# Building the Korg MS20 Filter
**URL:** https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071
**Category:** DIY STUFF
**Created:** [October 26, 2020, 7:28pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071 "2020-10-26T19:28:49Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@TTZ148](https://lookmumnocomputer.discourse.group/u/TTZ148)
#### Post date: [October 26, 2020, 7:28pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/1 "2020-10-26T19:28:49Z")
I have this schematic of an MS20 Filter for a filter module I’m trying to construct from scratch, but I’m having trouble understanding how it works exactly. Mainly, I was wondering what the function of the two pnp mirroring transistors do in this circuit.
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### Author:  [@BlackDeath](https://lookmumnocomputer.discourse.group/u/BlackDeath)
#### Post date: [October 26, 2020, 7:55pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/2 "2020-10-26T19:55:22Z")
LMNC to the rescue?
[ ](https://www.youtube.com/watch?v=q-2MkTAr4xk)
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [October 26, 2020, 8:22pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/3 "2020-10-26T20:22:20Z")
> **[Exponential converters and how they work](https://northcoastsynthesis.com/news/exponential-converters-and-how-they-work/)**
>
> Why exponential converters are important in music electronics, and how they work.
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### Author:  [@THOGRE](https://lookmumnocomputer.discourse.group/u/THOGRE)
#### Post date: [October 26, 2020, 8:29pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/4 "2020-10-26T20:29:12Z")
It would be so awesome if these texts were in German so that I could understand them better ☹
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [October 26, 2020, 8:31pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/5 "2020-10-26T20:31:27Z")
German texts don’t turn up when I google.
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### Author:  [@THOGRE](https://lookmumnocomputer.discourse.group/u/THOGRE)
#### Post date: [October 26, 2020, 8:34pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/6 "2020-10-26T20:34:04Z")
Yeah, I know, I think I just wanted to moan a bit because my English is so bad 🙂
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### Author:  [@SYNTHGUY57](https://lookmumnocomputer.discourse.group/u/SYNTHGUY57)
#### Post date: [October 26, 2020, 9:32pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/7 "2020-10-26T21:32:33Z")
they are precisely matched BC558C transitors for fine perfeclty tuned oscillation. So you have to find BC558C"s and get the hFe’s to be close to exact as possible or buy them already done for more money . Matched trannys usually have emitters attached together.
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [October 26, 2020, 9:49pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/8 "2020-10-26T21:49:05Z")
It’s not as critical for a filter as for an oscillator, unless of course you intend to turn up the resonance and use the filter as a 1V/oct oscillator.
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### Author:  [@TTZ148](https://lookmumnocomputer.discourse.group/u/TTZ148)
#### Post date: [October 29, 2020, 6:48pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/9 "2020-10-29T18:48:59Z")
Thanks for the exponential circuits link! That helped a bunch. I kinda undestand the relationship between the inputs and the exponential converters, but I have another question on how the TL074 Op-Amp that’s connected to the transistors effect the circuit. How is it drawing power? Is it acting as the low pass filter? If that’s so, I thought that the resistor and capacitor had to be connected through the positive terminal of the Op-Amp.
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### Author:  [@TTZ148](https://lookmumnocomputer.discourse.group/u/TTZ148)
#### Post date: [October 29, 2020, 6:50pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/10 "2020-10-29T18:50:21Z")
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [October 29, 2020, 7:24pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/11 "2020-10-29T19:24:46Z")
He talks about that in the article. The op amp’s job is to maintain a constant current through the transistor (of 15V/470kΩ = 32 µA). The 4.7k resistor is to limit current and to reduce the voltage gain of the transistor, and the capacitor is for stability.
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### Author:  [@TTZ148](https://lookmumnocomputer.discourse.group/u/TTZ148)
#### Post date: [October 29, 2020, 7:39pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/12 "2020-10-29T19:39:06Z")
Ah! Ok, I see now. Thank You!
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### Author:  [@TTZ148](https://lookmumnocomputer.discourse.group/u/TTZ148)
#### Post date: [November 10, 2020, 3:06am UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/13 "2020-11-10T03:06:42Z")
After running a few simulations on the schematic through Pspice, I realized that the circled potentiometer isn’t the cutoff knob for the VCF. Do you know how I can control the cutoff frequency in this schematic?
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [November 10, 2020, 3:34am UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/14 "2020-11-10T03:34:37Z")
Yes, it is the cutoff knob. All the control voltages are on the upper left feeding into the transistors which form the expo converter: Two CVs with attenuator, one “pitch” CV with no attenuator which is V/Oct, and one knob that adds between -15V and +15V, the cutoff frequency knob.
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### Author:  [@TTZ148](https://lookmumnocomputer.discourse.group/u/TTZ148)
#### Post date: [November 10, 2020, 4:54pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/15 "2020-11-10T16:54:48Z")
Ah, I see. Thank You!
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### Author:  [@TayBae](https://lookmumnocomputer.discourse.group/u/TayBae)
#### Post date: [November 10, 2020, 6:36pm UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/16 "2020-11-10T18:36:52Z")
Is the pitch CV there to add filter tracking?
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### Author:  [@TTZ148](https://lookmumnocomputer.discourse.group/u/TTZ148)
#### Post date: [November 12, 2020, 12:09am UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/17 "2020-11-12T00:09:55Z")
So I ran some simulations through Pspice of the Korg MS20 Filter and for some reason the resonance control isn’t manipulating the frequency content of the signal before the cutoff. Instead, it attenuates the beginning of the signal the smaller the resonance knob resistance is.
Is there any way to fix this to where the resonance knob manipulates the opposite side of the signal near the cutoff?
(Thanks in advance you’ve been a big help and I greatly appreciate the feedback.)
Here is the output signal with the resonance knob set to 10k ohms:
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### Author:  [@TTZ148](https://lookmumnocomputer.discourse.group/u/TTZ148)
#### Post date: [November 12, 2020, 12:10am UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/18 "2020-11-12T00:10:32Z")
And here is the ouput signal with the resonance knob set to 500k ohms:
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### Author:  [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)
#### Post date: [August 27, 2021, 9:18am UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/19 "2021-08-27T09:18:33Z")
Base on René Schmitz but with a LM13700 and its internal buffers
[ ](https://www.youtube.com/watch?v=eELTtd9Bv-k)
Cutting pieces of metal in old computers reminds me of doing this for my first machines / sequencers, now i really prefer to work with wood, more easy 🙂
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### Author:  [@Farabide](https://lookmumnocomputer.discourse.group/u/Farabide)
#### Post date: [August 28, 2021, 6:37am UTC](https://lookmumnocomputer.discourse.group/t/building-the-korg-ms20-filter/2071/20 "2021-08-28T06:37:19Z")
Love the simplicity in these vids.
That perfboard was perfect.
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