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.

There are PCBs available for this VCF. See 'PCB Service'

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. However the resonance is different in how it reacts to envelope pulses. 

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!

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 it should be possible to use it in this circuit but then you'd have to design your own layout because that is a DIP16 IC. 

I've had feedback from people using the 13700 that they get significant offset voltages on the outputs. For more on this please check out the comments below for comments by IDYLLM. He analysed this and came up with solutions.

For this particular project you'll need the AS3080 chips so please don't compromise on this and just order some AS3080's. They're easy to find and not that expensive. You only need two.

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.

A reader of this blog (Syboxez) made a great Falstad simulation of this filter which you can watch by clicking on this link: -- VCF-1 SIM --

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 using my Ian Fritz method matcher that I built on PCB but matching on Hfe with a multimeter should be good enough.

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. There's a little layout further down showing how to do this.

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.

EXTRA CV INPUT:

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 then 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:

As you can see it's very simple. I would really advise you to add this second CV input. Filters really need two CV inputs the get the most out of them. One for the Envelope signal and one for special effects like an LFO.

Stripboard only view:

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. 

Cuts and wirebridges:

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.

Cuts only:

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.

These are the pots I mean:

You can get them from AliExpress, they work fine. These pots are sealed so no dust can get in.

Adding extra's (like an LED and 2nd Audio 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.

   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.

However they must be set to the middle position so they don't add offset to the signal unintentionally.

Check your filter output on an oscilloscope (set audio in to DC) and make sure there's no DC offset.

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 as an experiment. I bent the plexiglass 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. It's over elaborate and I won't use this method again. It's just that I had no room for L-Brackets on the stripboard.

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.

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Synthesizer Build part-35: RESONANT LOPASS GATE (Buchla 292).

 An awesome sounding combination of a Voltage Controlled Amplifier and a LowPass Filter using Vactrols. It has three modes: VCA, VCF or Both. Prepare to fall in love with this one!! 

This is one of my favourite modules on this website because A, it sounds so good and B, it's versatility.

This module is not like your conventional Lowpass Filter. It's a combination of a VCA and a VCF. It helps if you're trained a bit in your modular synthesizer knowledge to get the best out of this module. As a beginner you might be better of building some normal filters first and leave this one for later. But then again, if you're feeling adventurous, then hop to it. You will certainly learn a thing or two as I did. Plus it's quite easy to build. I believe this design is equal to the Fonitronic version but Doepfer also has a Eurorack LowPass Gate for sale for around €100. It's the A-101-2. That's also the same one as in this project. I also have a Eurorack sized layout further down this article.

There are PCBs available for the Resonant LPG. See 'PCB Service'

THE RESONANT LOPASS GATE WILL RUN EQUALLY WELL ON +/-15V AS ON +/-12V. No extra changes are necessary.

A little bit of history:

When modular synthesizers were first being developed there were two people who became prominent in this world in the United States. Don Buchla on the West Coast and Bob Moog on the East Coast of the States. While Bob Moog preferred a more conventional way of playing the synthesizer by using a black and white piano style keyboard, Don Buchla chose to go an other route and developed a touch sensitive device that would react to the pressure human fingers would impose on it. Buchla didn't even like to call his instruments synthesizers since that name connotes imitating existing sounds and/or instruments. His intentions were to make instruments for creating new sounds. He wanted unrestrained artistic expression un-bound by the conventional chromatic scale used in western music. A completely different approach to modular synthesis but one that sounds out of this world if you get it right. However, piano style keyboards are instantly recognized by musicians as something they can work with, and therefore the Moog system became the most widely adopted system in the world. This module is one from Don Buchla's stables, in fact the first one from his design philosophy on my website. (Hopefully not the last one because I really like the West Coast approach.) The addition of the resonant feedback loop and the refinement of the original Buchla design goes to the credit of Thomas White. The module I built is the Thomas White version as presented on the website modularsynthesis.com. Click here to visit that webpage. 

Here's the link to the NatualRythmMusic website which features the same project.

(I'm not associated with any of those websites.)

Resonant Lopass Gate:

To be honest with you, I had never heard of Resonant Lopass Gates before I held a poll on Facebook to see what people would like me to build for future projects. This was one of the options that was mentioned. It instantly intrigued me  because I didn't know what it was. So I asked for schematics, did some research and started building one. 

This module consists of three parts and there's a mode switch to switch between them. There's a voltage controlled amplifier or VCA and a lowpass filter (12dB) and the option to have both on at the same time. The VCA is nothing more than a voltage controlled attenuator and with the switch in VCA mode that is what you get. Now if you set the switch to 'Both' mode, you get that same VCA function but unlike a pure VCA not all frequencies are attenuated equally. The amplitude will change in accordance with the frequency response. Depending on the height of the Control Voltage, the filter cuts off parts of the high frequency content of the input signal. If we now switch to VCF mode we have the full function of the lowpass filter including resonance (and it can self-oscillate) and the CV voltage determins the cut-off frequency of the filter. The VCA part is no longer working in this mode but we still get a mixture of changing cutoff frequencies and changes in amplitude driven by control voltage and the CV input also affects the amount of resonance that is put on the audio signal. It's very complicated and I can't explain it very well but it makes for a very special sounding module. Because it works best with a constantly changing CV inputs, the lopass gate really shines when used in more percussive typ patches (See demo video lower down the article for sound samples) but that doesn't mean you can't use it for other purposes. It'll work equally well as a VCF module. It just begs to be experimented with.

The CV inputs can be anything from Gate signals to Envelope signals or LFO's or any combination of those. You can experiment with what sounds best. I think it's better to have signals going into both CV inputs at the same time. The CV 2 input has an inverter connected to it in the form of opamp U2-A to form an attenuverter, The more you turn it clockwise the more the CV signal gets inverted. This is one of the changes that has been made (by Thomas White) from the original design as described in the 'modularsysthesis' article in the link below here, which I incorporated into the redrawn schematic. It works very well. The CV-2 control contributes a lot to the funky sound of this module. CV-1 is the more dominant input and if it is fully opened up it will somewhat suppress the working of CV-2 so you need to find the right balance between the two CV's.

Here's the schematic drawing that I re-made from the schematic on 'modularsynthesis.' It has all the changes that are suggested in the linked article implemented. (Click on the image to enlarge it and then right-click and 'Save as' to save it to your computer. Then you can zoom in on it.).

The schematic says to use VTL5C3 vactrols but the slower VTL5C4's will work fine too, maybe even better. It's a matter of taste and experimenting. I used home made ones myself. Somehow, slower working Vactrols make this Lopass Gate sound twice as good as with fast reacting ones. With slower LDR's in your Vactrols this module sounds really amazing. You get that snidy 'ripping the fabric of the universe' synthesizer sound from it.

C7 and C8 should be good quality, none ceramic,  capacitors. The rest can be ceramic although I myself always use film capacitors throughout the LPG. You know those green oblong ones.

I did not use any bypass/de-coupling capacitors on the two IC's but if you want them included, or if you're having trouble with noise from the powersupply, then just put a 100nF ceramic cap between the plus and ground and one from ground to minus 15V and as close to the chips as possible  You can also put some 10µF/25V electrolytic caps on the power rails to suppress any hum. The 'Deep' switch is a normal SPDT toggle switch (ON-ON). If you turn it on, the sound will be deeper with less high tones. It has the effect of turning the 'Offset' knob counterclockwise. You can set the amount with the trimmer Tp2. The MODE switch needs to be a 3 pole ON-OFF-ON switch and I have colour-coded the connections so you can easily see what goes where. The 3 by 3 diagram with red, green and blue represents the bottom pins of the switch and the colours match up with the colours in the schematic drawing. You can see it all connected in the layout below. The switch needs to have a middle position and in that position none of the 3 connections in the schematic are made, so they are all open. This is the 'Both' mode and is how it should be although it may look a bit weird at first. 

You can also use a 3 position rotary switch of course but it will have to be a 3 pole, 3 position rotary switch. I myself used a vintage 6 pole 3 way switch I had in my junkbox. I had four of them and used two of those in earlier projects. One in the Digisound 80 ADSR and one in the Steiner-Parker filter.

About the Vactrols:
The layout I made for this module worked rightaway but I did some experimenting with the Vactrols. 

I made my own Vactrols from 5mm red LEDs and LDR's that had an 'off' resistance of over 200MOhm and with a bright red LED shining on them the resistance was about 200 Ohm. I later soldered a 3mm red LED in parallel over the vactrol LED on the left to dim it a little, because I found out that sounded better. Later I mounted that LED on the front panel to have a visual indication of the working of the Vactrols. I only put a LED over one of the Vactrols, the top one going by the layout below.

I made some Vactrols earlier and used bright white LEDs in them but although they did work, the LEDs hardly came on because the maximum voltage over them was about 2,7 Volt which was too close to the threshold voltage of those LEDs. The red LEDs will shine full on with that voltage which works much better. (NOTE: because the LEDs in the Vactrols are part of the circuit and not connected directly to a powersupply they don't require their own current limiting resistors.) 

If you want to build your own vactrols using LDR's from the GL55** series then I refer you to a comment below posted by Tim who tried several LDR's from that type. He had the best results with GL5528's and GL5537's but read the comment below for his full review.

I now understand the function of the Vactrols a bit better. The characteristic filter sweep sound that we normally get from filters by applying an envelope signal to the filter cutoff is created in the LPG by the slowness of the LDR's inside the Vactrols. The LPG filter sweeps through as the Vactrols lower in resistance. So using super fast LDR's in your Vactrols would be counter productive. It sounds better if they're a bit slow reacting so you get a distinctive filter sweep.

LAYOUTS:

The picture below is the wiring diagram. The module is meant to work on a dual 15V powersupply but it will work fine on a dual 12V powersupply (Eurorack)  I built this module using two TL074 chips, not the TL084 as mentioned in the layout. It doesn't really matter which quad opamp you use as long as they're low noise types. It's up to you. As always the layout is verified. I used it to build my module and I already had confirmation from others who built this successfully. All potmeters in this layout are viewed from the back side.

Stripboard only. As you can see the components are quite spread out over the stripboard, so I'm sure you could design a smaller stripboard layout but I didn't bother with that because I now also have PCB's I designed myself, which easily fit a Eurorack setup. (see Menu: PCB Service):

Below are the cuts and wirebridges seen from component side. I marked the spot where you need to cut the copper strip between holes J3 and J4 with a vertical line, for the 500K trimpot to work properly.

As always, mark the holes on the component side with a Sharpie or equivalent and then stick a pin through the marked holes and mark them again on the copper side where the pin pokes through. Then cut the copper strips at the marked holes with a sharp, hand held, 6 or 7mm dril bit.

Bill of Materials:

The trimmers are listed as multiturn but you might aswell put in single turn (normal) trimmers because that makes tuning the circuit so much easier. There's no real need for precision here.

The layouts above are quite spread out so here is a more compact layout fit for Eurorack. I did not wire up the 3 pole switch to make it easier to view the layout. All connection points are numbered and colour coded. Refer to the other layout above if you can't work it out. The Eurorack layouts are not verified yet. I have not built a module with them but I'm sure they will work. Compare them closely with the ones above and the schematic if you're not sure. Please post a comment if you used these layouts so I can mark them as verified.

Stripboard only:

Cuts and Wirebridges seen from component side:

How to calibrate this module:

There are two trimmers on the board, the 20K trimmer directly influences the voltage that the Vactrols get so it plays a part in determining the sound. So you need to set it for best resonance, at least that's what I did. The influence it has is not that obvious though. 

The second one is for the 'Deep' switch and determins the 'deepness' or the low frequency emphasis of the circuit. It's a sort of tone control and the effect it gives is like turning the Offset knob down. You can set it to whatever you like best.

Here are some pictures from the build proces. The two black thingies at the bottom left of the stripboard are my home made Vactrols. Everything is in place only nothing has been wired up yet in these first two pictures:

I used a vintage 6-pole 3-way switch but unfortunately I drilled the holes for the screws in the wrong place but since they were 3mm holes I put some 3mm LEDs in them and connected them to a free pole of the switch so that the yellow LED goes on when the switch is set to VCA mode and the red one goes on when switched to VCF mode and both go on when in 'Both' mode. =)

Here's a sketch of how I connected the LEDs to achieve that. In 'Both' mode they are a bit dimmer because of the 0,6V voltage drop of the extra diodes but you hardly notice that. I could have used Schottky diodes to prevent that but anyway. It works perfectly fine:

Remember, the LED circuit above only applies to my own self built module. It's not something common to the Lopass Gate. It's not necessary for you to replicate this. It's just something I did to fill two holes in the faceplate. It turned out to be very useful though.

DEMO VIDEOS:

Here's a video demonstrating the sounds you can get from this module (listen with headphones to get the best effect). When I say "In 'Both-Mode' you don't get Resonance" what I mean is that you don't get self-oscillation in 'Both-Mode'. Resonance still works. When watching this video please keep in mind that I didn't yet know how to properly use this module. I'm simply turning knobs to see what happens, nothing more. Imagine what a skilled synth user could get out of this module when it already sounds so cool in the hands of a noob like me. ^____^

TIP: Try altering the pulse width of the squarewave going into the Lopass Gate. You'll get some really cool sounds that way.

Here's a more recent video of me playing around with the PCB version of the LPG behind a self designed faceplate. The LPG is connected to the Klavis Twinwaves mkII digital oscillator, using 8 sawtooth waves with phase shifting. (The Klavis twinwaves II is my alltime favourite digital oscillator.)

I have the feeling it sounds better than the stripboard version but that could just be me. It sounds amazing though. Listen with headphones if you can, to hear the deep bass it has:

Here's a video (not by me) from 2008 showcasing the Resonant Lopass Gate using the VTL5C4 Vactrols which are slower than the VTL5C3's. This gives a more vintage sound (according to some people). People nickname this version the Slowpass Gate. It sounds very TB-303 Acid House to me. I really love it! Slower reacting LDRs in the Vactrols are definitely the way to go with this module. Decide for yourself. Here's the video:

Here's an other one I found from 2015 demonstrating a dual lopass gate:

It would be very cool to have three or four of these Resonant Lopass Gates in a modular synthesizer set-up and to use them partly as VCA's with a twist. You can do some really cool things with this module, I know that. But I myself haven't figured out yet in how many ways you can use this.

Below is one final video that I posted here for people interested in the inner workings of the Lopass Gate. The video goes into all the electronics and their specific functions in the module. It's very interesting especially for electronics students:

Okay, that's it for now. As always, put any questions you might have in the comments below or on the facebook group.

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If you find this content helpful, please consider donating to keep this website in the air and to contribute to future projects. There's a Ko-Fi donation button underneath the main menu if you're on a PC or Mac. Otherwise use this PayPal link to cut out the middle man. Thank you very much for your help!

Synthesizer Build part-23: DIGISOUND 80.6 LOWPASS FILTER.

A very cool AS3320 design that sounds amazing! With verified stripboard layout and new schematics.

After having taken out the Sequential Pro One lowpass filter to make room for the Korg filter, I needed a new use for the AS3320 chip that was inside it. I found the Digisound 80 point 6 lowpass filter module on this awesome website that has all the schematics for the entire Digisound 80 modular synthesizer.
You can configure the filter for any type you want (it's all in the original text) but we are going to build the lowpass filter because for subtractive synthesis the lowpass is the best sounding and most useful of all the filters in my opinion.
I first made a new schematic drawing because the original had those zigzag lines for resistors and I find the rectangular way of drawing resistors easier and you can put the value of the resistor inside the box. Makes it less complicated to look at imho.
Anyway, here's the new schematic drawing:

So after that was finished I made a stripboard layout. It is verified because I used this twice and both times the filter worked perfectly. Furthermore it has been used successfully by others in their builds. Make sure you work accurately though because I wouldn't consider this a beginners project. The layout includes a second audio output with 3 times the gain of the original output. This is of my own design and is not included in the schematic drawing. It is this output that is wired up to the output jack-socket in the layout below. The original output is marked on the layout too. More about this further down the article:

(Last revised: 19-March-2020. Added a second audio output with 3 times gain compared to the normal output. 25-5-2023: Removed colour codes from resistors, added colouring to wirebridges.)

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 Sharpie and then stick a pin through the marked holes and mark them again on the copper side and then you can cut them with a sharp hand held 6 or 7mm drill bit. 

Bill of Materials:

The panel potmeters used are all 100K linear types but the value isn't that important. Since they are all connected to either a powersupply voltage or an audio signal you can use any value you like from 10K upwards. I myself used three 100K potmeters for the Coarse, Fine and Resonance and I used three 10K potmeters for the audio and CV level controls. This works just fine. 

You can choose to include the Frequency Fine control potmeter or leave it out to save more room on the panel. I personally never use it but it is there if you want to play the filter as an oscillator when it is in full self-oscillation mode with the Resonance turned up full. You'll need to tune the self oscillation pitch to the chromatic scale of notes so in that case a fine tune knob will be very useful. But I personally never tried this so I don't know how well this filter responds to that. If you have any experience with that then please put it in the comments below so I can share it in this article.

This is a 24dB/Octave, 4-pole LPF and it is self oscillating unlike the Prophet One filter I used this chip in earlier. That one refused to self oscillate. I used simple ceramic capacitors for the 220pF caps and this works fine. There's no need for fancy polystyrene caps ^___^.
The 1µF electrolytic cap C7 at the input may seem to have the wrong polarity. Usually a cap like that would have the positive pole connected to the point where the signal comes from and negative to where the signal needs to go. In this case it is mounted correctly because the input opamp is an inverting buffer with a negative gain reducing the amplitude of the 10Vpp input signal by a third to an amplitude the chip can handle. In the output buffer the signal is then inverted again to a positive signal with a gain of 3 to give us the original amplitude.

Calibrating the filter:
There are three trimmer potmeters on this print and you can set them as follows:
RV8, the 100K trimmer, is used to trim away the DC voltage on the audio output. Measure the DC output voltage with nothing connected to the input and turn RV8 until it reads zero.
RV7, the 20K trimmer is an interesting one. It's used to have the filter track 1V/Octave oscillators correctly but I simply tune it for best sound. If you have a squarewave on the input and you turn this trimmer you can clearly hear the over-tones, the harmonics, change in pitch. You should be able to hear the frequency beating effect of the note from the VCO against the tone of the resonance. Trim until there's no frequency beating but also listen to the tone while changing the cut-off frequency and trim until it sounds right to you. There's a full description of the proper way to calibrate this filter in the original text, which is in the link I mentioned earlier in this article.
The last trimmer is the one in series with the current limiting resistor for the AS3320. Simply measure the resistance and set it so the total resistance of the trimmer with the 1K resistor equals 1,5K. You could also just put in a 1K5 resistor but turning this trimmer does have a little influence on the sound but you'll have to try it to know what I mean. I just set it to 1K5 and left it at that. Turning this trimmer all the way to zero resistance won't damage the chip though, eventhough it needs a 1K5 current-limiting resistor, 1K won't hurt it. The one thing I learned building this filter is that the AS3320 is quite a robust chip. I made a few mistakes building it the first time and the chip has had voltages (through resistors) placed on the wrong pins, short circuits and all sorts of other mishaps but it survived all that without a scratch. Thank goodness because I only have one of them at the moment :) Luckily I was able to test if the chip still worked by placing it back inside the old Prophet One filter and seeing if that still worked. That was very useful.
Anyway, you can use this filter with a dual 12 Volt power supply, but in that case the current limiting resistor should be 1,2K in total. But it's really not that important. Simply connect it to +/-12V and it should work fine.

This filter sounds amazing! It has its own distinctive sound and I can not say it sounds like the Korg or the ARP or the Moog Ladder filter. It sounds like a Digisound 80 filter, although it comes veeeeeeery close to the ARP in sound. This one sounds a bit more well behaved, if you know what I mean. The sound of the Resonance is clearer than in the ARP which has a Rensonance that is sharper and rougher in sound. But that's the only  difference I could hear so it occupies a solid second place over the Korg-MS20 and the Moog Ladder filter in my personal top 5. The ARP filter is still number one because it's a real rebel and I love it. But hey, remember, this is all just my personal preference. You may judge it quite differently. Actually, I find myself using this filter more often than the ARP filter somehow.
The output from the Digisound 80 LPF is a bit more attenuated than the other filters I built and that's why I used the left-over opamp in IC-1 as an output buffer with a gain of 3. There's a 150K resistor from pin 6 to ground and double that value, 330K, as feedback resistor from pin 6 to pin 7. (You can use any value over 10K for both resistors, as long as the feedback resistor is twice the value of the resistor to ground.) This brings the volume up to the same level as the other filters I made. As I mentioned earlier, the amplitude is first divided by 3 and then multiplied by 3 again in the output opamp but, at least in my filter, I found the sound still lacking in volume compared to the other filters. That's why I wired up the left-over opamp as an amplifier with an extra gain of 3 to bring it up to normal. I'm not sure if it's just my filter or if this is normal, that's why I left the original output un-touched so you can use it if you think my solution is too loud. You could also install a potmeter of 500K instead of the feedback resistor so you can manually set the gain. (Put a 50K resistor in series with the potmeter so the feedback loop can't go to zero Ohm.)
The original audio output is marked on the layout so you can choose which one you want to use.
The first time I build this filter I had used a coupling capacitor of 4,7µF over the audio output because I measured a big DC offset voltage on the audio output, but then I read the original text and found out you can trim that away with trimmer RV8 so I took out the cap and trimmed the DC away and now it's all as it should be.
This filter has an input for 1V/Octave but unlike the ARP filter it's not necessary to use this. The filter will work fine without it but if you connect a 1V/Oct. source to it, the filter will track the octaves better. The sharp synthesizer sound we all love, will be more prominent if you use the 1V/Oct input. It actually makes the filter sound better.
Like I mentioned before, this filter has 2 potmeters for the Cut-Off Frequency but I advise to only use the 'Coarse' control. Fine is only for Polyphonic synths. I included it in my build so I could hear its effect and write about it here, but I normally don't use it. It stays in the middle position because turning it just changes the sound a tiny bit. It can be handy though to tune it into a certain harmonic frequency because this filter brings out the harmonics of a square wave really well, but all in all; leave it out.

Here's a picture of the finished panel built into the synth.:

As you can see in the picture I also included a bypass switch on my panel so I can put other filters in series with this one and if I only want to use one filter I can bypass this one and send the signal straight to the next one without having to change the patch cables. For instance, having this filter in series with the Korg MS-20 in High Pass mode sounds pretty amazing too! That way you have a Band-pass filter made up of two different filters. The bypass switch is only connected to the 'Audio-1' input though. If you want to see the wiring diagram for this switch, you can find it in the article about the Moog Ladder Filter, in which I also installed a switch like this.

Here's a little video with a demonstration of the sound of the Digisound 80.6 LPF (EDIT: at this stage in building my synth it still hadn't occurred to me that you need to connect an AD or ADSR to the CV input to get that typical synthesizer sound. #facepalm (We live and learn LOL):

Lastly I want to share with you the efforts of LookMumNoComputer Forum member Doolang who successfully built this filter using my layout. He made it so it fits the Eurorack standard by cutting the print in half and connecting the copper strips together with wire. This works like a charm and he did the same with the Steiner-Parker filter which also worked fine.

So that's the Digisound 80.6 lowpass filter done. I can really recommend building this. It has some very recognizable synthesizer sounds that you should really have available in your synth. Make sure you use good quality stripboard though. The first one I built had problems because strips of copper would become loose and break. So I rebuilt it with better quality stripboard. Make sure you use the filter with the 1V/Oct connected to get the best out of it. This will make resonance follow the notes you play (filter tracking). It'll also work with out 1V/Oct of course.

Okay, thanks for being here and if you have any comments or questions just put them in the comments below or in the special Facebook Group for this website.

Synthesizer Build part-15: DUAL KORG MS-20 HP/LP FILTER.

This is just two Korg MS-20 filters behind one panel with switches to go between using them as individual filters or switching them in series.  This is one of my early projects so the implementation is a bit clumsy with how I use the switches etc. but it'll be easier to use socket switches instead of toggle switches but in my early building days I didn't know these things. 

I'm keeping the article up for archival reasons plus this dual configuration actually sounds pretty awesome but in hindsight, I wouldn't build it like it's presented below. Just so you know. 

I wasn't too pleased with the performance of the Prophet 5 lowpass filter so I decided to remove it and put a new filter in its place. I've seen lots of videos about the Korg MS-20 and really like the sound of it. I noticed that synthesizer has two nearly identical filters next to eachother; the highpass- and the lowpass-filter, so I wanted to emulate that in my own synth. So I set out to build two of the 'Late MS-20 filters' by Rene Schmitz, and fit them behind a single panel that was the size of the old Prophet 5 filter that I took out. It was a tight fit to put all the knobs and switches on but it worked out beautifully in the end.
The schematics and layout I used are just the same as the ones I used in article 12 of this blog, so if you want to build your own dual filter arrangement you can go to the Korg MS20 filter page and build TWO of those. Build both filters with the HP/LP switch but do not include the bandpass switch. You build two MS-20 HP/LP filters and put them behind one panel. Then, as extra, you add switches to the inputs so you're able to put them in series or use them independently of eachother. The wiring diagram for those switches is further down the article.

Here's a picture of what that looks like. You can see I have filter one on the left side and filter two on the right. Each has its own Cut-Off Frequency and Resonance controls and each has audio and CV-in level controls and each has it's own Highpass/Lowpass switch. Beneath those switches you see two more switches which enable me to switch both filters in series without using patch cables to connect them to eachother.:

The wiring of these two stripboards was a bit of a nightmare but I got it done in the end. I made some initial mistakes and had to re-wire some potmeters so that's why the wires look like such a mess. Luckily it doesn't affect the working of the filter.

In the first filter I used the LM13600 chip and in the second the LM13700 chip. And having them side by side is a good opportunity to compare them and the LM13600 is a bit tamer than the 13700. So if you have both chips in stock you can decide whether you want your filters to sound aggressive or a bit less aggressive. It's not a big difference though.

You can see in the pictures above that it's a tight fit but I did managed to include two volume or level potmeters for the audio inputs, which are not included in the original build but are very useful to have. I'm going to make sure that every filter I build in the future has input level control.
Beneath the HP/LP switches are the two switches that give you the option of using these filters as two stand-alone filters, so not internally connected, or if you want to put them in series so the output of filter 1 goes into the input of filter 2.
One other weird option would be to switch the output of filter 1 to the output jack but leave the input switch for filter 2 as is, so filter 2 has no input. Because these filters are self-oscillating you can now use filter 2 as an oscillator. Put the output into the 4 channel mixer described in article 17 and put the resulting wave through filter 1 or wherever you like. Connect a 1V/Oct voltage to the CV input. Just an idea, but you see there are endless possibilities. That is the beauty of modular synthesizers. :)
However, it would be better to use a single DPDT switch here and use it just to switch between two filters in series or both separate. That makes it easier to switch but that way you can not use one of the filters as stand-alone oscillator, but you won't use that function much anyway I'll bet.

Here is the wiring diagram for those two switches. By all means try and think of a better way to do this. Like I mentioned in the beginning, this was one of my early projects so not everything is perfect. I still get confused by these switches when I use this module after 4 years of using it LOL (I don't use it any more)

Use the switches inside the sockets instead of toggle switches. Much less confusing. But I used the wiring diagram below. 

Finally I want to show you a little video I made which demonstrates the sound of these two filters in series with eachother. I filmed this just after I had finished the build and I was still figuring out what the filter could do but it shows the added benefit of having two of these in series. It can do really deep and full sounding bass tones and it can also scream and distort and sound really weird. I am glad I fitted these and they are certainly a big improvement over the Prophet 5 filter although I will use the AS3320 chip inside it for a future build.
Plus all those knobs and switches so close to eachother look really cool I think ^_____^

Here's a look at the different sounds this filter can produce. (The phaser effects come from the special effects unit and not from this filter):

Okay, that's it for this one. If you enjoyed this article please check out the rest of my synth build and leave me a comment if you have any questions, or even to just say hi. Please also subscribe to my YouTube channel. That would be a great help. THANK YOU!!

Other websites that deal with the DUAL MS-20 configuration:

https://www.modulargrid.net/e/befaco-sallen-key-filter-bf-22

https://www.perfectcircuit.com/signal/korg-ms-20

Synthesizer Build part-12: THE KORG MS20 FILTER.

A good working version of the famous Korg MS-20 filter by Rene Schmitz, with updated stripboard layout and wiring diagram with LP & HP .

As popular as this filter is, I would not recommend it as your first filter build. Just because of the audio input level behaviour. For your first filter, build the Steiner-Parker diode filter. Instant gratification. 
Now on with our regular program:

There are PCBs available for this VCF. See 'PCB Service'

This is a filter I absolutely had to include in my DIY synthesizer project, for one because Sam Battle from LookMumNoComputer raves about it and it sounds amazing in his videos and the other reason is that it has the option to go between Low-Pass and High-Pass and I didn't have a High-Pass option in the synthesizer yet. I also added the Band-Pass option in the layout drawing however I tested it and it adds no extra benefit to this filter. 
There was a discussion on the Synth DIY Facebook group a while back about the possibility of switching this filter between 6dB per Octave and 12dB per Octave. I've tried it and it works but only in LowPass mode. More about this further down the article.
Btw, I tested this filter on dual 12 Volt and it works just as well as on 15 Volt so no need to change anything if you are feeding it from a +/- 12V powersupply. Of course you need to open Resonance a little more than on 15V but it's all within the normal throw of the potmeters so no problem there.

Btw, eventhough this is THE going design for the OTA version of the MS-20 filter, in my humble opinion it doesn't sound much like a vintage MS-20. But that's just me. It does sound good though so don't get me wrong, you're not waisting your time.

SCHEMATIC:

For this filter I used the 'Late MS20 Filter' schematic from Rene Schmitz which you can find by clicking here. I think he called it the 'late' version because the earlier versions used the Korg MS35 module which was sealed off and what was in it was only revealed in 2000. It was transistor based. The later versions used the OTA chip the LM13600.

This filter is definitely different from other filters. It doesn't sound like the Moog Filter but it does have that 'ripping the fabric of the universe' synth sound and it's a real Speaker Ripper!. It's a 'Sallen-Key' type filter and it produces really divers sounds. (I always think of this filter as the 'Heavy Metal Guitar' pedal of the synthesizer world.) You can get deep bass tones out of it and if you connect an Envelope Generator like the little AD/AR to the CV1 input you'll get a squarewave changing into a really bassy sinewave as the note progresses. It's awesome to experiment with this filter.
In HighPass mode the Cut-Off Frequency potmeter doesn't work over its complete throw. I found it usually only works over the first 50%. This is also true of the other MS-20 filters I built for the Dual version so this is normal behaviour for this design.
This filter is self oscillating in both the Low and High-Pass configuration. The more Resonance you give it, the more the two yellow LED's light up. I found that by using an LM13600 instead of a LM13700 you can  tame the filter a little. The 13600 seems to be a bit less aggressive although the difference is really small. Btw, instead of the TL074 you can also use the TL084 or the LM324. They are pin for pin compatible and work just fine. I personally tested them all successfully.

Here's the original schematic with the pinout numbers for the LM13700:

Here's a link to an image I made showing the switch connections for HP and LP mode: --- click here --

About Resonance and Self-Oscillation:

On the oscilloscope you won't see much Resonance ringing on top of a squarewave in Lowpass mode. I've noticed this with all 3 Korg filters I built so far. Maybe one little sinewave bump on the top left corner of a squarewave but it's not like the ARP or Steiner filters where the whole top and bottom of the squarewave is full of Resonance or self-oscillation. (It gets better with lower input levels though.) But in HiPass mode you get much more. However, this is normal for this filter and it still sounds pretty amazing so this doesn't matter.

Beware this filter does not like high input levels! When I feed it 0-10Vpp squarewaves directly from the Digisound-80 VCO it doesn't work right. It is much happier with the -5/+5Vpp levels of the Thomas Henry 555-VCO. It is always a good idea to put an input level potmeter on the audio input of this filter so you can regulate the input volume. You can try to put a capacitor between 470nF and 1µF on the audio input to block the DC offset voltage of a 0 to 10V signal and turn it into a -5/+5V signal. That should work.

This is a well known characteristic of the Korg MS20 filter. If you increase the level of the audio input you will drown out the resonance of the filter. So you must find a balance between input level and resonance.

6dB vs 12dB.
Although this is a 12dB per Octave filter (2 pole) it is possible to get an output with 6dB per Octave roll-off (1 pole) if you tap the signal from pin 7 of the TL074 (A1 in the schematic drawing) with a 470nF capacitor, just like the normal output. The 6dB however won't work in Highpass mode because the signal is output from the stage before the one where we input the Highpass audio signal.
I have experimented with 6dB and at first I dismissed the option but having recently rebuilt my filter using (vintage) Polystyrene capacitors (and Polyester box caps for the 470nF output caps) it now sounds much better and the 6dB function sounds better too. So I made new layouts which includes the 6dB/Octave function with a switch to give you the choice between 6dB or 12dB. In the rebuilt filter I also used two BC558 transistors that were roughly matched. I'm not sure if this is necessary but it can't hurt. The 6dB/Oct setting sounds more distorted an rough around the edges if you know what I mean. It's a heavier sound. 

LAYOUT:

Below here's the stripboard layout. I made my own version from the ones that are circulating on the internet to which I added the potmeter connections and the audio in and out, CV in, plus the switch connections for Low-, Band- and Highpass with the altered position of the 1nF cap at pin 12 of the LM13700. So with this and the schematics you should have all the info you need to build it right the first time. BTW, the 2 transistors used on the layout below are BC558 PNP's but you can also use the 2N3906 but those have to be put in the other way around. In subsequent builds I used the BC557.

ABOUT THE LED's: 

Make sure you use LED's with a voltage drop of around 1,8 Volts. That means Yellow, Green or Red LED's are okay but do NOT USE Bright white LED's or Blue LED's. They have a much higher voltage drop and won't work well in this filter. Remember the LED's replace the original 3 diodes in series. These diodes have a voltage drop of 0,6 Volt so 3 in series means 3 x 0,6Volt = 1,8 Volt.

(Last revised 7-Oct.-2023 Cosmetic changes to make layout clearer.)

Please note: I did add the Band-Pass option to the layout but if I were you I would just leave it out. But do some tests and decide for yourself.

About the DPDT switch wiring for HP/LP mode: 

Connect the top two pins and the right middle pin of the High Pass/ Low Pass switch together and connect that to the High-Pass input. You could even just forget about the top right pin and bypass it but I thought it was neater to include it. The lower right pin goes to ground. The audio signal goes into the middle left pin, and the lower left pin goes to the Low-Pass input. The Band-Pass switch is simply connected to the Low-pass input and the audio input. If the filter is in Low-Pass mode the BP switch won't have any effect but in High-Pass mode the switch will connect the audio to the Low-Pass input aswell so both inputs get the audio signal thus creating the bandpass characteristic.

A little sidenote about the writing next to the switch in the layout: the HighPass and LowPass marking next to the switch on the wiring diagram reflect the function of the actual wires connected to that part of the switch. A Toggle switch works the other way around. If you flip the switch upwards, the middle and bottom pins are connected to eachother. If you flip it downwards the top and middle pins are connected so on the eventual panel, the switch should have LowPass marked at the top and HighPass at the bottom.  This way of marking the switch functions goes for all the layouts on this website that have switches in them.

Close up of just the stripboard layout (Print this one and use it for your project. The lay-out is guaranteed, tested and verified faultless. It has been used by hundreds of people by now). Don't forget to cut all the copper strips under the IC's.

(Last revised 7-Oct.-2023 Cosmetic changes. Removed colour codes from resistors, added colour coding to wirebridges.)

Cuts and wirebridges as seen from the COMPONENT SIDE! As always, mark the cuts to be made with a black waterproof Sharpie on the component side. Then put a pin through the marked holes and mark them again on the copper side. Then cut the copper at the marked positions with a sharp, hand held, 6 or 7mm drill bit. Check your cuts with a continuïty meter or a powerful magnifying glass before you proceed.

Bill of Materials:

About the components:
I used fairly cheap LM13700 chips I got out of China via eBay and I think I got lucky and got real ones instead of fakes. But play it safe and get your chips from a reputable source.
I used a DPDT toggle switch (Double Pole Double Throw) to switch between High-Pass and Low-Pass but you could also use a jack socket for High-Pass input with a build-in switch that connects C4 to ground when nothing is plugged in there.
The Cut-Off Frequency potmeter in this build is a 100K one, but you can use any type you wish because pins 1 and 3 are connected to the + and - of the power supply so it is nothing more then a voltage devider. I saw that Sam Battle uses a 4K7 pots for this in his layout so use what ever value you want. (Beware that the voltage difference over that potmeter is 24 Volts so don't go too low with the value or you'll fry your potmeter. Remember Ohm's law!)
I don't think the Resonance potmeter is that critical either but you better stick to the schematics for that one. Keep it a 100K potmeter. I used a logarithmic one but linear will work fine too. As mentioned earlier, you can use an LM13600 instead of the LM13700 and instead of the TL074 you can use the TL084 or the LM324. They all work just fine.
I rebuilt the filter recently and used polystyrene caps instead of ceramics. It did make a difference in my filter but the first version I built of this filter was a mess to look at. Building something neat and tidy always makes it work better I found out. So don't expect miracles by changing the caps from ceramic to polystyrene.
One thing that might be worth experimenting with is the 10K feedback resistor over opamp A3 in the schematic. The one by the LEDs. You could put in a 15K trimpot and see what it does if you change the feedback resistance. I haven't tried this yet myself but it might be worth experimenting with.

Here's a picture of the panel I made for it. Because I recently re-built the filter I had one unused mounting hole left in the panel so I put a white 3mm LED in there. It's a bit bright =)

Here's a picture of the insides. Those big brown Mullard 'tropical fish' caps (so called for their colourful stripes) at the top and bottom are just 100nF caps for de-coupling. They're spread out a bit because they're so big. I advise to use polystyrene caps for all the other capacitors although that's not necessary for the filter to work. I just found polystyrene caps to sound better but you mustn't attach too much importance to the use of polystyrene caps. They are difficult to find. Any type will do.:

DEMO VIDEOS:

Here's a new demo video I recently made. Just trying out the different functions. I had it connected to the 8 step sequencer. It sounds really awesome but it needs a snappy ADSR input and attenuation on the audio input to make it sound like this:.

This is an old test video, made right at the beginning of when I started building modules. It has the Korg filter (in Highpass mode) and the Moog Ladder filter (in Lowpass) in series and using my 8 step sequencer and reverb from the CaraOK effects module:

Sounds quite good doesn't it? Especially with added echo or phase-shift effects. 

WARNING: Beware your speakers!! This filter can oscillate at below audible frequencies and the cones of your bass speakers will take a hell of a beating if you've got some serious amplification going. If ever a filter could be called a 'Speaker Ripper' this one is it. Quite literally. (I added this warning because tonight I almost blew my speakers up with this filter.)

EXTRA DOCUMENTATION:

I want to direct your attention to a very useful page from Scott Stites' website. He talks about all the different aspects of this filter, using two of these filters in tandem and his approach to adding a Band-Pass mode to it. If you want to build this filter, you have to read this text I think.: Click here for Scott Stites website.

Here is a link to a PDF file called: "A Study of the Korg MS10 and MS20 filters by Timothy E. Stinchcombe --- click here for that PDF ---
It's an in depth study with formulas and schematics, excellent if you're an electronics student and want to get into the details. I must admit it goes way over my head ^__^

Lastly, here's an other very interesting document I found by Sound Semiconductor entitled "Designing Voltage Controlled Filters for Synthesizers with the SSI-2164."
It goes into great detail into how filters work and how to design them and places specific emphasis on the Korg MS-20 filter.

Okay, that's it for this one. If you have any questions or comments then please put them in the comment section below of post them in the special Facebook Discussion and Help Group for this website.

Fun fact: this article is on a solid number two listing of most popular builds on this website, number one being the AS3340 (Digisound-80) VCO. (you know, the really good one.. ^___^  )