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 recently was a discussion on the Synth DIY Facebook group about the possibility of switching this filter between 6dB per Octave and 12dB per Octave. I've tried it and it works up to a point but I don't find it useful at all and in order for it to work properly you'd have to make several changes to the original schematic. As this is not the point of this article, this option is not included here.
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.

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 the other two. 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.) This filter is self oscillating in both the Low and High-Pass configuration. In Low-Pass you can get oscillations on the low end (and they go very low, like 10Hz low) and in High-Pass the oscillations occur on the high end. When it's overdriven the two yellow LED's light up. It's possible to overdrive this filter so much that it switches off. With my filter in Low-Pass mode, if I have Resonance turned all the way up and I turn Cut-Off all the way up too, it will be too much for the filter to take. But that's no problem, I simply turn Cut-Off back and turn Resonance back by about a quarter turn, and the filter will kick in again. Of course this will vary with component tolerances etc. (The dual MS-20 filter I describe in article 15 suffers way less from this problem). You simply have to learn its limits. But it does mean you can get everything out of this filter that it has to offer. It is also greatly dependent on what you feed to the filter. The volume levels can be too much to take for this filter. That's probably in part due to the hightened gain of the buffer opamps, but I don't mind it. You can tell beforehand when the filter is about to clip and adjust the potmeters accordingly. The brightness of the LEDs is a great help in this. Use a little bit of attenuation and it all works perfectly fine. I found that by using an LM13600 instead of a LM13700 you can also tame it 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.

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. But the filter won't work normally in LPF mode and almost not at all in HPF mode because the signal is output from the stage before the one where we input the Highpass audio signal.
You can get decent sounds in 6dB/Oct mode but you'll need to constantly work the controls and the resonance will also work as a gain potmeter so the audio levels are all over the place. I have experimented with this but my conclusion is: Don't bother. It's not worth the trouble. If you want even cooler sounds than the normal MS-20 filter then build the Dual Korg MS-20 filter or the ARP2600 filter. ^____^

LM13700 pinout:
I took the part of the Rene Schmitz schematic that shows the CA3080 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.)

About the DPDT switch wiring: 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.

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). Don't forget to cut all the copper strips under the IC's.


(Last revised 1-March-2020 Corrected mistake with 470K resistor.)

Bill of Materials:



About the components:
I used fairly cheap LM13700 chips I got out of China and they worked just fine. Here's the link from where I got them: click here.
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.

Here's a picture of the panel I made for it.



Because I'm running out of space in my synthesizer cabinet, the PCB is mounted at 90° to the panel with two L-Brackets I made from coppersheet. That way I can make the panel smaller. I would have used smaller knobs too but I'm fresh out of those. They're in the mail though.

These are my own recordings of the first tests of the filter after completing the build. I had left the CaraOK effects module on, so that's where the echo comes from. I also had the LP/HP switched labeled wrong. The video starts with the filter in High-Pass mode:



Here's a second video I made 6 months after the first one giving some more in depth samples of this filter without 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. It sounds very 'Giorgio Moroder' to me ^____^

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.)

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!
Right, see you on the next one!



2 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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    Replies
    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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