Sunday, 15 December 2019

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 .

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. Of course, combining this filter in High-Pass mode with one of the other lowpass filters gives you the bandpass option too and with a much better sound! (Check out the video at the bottom of this article to hear this filter in series with the Moog Ladder Filter. A really cool combination.) And if you build the Dual Korg Filter you'll have an even better bandpass option but that's for an other article.
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.

SCHEMATIC:
For this filter I used the 'Late MS20 Filter' schematic from Rene Schmitz which you can find by clicking here.
As he mentions in the text with his schematic, the gain of the opamps is hightened so you can get some weird sounds out of this. That's certainly true. :)
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' 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.

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.

6dB vs 12dB.
Although this is a 12dB per Octave filter it is possible to get an output with 6dB per Octave roll-off 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 won't work in HPF 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 choise between 6dB or 12dB. In the rebuilt filter I also used two BC558 transistors that were matched to within 4 points of their Hfe using a simple multimeter transistor tester. I'm not sure if this is necessary but it can't hurt. See the video below for a demo of the 6dB sound.

LM13700 pinout:
I took the part of the Rene Schmitz schematic that shows the CA3080's and I added the pinout numbers for the LM13700 chip which is just 2 CA3080's in one chip with added buffers which are not used in this filter. I'm not showing the whole schematic on my website because I don't want to draw visitors away from his website. It's Rene's schematic, not mine. Here are the pinout numbers:


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.
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.


(Last revised 1-March-2020 Corrected mistake with 470K resistor. 07-07-2020 added 6dB/Oct option.)

About the DPDT switch wiring for HP/LP mode: 
Connect the top two pins and the right middle pin 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: the HighPass and LowPass marking next to the switch on the wiring diagram reflect the function of the actual wires connected to 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 strip-board lay-out (Print this one and use it for your build. The lay-out is guaranteed, tested and verified faultless). The green wire-bridges are coloured green because they indicate a connection to ground. Don't forget to cut all the copper strips under the IC's.


(Last revised 1-March-2020 Corrected mistake with 470K resistor. 07-07-2020 Added 6dB/Oct option.)

Bill of Materials:


About the components:
I used fairly cheap LM13700 chips I got out of China and they worked just fine, however I'm convinced using the real thing will make it sound better. 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 for this in his layout so use what ever value you want. (Beware that the voltage difference over that potmeter is 30 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.
As I mentioned earlier, 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 I'm going to and I'll report here if I get any results.

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 ceramic caps at the top and bottom are just 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. Any type will do.:



The video below shows a demo of the filter after I had re-built it in July 2020 and added the 6dB per Octave option. The audio in this video is a bit crap and I'm just ploinking along the keyboard to test the sounds from low to high notes. (If I could play properly I would have done that, but there we are... =) 


Here's an other and older video demo-ing some  extra effects. In Low-Pass mode the Resonance Control has very little influence on the sound. In High-Pass mode it has much more effect but in this filter the Cut-Off frequency is what it's all about. That's what you use to get the cool sounds:



This is a test with 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 amazing 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.

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. I hope you enjoyed this and other articles and if you did why not leave a comment. I'd love to hear your thoughts or questions in the comments below. Any questions will be answered by me asap!
If you like this website and would like to sponsor future projects, you can 'Buy me a Coffee'.  There's a link for that underneath the main menu if you're on a PC or Mac. All donations will go towards components for future projects. Thank you!
Right, see you on the next one!



32 comments:

  1. Hey
    I just built this thing. And I got really excited when it worked, but I soon realized that the cutoff effect decreases over time and after 2 minutes you can't hear anything changing anymore, although the resonance keeps working. Any idea why that could be?

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    1. That is weird. I've never heard this sort of problem before. Could there be a faulty capacitor somewhere? I've built 3 of these and they all work fine.

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  2. Just found this article after receiveing some PCB's I designed using the Rene Schmitz late Korg MS20 design. Thanks for the info, now I have to try and find some polystyrene caps.

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    1. Thank you Leonardo. You don't "absolutely must use" Polystyrene caps. You can use those plastic Polyester caps or silver mica. As long as it isn't Ceramic.

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    2. Thanks Eddy. On the hunt for some non-ceramic caps! Love the sound of your filter by the way!

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    3. Thank you! Good luck with the build!

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  3. This comment has been removed by the author.

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    1. I'm sorry you're having so much problems with it. I wouldn't know what caused it. Maybe you should abandone this project and build the steiner parker filter instead.

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  4. Hi Eddy. Do you think a 470nF MLCC C0G cap will work in place of the styrene?

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    1. Yes, that should work fine. You only have to solder some wires to it.

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  5. Hey Eddy,
    Me and a Friend built quite some of your modules and we are very happy with the results. (we had quite some starting issues and are currently tackling most problems, but all in all managable)
    So I first wanted to thank you for all the lovably detailed information through your journey of building them! :)

    Also I wanted to ask a quick question on the dangerousness... dangerability or whatever of this filter.
    I tried it today with your CEM3310 envelope ADSR triggering resonance oscillations (theremin laser gun sounds)
    When turning up the resonance with low filter I can hear the low frequency oscillations you were talking about (I think)
    I was surely worried if that would harm my speakers, so I was thinking to add a high pass filter into the circuit cutting the frequencys at approx. 40Hz (just a CR filter after the output)

    I will try it and wanted to ask for your opinion on this, and maby if you had additional suggestions, as I think many would ask this same question.
    I think it could interfere with the whole "Symmetric supply amplifier chain" in all of synthesizer modules, where there is no capacitance in the audio signal chain, like in guitar pedals. Maby this could be bad for sound or signal impedance voodoo or something :)

    Thanks a lot!
    - A Fan (Frederik)

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    1. Hi, I'm glad you're finding the website useful. As for your question, I wouldn't put in a highpass filter. Just make sure the speakers you're using are up to spec. That way they can handle the low frequencies and you get to listen to awesome low bass :)

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    2. Thanks a lot, I will do that :)
      Another question arose until recently, I wanted to make this circuit track 1V/oct properly, but I couldnt I tried to use an extra input without the potentiometer, directly connected to the cv input with only a 100 k resistor (in the schematic of rene it is described as "pitch".
      Though I only get 1V per 15 notes approximately.
      How would I tackle this issue properly?

      Thanks a lot!
      Frederik

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    3. I don't think you can Frederik. Most filters won't track properly over more than one or two octaves. Remember they're not VCO's.

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    4. Okay I still tried it, i exchanged the 100k resistor on the "pitch" connector with an 100k potentiometer, tuning it down to approx. 82k I received pretty good results! I could track over many octaves :)
      the only problem is tuning stability I guess, because it will detune with temperature.
      Thanks still! :)

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    5. Okay cool! That's good to know. I have a question. Do you get a lot of self oscillation in low pass mode with this filter. Because mine only really squeels in High Pass mode but in LP mode the self oscillation is almost non exsistent.

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    6. yeah I start to get self oscillation with following settings in 6dB
      Resonance pot: 50% 12o'clock
      And 12dB
      Resonance Pot: ~30% 9 o'clock

      And besides I added a Linearity Pot on the plate to adjust the cv for 1/8,
      I have drawn a eagle schematic and would draw and describe it there, would you be interested in a Pdf of it?
      I plan to order them from a pcb fab with smd components and all :)

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    7. Yeah that'll be very helpful. Can you email it to me? Just write my name in small letters and add @gmail.com

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  6. I will, itl take maby some time :)

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  7. Hi Eddy! I just built this filter and it works! The only issue i found is some instability in the final buffer stage, the 470n capacitor is connected directly to the output of the opamp and looks like the capacitive load is making the stage oscillate, adding a 100ohm resistor before the capacitor seems to solve the problem

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    1. Glad it worked. I didn't design this filter but it's good to know you found a solution in case anybody else has this problem. Thanks for your feedback.

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  8. Hi Eddy! Thanks a lot for sharing. I was looking to start building the version LMNC published but when I saw your page I decided I'd give yours a shot!
    Everything seems to be working just like yours except one thing: on the 12dB Lowpass when reaching half of the cutoff knob range the output gets totally silenced and comes back when turning back the knob. Do you know what could cause this problem (btw I'm quite new to DIY synth so this might be a common mistake I made). Thank you

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  9. Hi Eddy, Just built this filter. I tried using a rotary switch or stepped attenuator for the frequency cutoff just like an old altec filter does. However, when I tested it for the first time, for some reason the cutoff CV level pot started smoking! It happened when I was moving the frequency cutoff knob which as you mentioned is a voltage divider. I used the SparkFun BOB-13099 Rotary Switch breakout board with the 10 position single pole rotary switch from proto pic. I used the resistors in this order, 2x 1k8 then 4x 4k7 then 3x 10k which adds up to 52.4K in total. The breakout board has the normal CCW, W and CW ports which I hooked up accordingly to the diagram (CCW being the left when pot meter facing towards you). I noticed one of the LEDs on my power supply (FC Power supply [running at 12v], same one LMNC used) dimmed as I was turning the knob. Something must not be quite right with the values I chose. I know this isn't related to your design but just wanted to ask if there was a way to get the filter working with the rotary switch instead of a traditional pot meter and if so what values of resistance should I use? I was unable to get any sound through the filter, when I switched the 6/12dB switch I got constant high pitched noise, followed by smoke from the pot meter. Thinking of rewiring the components in a new enclosure as my first attempt was quite messy. I checked the vero board layout multiple times to make sure all components were correct, so I am certain the issue lies in the wiring and/or the resistance values of the rotary switch. Apologies if this is confusing or unrelated to your designs. Any help will be much appreciated!
    Chris

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    1. That smoke is deffo not okay (obviously). That means there are high currents flowing. High enough to heat the resistor in your stepswitch up to smoking. Normally the currents in this filter are in the milli-amp region so check all connections that go to the power rails. There must be something wired up wrong. Has to be. As for the resistor values. The freq cutoff potmeter is a linear one so you can just divide the number of steps by the total resistance you want. Like 100K / 10 steps = 10K per step. That should do it. I'd put in a potmeter if I were you.

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  10. Hello Eddy ! Thank you so much for sharing your circuits experiments process, its really a great blog! I have built this filter and add it to a circuit that i have made with 3 oscillators, so its a standalone instrument not modular.. Im powering it with 12v. Im a newbie trying to experiment and learn with my buildings, i would like to understand better what kind of voltages I can send via the cv jack on this filter, do i have to be carefull with the values or negative voltages? thank you!

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    1. Hi, I'm glad you're having so much fun with my website. This filter is quite stirdy so the CV inputs can handle all voltages between + and - 10V. The CV voltages go into transistors first and are attenuated with 100K resistors so there's not much that can go wrong. By all means, experiment to your hearts content ^___^

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    2. oh thank you the fast reply!! :+) ohh ok! good!! I will experiment more yes! Also I want to add a delay to this instrument, i already have done some delays pedals with the pt2399, but i found them to be a bit noisy on long delays..., i will try to search more about it. have a great weekend!

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    3. I just commented about pedals but I hadn't read that you already tried them haha. Well I just got the Bloodmoon Phaser and love it but circuits with the PT2399 can be noisy because they stretch the chip over it's normal working area. It doesn't have enough built in memory for longer delays or reverb.

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    4. :) yes! thats the problem with the Pt2399, i will search about others possible chips. The Bloodmoon Phaser looks nice!!

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