on when the dual-gang output level potmeter is at the ten o'clock position or higher. At the lowest levels the LEDs don't light up. That's just how the circuit works.
Instead of the LM13700 you can also use the LM13600 and the TL074 can also be a TL084. From testing I also came to the conclusion that the 10nF caps don't necessarily need to be matched so closely. But remember they can NOT be ceramic capacitors. They don't work in this circuit. The two PNP transistors do need to be matched but you can use your multimeter's transistor tester for that and matched them to within 2 points of their hfe or amplification factor.

Here's the schematic I made the layout from:

I made a Falstad simulation of this circuit and it shows the start-up time really well. When you drag the 'RATE' potmeter to the left (faster rate) it will start up quicker. If you drag it the other way it will take quite a while.

---- CLICK HERE for SIMULATION ---

Here's a picture of the finished panel:

Here are some screenshots from the oscilloscope:

Here you can see the beautiful and absolutely perfect sinewaves this LFO produces:

Here's the feature I added myself. Trapezoïdal waves. You can see the amplitudes differ a tiny bit from eachother but that's not a problem in normal use. It's the same with the Sinewaves btw.

Fast Fourier Transform (FFT) image of the sinewave at 11Hz. As you can see, it's a near perfect sinewave at this frequency. Almost no harmonic spikes to the right of the main wave. When the frequency gets higher the sinewave becomes a little less perfect but that's only visible on a oscilloscope's FFT readout, not with the naked eye:

Here's what happens when you add a Ramp wave from an LFO to the FM Input. You get a frequency sweep:

Finally a little video showing all the different aspects of this module.

Here's a video of the first test I did with the Quadrature LFO. The patch is set up as follows:
3 VCO's are feeding 3 filters with a squarewave. The Steiner, Korg and the ARP filters. The Steiner-Parker receives two sinewaves from the Quad-LFO, one on CV-1 and one on CV-2 which are 90° separated in phase. The FM input of the Quad-LFO is being fed by a 0 to 10V ramp wave from the other LFO. Each filter output goes in a different channel of the mixer where they are summed and the output then goes through the CaraOK effects unit set to preset 61 which is a chorus effect. From there it goes to the speakers. The "da-daa-dang da-daa-dang" drone you hear is produced by the waves from the Quad LFO. :)

Here's an other short sample of the Quad LFO. It's being fed by a slowly rising ramp wave and two of the outputs are going into the Steiner Parker filter. The ARP and the Wavefolder are also in the mix:

Just for my own record keeping, here is an image of the state of readiness my synthesizer is in now. I have 8 free power-buss connections left so plenty of room to build more modules in the near future.

Okay, that's it for this one. With special thanks to the folks over at the Synth DIY FaceBook Group for all their help in the initial troubleshoot.
As always, if you have any questions or comments please put them in the comments below. 

If you find these projects helpful and would like to support the website and its upkeep then you can buy me a Coffee. There's a button for that underneath the menu if you're on a PC or Mac. Or you can use this PayPal.Me link to donate directly. All donations go towards the website and projects. Thank you!

Synthesizer Extra's No. 2: JOYO Instrument Tuner Hack.

This article explains how to connect a JOYO Instrument tuner to the VCO('s) of your modular synthesizer.

Inspired by LookMumNoComputer's Preformance VCO with built in tuner I bought three of these small Joyo tuners on eBay to see if I could adapt them to work with the synthesizer. I managed to do it and it's not difficult to do at all. Many people seem to do this too going by the Facebook reactions I got on this hack. :-)
The JOYO instrument tuner is a very useful bit of kit that you can buy on eBay or AliExpress etc. for around 5 US Dollars including shipping. It's pretty easy to hack these things so you can connect a jack socket to them and so connect it to the VCO's in your synthesizer. It makes tuning VCO's or Sequencers very fast and very accurate.

Now, before we begin I want to mention that the method below is just the way I did it. I've had lots of reactions to this hack and people do it in all sorts of ways, for instance connecting a jack straight to the piëzo wires without a resistor divider and not having the piëzo mic connected. Using a capacitor instead of resistors and it all seems to work so this is just my way of doing it but it's certainly not the only way, or the best way, to do it! Just so this is clear.

Here's what we do. You'll need to carefully open the tuner case by unscrewing 4 little screws at the back and then take out the two screws that hold the little circuitboard in place and now you can lift out the circuitboard. Leave the actual LCD display safely inside the case and make sure not to get fingerprints all over it. On the back side of the circuitboard, you'll find a very thin Piëzo-electric disk-microphone. Carefully push the white display lighting out of harms way and carefully solder a thin wire to the earth connection, which is the solder blob on the outer part of the Piëzo electric microphone. Solder an other wire to the center of the Piëzo disk microphone. Do not disconnect the red wire from the center of the microphone otherwise this won't work. 
Now make some small notches in the side of the plastic tuner case so you can lead the wires outside. Put the little circuitboard back inside the case first, to make sure where the notches need to go, then mark the right spot and take the circuitboard back out and make the notches in the case. Assemble the tuner and put everything back where it was. Now you can solder on the resistors following the diagram beneath. Solder them straight onto the input socket. Now solder the wires to the input jack socket (3,5mm) and the resistor and then hot-glue the input socket, with the resistors and wires attached, to the side of the tuner case. Make sure when glueing that you don't go past the bottom of the tuner case so the underside stays straight and flat. I myself then hot-glued the tuners to my synthesizer case by putting hot glue on the battery cover, which covers about 3 quarters of the backside, so the tuners are held in place by the battery cover. I later carefully drilled a 2mm hole through the covers into the wooden panel of my synthesizer and put a small screw in each one, that sits flush with the battery cover inside surface. That way it won't fall off at some point.

Powering the tuners:
I soldered two wires to the battery connectors. The spring connector in the middle of the battery compartment is the negative pole and the metal contact at the side is the positive pole. I have a 5V power rails in the powersupply of my DIY synthesizer and I connected the wires from the tuner(s) to that. I put three 1N914 diodes in series into the positive voltage rail to get the voltage down to just over 3V which is what the tuners need.
It's much better to give them their own internal power source because if you run them on the batteries the screen will flicker and vary in brightness. This way is all nice and stable.

SCHEMATIC DRAWING:

Here is the way I wired up the audio inputs. I believe the input of the Joyo tuner is capacitive so a resistor voltage divider like this will work fine and there will be no need to use a capacitor. This resistor voltage divider brings signals with an amplitude of 10Vpp down to 2Vpp.

This is the little circuitboard lifted out of the case. The white panel on the left is the LCD back-light. The gray and pink wires are the ones I soldered on myself:

Here's one of the finished hacked tuners without the battery cover because that is glued to my synthesizer. I blackened the glue with a permanent marker. As you can see I cut off the mounting eye for the clamp and made sure it was all sandpapered nice and flat. The fact that this particular model of tuner has a straight back and sides is why I chose this one. There are models out there with curved sides which make mounting more difficult. Bear that in mind when you buy one of these tuners.

Here are two of the three tuners mounted on my synth above two VCO's:

And finally a little video showing the tuners in action. As you can hear at the end, I was really pleased with the result. It looks very cool too on the synth I think.:

Oh and one final thing. You can use the left over clamps, from the backs of the tuners, to make a so called "Third Hand" to help with soldering :)

Leave the little screws in there and take some thick electrical copper wire (about 50 Centimeters long) and bend an eye at each end and mount the clamps to them with the little screws going through the eyes. Then hot-glue them in place and glue and/or tape that to a heavy enough metal base and with the copper wire you can bend them any which way you need them. And there you have it. Super useful! ^___^

EDIT: 6th of May 2022: Today I did two more of these hacks to have some tuners for my Eurorack system and they work fine :)

These were of a different make but they turned out to be even easier to hack. Here's a picture. I did use a lot of hot-glue to secure the sockets in place but it all works fine. In these I used a 120K resistor and a 6K8 resistor. which reduces a 10V input signal to a 0.54V output signal going into the tuner, making sure it's not damaged by the high synthesizer voltages.

In this case I left the clamps on the tuners because I can then clamp them to the powersupply wire of my Behringer Go Eurorack case. I drilled some holes in that case and connected the powerbrick straight to it so it won't dangle off the case which is very annoying.

Finally I want to show how one of our Facebook members (Martin Nyborg Sørensen) turned this idea into a very cool Eurorack Tuner module. He buffered the in and output so he can route signals through the tuner without any problems. I think you'll agree he did a very professional job.

The other side:

Okay, that's it for this one. If you have any questions please put them in the comments below or post them in the FaceBook Group for this website.
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Synthesizer Build part-26: STEINER-PARKER VCF (Yusynth Design).

An awesome sounding filter with LP, BP, HP and AP modes and a resonance that can be very agressive! (Beware your speakers, LOL). It's way up there with the ARP filter as one of the best sounding filters I ever built on stripboard.

Let's see, we've done the Moog, Prophet One, Korg, Dual Korg, ARP and Digisound so this is filter number six not counting the dual Korg. This is an other Yusynth design and once again I managed to make a Stripboard layout that turned out to work straight-away.

With this filter we get all the different options for filtering sound in one module; Lowpass (LP), Bandpass (BP), Highpass (HP) and Allpass (AP) which is more a phase shift effect although not very pronounced. The filter is self-oscillating in all modes (!) which is not usual for a 24dB/Octave filter I believe.
This filter has a very distinctive sound so it's an excellent addition to the synth because I want as much diversity in sound as possible. Especially the Bandpass and Highpass can have a very cool distortion if you turn up Resonance so it almost sounds like a heavy metal guitar. The filter was re-designed by Yves Usson (YuSynth) to do away with some of the erratic behaviour of the original design, but I noticed he didn't manage to get rid of it completely. The resonance can give a very loud whistle and if you turn it past that, with cut-off freq turned down, the filter stops working with a deep pop that can't be good for the speakers. I've included a 5K trimmer to replace the 3K9 resistor, R18 in the schematic, so I can set the reach of the Resonance control a bit better. This works up to a point but it's not possible to tame the resonance completely this way. The resonance gets better as you reduce the resistance of the trim pot but this reaches zero Ohm before resonance is completely tamed. But then again, that's the character of this filter so you don't want the agressive behaviour to disappear completely. 

One way to tame the resonance is a method mentioned in the comments below and recently tried out by one of our Facebook group members. It is to put a 10K resistor from pin 3 of the resonance potmeter to ground. In other words, you put the 10K in parallel over the potmeter, from the left pin to the right pin. You only have to make this change if you find the resonance potmeter to work too aggressively. If it's not a problem for you then you don't need to install the 10K resistor. (I myself don't use it).

Here is an illustration of where to put the 10K resistor. All the other connections are left as they are.

The panel potmeter needed for Resonance is a 50K reversed logarithmic one but in the Yusynth article he explains that you can use a 100K linear type with a 100K resistor soldered between pins 2 and 3 of the potmeter, so that's what I used and it works just fine. That's also the way I drew it in the layout because I figured not many people would have a reversed logarithmic 50K potmeter lying around. If you do have one though, use it.

I didn't have a 2-pole 4-way switch but I did have a 4-pole 3-way switch, a vintage 1970's one too, which I was dying to use in a project. To compensate for missing that 4th position of the switch, I added a second DPDT switch so I can switch from High-pass mode over to All-pass mode. In the layout it's a SPDT switch but I used a Dual Pole DPDT switch and I used the left over pins to switch on some 3mm LED's to indicate visually if the filter is in HP or AP mode. This works very well. The LED's are normally off and only if the main switch is set to HP does the 2nd switch get power to light up one of the LED's. I used one of the left over poles in the 4-pole switch to switch the power for the LED's on or off. This looks really cool and it works perfectly so I now have all 4 options available. I did not add the LED's etc to this layout because I added it as an after thought but it's easy enough to add something like this yourself. Anyway, you would normally use a 4-way double pole switch, so there would be no use for LED's then.

One thing to be aware of, make sure the TL074 you're using is not a cheap fake from China. I'm becoming aware now that I've got a lot of fake opamps installed in my modules. I recently got a batch of good ones from a reputable source and it makes such a difference in the sound this filter makes. The fake ones will work and for low frequency stuff like LFO's you won't notice the difference but the fake ones can not handle the high frequencies that resonance produces very well. So make sure you invest in good opamps.

One other thing I'd like to advise is measure the value of every component before you solder it in. I always do this myself too. Some components, like resistors but especially capacitors, can be way out of spec, especially if you use a second hand stock de-soldered out of other PCB's. This can present you with big problems when you need to troubleshoot the filter. So save yourself a headache and measure before you solder. This goes for all the projects on this website naturally. 

One more important issue I need to address with this filter is that it is susceptible to hum from your powersupply if you are using a switching powersupply. In my DIY synth I use two linear powersupplies and they deliver a noise free Direct Current but switching supplies can have a lot of high frequency noise on it which this filter doesn´t like. A good solution is putting some big electrolytic capacitors on the powerrails, on the stripboard. I myself used 1000µF caps for the filter I built for my Eurorack system (which does have a switching powersupply) and the hum is gone!  

Here's a link to the Yusynth Steiner project with the schematic and panel design. 
Here's the stripboard layout and wiring diagram I made for my specific needs with the somewhat odd switch arrangement. This is posted here for my own reference. Use the other layout beneath this one:

The Steiner-Parker filter will work fine on a dual 12V power supply. No extra changes are needed.

LAYOUTS:

Here is the version with a normal 4-way double pole rotary switch. These layouts are verified, I used them for my own build and they have been successfully used by many others, so you can print this one and use it as the wiring diagram. Please note in this layout I left out the second audio input jack and potmeter, the second CV input jack and potmeter and the 1V/Octave input jack, so remember to put those in! They are just copies of the first inputs with a level potmeter and an input jack for each. Except for the 1V/Octave input which is just an input jack. So in total we have 6 jack-sockets and 6 potmeters.:

(Last revised 03-Nov-2021: Cosmetic changes)

All my stripboard layouts are made on 24x56 stripboard and usually I use the whole board. Because I use the Kosmo format of 20cm high panels I have room enough to accommodate them behind the panels I make. So the builds on my website are not very Eurorack friendly. I keep the components spread out to make troubleshooting easier should that be necessary. There is now also a Eurorack version of this filter available on my website. Just click here to go to project 45.

Stripboard only. (Print this out and use it for your build. Don't forget the cuts underneath the chip.).

Beware that some stripboards are sold with 56 instead of 55 holes horizontally. The layout is 55 holes wide:

Schematic:

You can see from the schematic drawing how simple the design is. It's in fact just a diode ladder with 4 capacitors and a few buffers around it. A very effective set-up as you will notice after you successfully built this filter.

An overview of the cuts and wirebridges seen from the component side:

And here's a look at just the cuts that need to be made. This is viewed from the COPPER SIDE!!

Bill of Materials. The diodes and transistors need to be matched so I suggest buying a batch of 100 of each (or at least for the transistors. I didn't match the diodes because they were all from the same batch and close enough). They only cost pennies on eBay anyway. That way you have some stock from which to find matched pairs:

(Last revised: 15-May-2020 Added extra potmeters and input jacks that are not shown in the layout. 14-Jan.-2021 Corrected spelling errors.)

Calibrating the filter:
There are three trimpots on this circuit and the way to set them is just to turn them and listen to the filter's reaction. They mostly influence the cutoff point of the filter. The 5K trimmer, as mentioned before, is one I added myself. On my print it is turned to zero Ohm to give the Resonance potmeter as much throw as possible. So you could actually replace it with a wire bridge but I advise to just put in a 5K or 2K trim potmeter.
For the 50K logarithmic pots for audio in, you can use linear types if you wish and the value for the potmeters for audio and cv level is not important either. You can use any value from 10K to 1M for those potmeters. I put in 10K ones myself because I have a lot of those in stock. But do use the right value potmeters for the Cut-Off Frequency and for Resonance. (I guess you could use a 100K for Cut-Off but then the 47K resistor needs to be changed to a 100K type but I have not tried this and I don't guarantee that'll work. So just keep to the recommended values for those two potmeters.)

Here are some pictures of the finished module:

The 'finished' stripboard. Notice anything wrong? Yep I managed to forget 4 components, one 220K resistor, two 10µF caps and the earth connection for pin 3 of the TL074. So when I first tested the filter it didn't work like it should. (Even without the input caps I was still getting audio in. I was very surprised because without those caps the inputs are effectively cut-off from the circuit.)

In the first instance I didn't notice a big difference between the different modes of the filter. That was until I discovered I had forgotten to earth pin 3 of the TL074. Now that I've done that the filter sounds even better than it already did. I noticed that with this filter you can hear the original sound coming through the filtered sound if you turn Cut-Off and Resonance way back. I've done some research and this does seem to be exactly how this filter should sound so my build is spot on. Here's the filter in action in the Arturia MatrixBrute:
https://www.youtube.com/watch?v=7ul95vmNFwM
The Arturia MiniBrute was designed by Yves Usson aka Yusynth with help from Nyle Steiner, the original developer of this filter from whom it gets its name..

Here's a video demonstrating the sounds this filter can produce. It's my usual 'ploink-ploink' on the keyboard, but even so it sounds just amazing! I especially love the Bandpass mode. You can instantly see the squarewave disappear when switching from lowpass to bandpass and you're left with all the harmonic content on top. I LOVE this filter. (Perhaps even more than the ARP filter because of the these different modes.). You can hear my comprehensive conclusion at the end of the video; a single word... WAUW!! (<- That's Dutch for WOW!! ^^)

In case the video below doesn't show up on your mobile device, here's the link: https://www.youtube.com/watch?v=kGf4HB5miJY

Here's an other video, NOT by me, with a very methodical demonstration of all the functions of the filter. Even-though this filter was built with a ready made PCB, it reacts exactly like the the one in this article coz it's the same Yusynth circuit: -- LINK TO VIDEO --

And here's a YouTube video by Alan Wolke (W2AEW), demonstrating how to match diodes. Btw, his YT-channel is full of interesting electronics videos. They're not synthesizer based, more radio frequency stuff, but very interesting none the less. (He mentions a DMM a lot. That stands for Digital Multi Meter, just in case you didn't know ;):  -- LINK TO VIDEO --

Finally I want to share with you a picture by a member of the LookMumNoComputer Forum, Duane Kelly (Doolang), who successfully built this filter using my layout only he made it Eurorack size by cutting the print in half and connecting the necessary copper strips together. He did the same with the Digisound 80 filter which also worked perfectly.

As of December 2021 there is now a Eurorack version of this filter available on my website. Just -- click here -- to go to project 45.

Okay, that's an other filter to add to our collection. I have a few more on my wish list and now that I have the second case finished I have enough room to accomodate all the modules.

If you have any questions or comments, please leave them in the comments below or post them in the special Facebook Group for this website.

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