MS-20 Voltage Controlled Filter for Cheap

What you need:

All the parts for this build

A clean, well-lit working surface

Your soldering iron

Nice solder

Pliers, wire strippers, tweezers, whatever

A large hunk of poster putty to hold your work in place

This Instructable!

Remember, you’ll need a bipolar power supply to run this circuit. Mounting it to a panel and in an enclosure is up to you. If you want to see how I do it, in tin cans, check out my video about that on Youtube. Search for ozerik -- that’s me.

This project is based on a lightly modified version of René Schmitz' version of the very well-regarded Korg MS-20 VCF. This circuit has so much potential for modification, but the purpose of this project is to let anyone with enough patience and dexterity build themselves a pro-quality VCF module for literally a few dollars.

Find René's project here

My own schematic is here

BOM (Bill of Materials)

(all the parts you’ll need)

• 1 x TL074 quad op amp
• 1 x LM13700 dual OTA
• 2 x 2N3906 PNP transistors
• 2 x green LEDs 2 x 100K potentiometer
• 1 x 470K resistor
• 2 x 100K resistors
• 7 x 10K resistors
• 1 x 4.7K resistor
• One more resistor, from 2.2K to 20K… see text!
• 4 x 220R resistors
• 1 x 1uF electrolytic capacitor
• 1 x 100nF ceramic disc capacitor
• 1 x 4.7nF ceramic disc capacitor
• 2 x 1.5nF film capacitor

Okay, here’s the two chips you’ll need. The cutouts in the near end indicate that that’s the “north” or “top” end of the chip. These two chips also have a small circular depression on that end of the chip. The pin nearest to that dip is pin one (1). The pins are numbered from there, going counter-clockwise down, across, then up.

The TL074 has 14 pins. The LM13700 has 16 pins. This makes the pin across from pin 1 of the TL074 pin 14, the pin across from pin 1 of the LM13700 is pin 16. The reason the pins are numbered that way is because when electronics were all round glass tubes, there would be pin 1, and the bottom of the tube would be numbered clockwise around the circle. In this document, I will be using the pin numbers to help you get the wiring exactly correct.

Here’s the LM13700.

Cut these pins short: 1, 3, 4, 13, 14, 16. Cut these pins right off: 2, 7, 8, 9, 10, 15. You’re going to do the same thing to both sides of the chip. Both chips we’re using in this build are symmetrical, besides the power connections.

Here’s the TL074. You’ll bend the shown pins together like that, and do the same thing to the other side. The pin numbers are 6, 7, 8, 9.

Our first solder!!!

Put the LM13700 directly on top — and the other way around from — the TL074. The notches in the chips will be on opposite ends of the build. This is very important, since the power pins on the chips are backwards from each other.

The pairs of pins that will be soldered together, listed with the LM13700 pin first, then the TL074: 5 and 10. 6 and 9. 11 and 5. 12 and 4. Hope that made sense, just copy the picture carefully and solder these pins together, and the pins on the other side as well. So far we have stayed symmetrical — what you do to one side of the project you also do to the other.

Our first resistors!!!!! And so far, we’re still symmetrical!

These 220R resistors go to pins 3, 4, 13, and 14. Leave the shorter leads about that long, no shorter since these resistors need to bend like in the next step:

Bend the leads down away from the notch in the LM13700 and twist them together. No need to solder them yet, we still want them slightly flexible and many other connections will be made to those leads.

The long leads of these 220R resistors are going to be our circuit ground point. Everything that needs to be grounded will be connected to that long set of twisted leads.

This is the project flipped upside down. Bend the middle pins of the TL074 out (pins number 4 and 11), and twist the leads of the capacitor around them. Be careful with this part of the circuit. The ends of this capacitor will be carrying power to the project, and if there is any short circuit here, the project won’t work and might burn up.

Be sure to use a little ceramic disc capacitor here, since they are actually better than fancy bigger more expensive capacitors in this role.

It doesn’t matter at all where the capacitor’s lenticular body lays. The important bit is keeping the bits carrying power from touching any other metal.

This 10K resistor goes from pin 13 of the LM13700 to the two bent-together pins of the TL074. You’ll do the same thing on the other side of the build.

It’s a good idea to keep the bulging parts of resistors from pressing up against other metal parts. The bulges are little metal cups that are part of the leads. There’s just a layer of paint insulating that part, so in this instance, if the upper part of that 10K resistor were to scrape against the pin next to where it’s connected, the paint might scrape off and make surprise contact. This has happened to me before, so don’t let the resistor bulge scrape other metal parts!

Here’s a view of the other end of the 10K resistor that is also connected to pin 13 of the other chip.

Here’s the other side. Connect the 10K resistor to pin 4 of the LM13700, with the other end connected to the bent-together pins.

Get ready for a record scratch, because so far, everything has been symmetrical. But next up!?!?!?

GRRRrrtchchchc!!! We’ve gone and destroyed the symmetry of your project. Also we scratched the hell out of my vintage Steve “Silk” Hurley EP dang it.

Here is the 10K resistor that goes from one half of the circuit to the other half. Attach one end as shown to the bent-together pins of the bottom chip. Notice the view angle here and be careful to get it right. When you’re happy with the solder joint, you can trim that lead right off.

The other end of that 10K resistor goes to pin 14 of the LM13700. Yes, one of the 220R resistors is also connected to that pin, but if the other end of the 220R resistor is securely twisted into the bunch, it should stay put when you remelt that solder joint.

Moving on!

These two pins need to be bent like that. This is the TL074, which has 14 pins, and these are the last two pins: 13 and 14. Bend 13 up with a little kink, and pin 14 out slightly with a little kink.

As long as you bend the pins just once, and are not rough, they don’t mind being bent like this. If you bend one back and forth just a few times, it is likely to break off, so be tender.

Okay, here’s a surprise. This circuit uses LEDs as part of the audio circuitry. The LEDs prevent the filter’s resonance from becoming uncontrollably loud. Green LEDs are what I usually use, but any other color will work too, but they may change the character of the resonance. Generally red LEDs will make the resonant feedback quieter, blue or white (or pink or UV) will be loudest, yellow and green are a nice middle ground.

Take two matching LEDs (or not matching, get crazy with it if you want) and bend them the same way as each other meaning, if the LED is a person sitting down, their same leg is the short one. Doesn’t matter which, as long as it’s the same one. If the LEDs are people sitting down, they'll sit in the next step butt-to-butt, or "heel-to-toe", basically their polarity must be flipped from each other.

Connect the first LED facing this way, with the top leg connected to pin 13 of the TL074 (the bottom chip) and the other leg of the LED connected to pin 14.

Try to work quickly here. LEDs are a bit heat-sensitive, so if you linger for 10 seconds on the solder joint, you may break the LED.

Here’s the second LED. It “sits” right on the other one and is connected leg-to-leg with the other one. In this picture, I’ve already trimmed the leads.

Again, try to work quickly. With both leads of the first LED held in place, you should be able to get the second LED attached one leg at a time without the first LED moving.

This is a view of the LEDs. The “anvil” or “cup” shape is the cathode, or “more negative” side of the LED, and as you can see, the cathodes are turned around from each other. That’s the way it needs to be!

Here’s the 10K resistor that goes between the pins we’ve been working with. It goes between pin 13 and 14 of the TL074, (the bottom chip).

This is a crowded part of the circuit! There is one more connection that will go to each of these pins, but that will come in just a moment.

All right!!! This is our first audio capacitor! This part does the magical filtering part of this circuit, so people who care about audio quality usually use film capacitors like this.

This is a 1.5nF capacitor, which will be marked with the number 152. 152 means 15 with two zeros trailing, so 1500 in picofarads equals 1.5 nanofarads. The power bypass capacitor under this project is a 104, meaning 10 with 0000 trailing, for 100,000 picofarads: 100nF.

Anyway, attach one leg of this capacitor to the pins that are soldered together between the chips that are not the power pins. This means pin 10 of the bottom chip and pin 5 of the top chip.

The other leg of this capacitor goes to pin 14 of the TL074 (the bottom chip). This is the last thing we’re going to connect to that poor pin!

Be careful that the relatively long uninsulated lead going from the capacitor to that pin is as short and straight as you can make it. You don’t want it bending around and touching other parts.

The second magic capacitor!

This is an identical 1.5nF capacitor. Connect it to the pins on the opposite side of the project, pin 12 of the top chip, pin 5 of the bottom chip.

Make sure to route the capacitor leg carefully so it doesn’t touch any of the pins or leads near it.

The other side of the capacitor connects to the long twisted bundle of leads. This is, as you’ll recall, the ground point of the whole circuit.

Look at it. Loooook at it.

This is on the same side of the project as the previous step. This is pin 3 of the TL074 bent out and up like that. In the next step we’re going to connect that to the ground bundle, so that’ll help you know how to bend it.

Attach a bit of wire (a trimmed resistor lead is what I used) to the pin. Twist the other end of the lead around the bundle of ground wires. Again, this is pin 3 of the TL074 (the bottom chip).

Here’s another place you can use a cheap junky ceramic disc capacitor! This is a 4.7nF capacitor between pins 1 and 2 of the TL074 (the bottom chip). If you don’t have a 4.7nF capacitor, anything between 500pF (0.1nf, or code 501) and up to maybe 10nF (maybe even more?) should be okay.

This area of the circuit is always the most confusing for me, so let’s dive in!!! First up, some PNP transistors!!!

Here they are, all outlined and with one leg bent. I use 2n3906 transistors, but any PNP transistor will do fine. Be very aware that different transistors often have different pinouts, so to be safe, just use 2n3906 transistors.

PNP stands for Pointing iN Please (no it doesn’t) so the arrow in the schematic symbol points in. The lead that I bent up here is the lead that, in the schematic, has the arrow. If you select a different PNP transistor, make sure to bend the leg that has the arrow.

Okay! The transistors go in for a weird flat-to-flat hug, with their bent arms holding on to each other. Aww cute, right? This way, they’re thermally coupled (hot!) which is important for some analog synthesizer circuits, and it will definitely help this filter’s cutoff frequency not drift when the temperature changes. Trim those hugging arms, and let’s move on to the next step!

This one may be tricky.

You’re looking at the LED end of your project. Point the hugging arms of the transistor pair toward the nearer end of the project. Eventually those hugging arms will be connected to pin 1 of the TL074 with a resistor, so that’s where it needs to be situated. The other outside pin of the down-pointing transistor attaches to pin 2 of the TL074 (the bottom chip). The middle pin of that down-pointing transistor gets bent straight out. Follow the picture carefully!

Bend the middle pin of the up-pointing transistor in to touch the ground-bundle. The non-hugging pin of the up-pointing transistor is trimmed already in this picture.

Here’s another view of this step with the joint soldered.

Here’s a 1.8K resistor going from the middle leg of the down-pointing NPN transistor. If you know your resistor color codes, you’ll see that it’s not actually a 1.8K resistor. I messed up.

But do use a 1.8K resistor, attach one end to the middle leg that you already bent outwards. The other end of that resistor goes to ground…

...like this! It looks almost like the hugging-arms of that PNP transistor pair are also connected to ground, but they’re not. The middle leg of the up-pointing transistor is grounded, as well as the end of the 1.8K resistor.

We’re not quite done with this section of the circuit, but let’s move to something a bit different:

Here’s two 10K resistors twisted and trimmed just like this. They look like marshmallows on a campfire fork ha ha ha ha ha ha ha (breathe) ha ha.

Attach the short ends of the 10K resistors to pins 1 and 16 of the LM13700 (the top chip). These resistors are involved in changing how much the LM13700 amplifies the signal coming in to the circuit.

The twisty ends of our campfire marshmallow fork goes to the non-hugging pin of the up-pointing PNP transistor. Bend the leads towards each other and solder them up!

Of course here’s another area of the circuit with uninsulated leads stretching a ways. Make them as short and straight as possible so they’re not going to bend around and touch other parts of the circuit.

Eagle-eyed readers will spot that by this time I noticed that I had used the wrong value for the resistor that goes between the middle pin of the down-pointing transistor and ground. In this picture it’s fixed, in the previous picture it’s still wrong.

Here is the 4.7K resistor that connects the hugging arms of the pair of PNP transistors to pin 1 of the TL074. Hook it up like this!

Bend the 4.7K resistor lead over so it can touch the hugging-arms of the PNP transistor pair. This part will be close to the potentiometer in the next step, so make sure it’s tidy and snug.

We’re done with this part of the circuit! If you’re still with me, you’re doing great!!!

This is a 100K potentiometer. The outside pins of a potentiometer are the two ends of a longer-than-usual resistor. The middle pin connects to a “wiper” that makes contact with the resistor at different points, depending on where you turn the potentiometer. I always think of potentiometers having a “high” side and a “low” side. When you turn a potentiometer all the way “up” (as in, higher volume), I think of the wiper moving towards the “high” pin.

This potentiometer (which I’m reusing from an old project -- look at the paint and glue on it!) has the “low” side connected to ground. It’s attenuating the signal feeding back into the filter, increasing the resonance of the filter. Depending on choices you can make later, this potentiometer will change this circuit from a nice mild low-pass filter into a screaming monster of sonic disturbance.

Bend the pins of your potentiometer to point up like this. Trim the long bundle of ground leads, and make a very sturdy solder joint from the “low” pin of the potentiometer to that bundle of grounds. This solder joint will hold the structure of your circuit in place, so do take care to make it strong.

Also, to make it easier to follow along in the next few steps, turn your project until the LED pair is hanging out near the “high” pin of the potentiometer.

Basically, copy the picture.

Here’s a 1uF electrolytic capacitor. Electrolytic capacitors are polarized, so they have a + leg and a - leg. The - leg is usually marked with a stripe that features little minus signs inside it.

Connect the + leg of the capacitor to pins 6 and 7 of the TL074 (the bottom chip). The - leg of this capacitor is this projects audio out, which means we are making serious progress!

Here’s a short piece of wire between the middle pin of the potentiometer and pin number 12 of the TL074 (the bottom chip). At this point, pin number 12 will be the only pin on that bottom chip that has nothing connected to it at all.

Connect another short bit of wire from the “high” pin of the potentiometer to the - leg of the 1uF capacitor. Leave the - leg of the 1uF capacitor a bit longer, since that’s where we’re going to be getting the signal out of this project.

This picture also shows the shorter wire going between the middle pin of the potentiometer and pin 12 of the TL074 (the bottom chip).

In this step you have a choice to make. This resistor goes between pin 13 of the TL074 (the bottom chip) and ground. Pin 13 is the up-bent pin that the LEDs and 10K resistor are attached to. This is the last connection we’ll make to that pin!

In this picture, it’s a 20K resistor. You can choose any value between, say, 20K and 2.2K.

The lower resistance (2.2K) will cause this circuit to self-oscillate sooner when you turn up the resonance knob (the potentiometer in this picture). If you choose that value, the circuit will begin to resonate with the knob about halfway up, and oscillate more as you turn up the knob, with the waveform changing as the amplitude gets higher and therefore more clipped by the two LEDs.

The higher resistance (20K) will not let the circuit oscillate at all. It will still be resonant, but you’ll just hear the spike in frequency response when you change the cutoff frequency, but it will never trip over into runaway oscillation feedback.

A nice compromise is between 4.7K and 8.1K.

Oh whoops, I forgot this resistor. It’s a much higher resistance part than any other in this circuit. Attach one end to pin 6 of the LM13700 (the top chip), pin 11 of the TL074 (the bottom chip). It needs to be connected where the negative power rail enters the project. In my build, it goes right across the 100nF power bypass capacitor. The other end goes to...

Pin 2 of the TL074 (the bottom chip)!!! If both ends of the 470K resistor attach to a part of the circuit with a ceramic disc capacitor (not the same ceramic disc capacitor), you’re in good shape.

I can’t believe I forgot this resistor until this point in the project. I’ve done it before, and the circuit doesn’t function without it! Next up: POWER!!!!

I get my power cables from Cat5 network cables. In all my projects, orange is positive, green is negative, brown (or white) is ground.

Get yourself some wires of whatever color you choose (but for real don’t forget which colors) and twist them together to make them tidy!!!

Okay, don’t twist them all the way together. Leave a hand’s-width untwisted, because the cutoff potentiometer needs to be attached to this wire as well as the main part of the project.

Here’s where the positive connection is made. Pin 4 of the TL074 (the bottom chip) and pin 11 of the LM13700 (the top chip). Be careful. Wire this up backwards, and stuff will burn up.

Also indicated is where the ground wire is attached, but that’ll be in the next picture too.

The negative power connection goes in the other side of the project. That will be pin 11 of the TL074 (the bottom chip) and pin 6 of the LM13700. Examine your power connections closely. As long as the power goes in to both sides of the 100nF ceramic disc capacitor on the bottom of the project, you’re probably okay. As long as you put that part in the right spot!

You can see where the ground is attached also. Get an even better look at it in the next picture!

The ground power connection goes right there!

Use wire strippers to mangle-strip the insulation of the positive and negative power wires a short length away from where the power wires go in to the project.

Here’s the power wires connected to the high leg (the positive wire) and the low leg (the negative wire) of this 100K potentiometer. The middle leg of this potentiometer doesn’t have anything connected to it right now.

Look at that potentiometer! Another used one!

Twist the ends of a couple 100K resistors together. Trim the twisted ends short, this isn’t a campfire marshmallow stick, it’s the opposite of that. Whatever that is.

These resistors are where the voltage-controlled filter has the voltage part coming into the circuit. One of these connects to the middle of the “Cutoff Frequency” potentiometer, and the other connects to an external CV input.

Okay, remember the down-pointing transistor in the pair of hugging NPN transistors? Attach the twisted leads of the pair of 100K resistors to the middle pin of the down-pointing transistor. Remember the 1.8K resistor that I got wrong earlier in the build? One side of that resistor goes to ground, the other goes to the middle leg where you’ll need to connect the 100K resistors.

Go ahead and trim the long ends of the pair of 100K resistors. Solder one of them to a longish bit of wire -- long enough to reach the middle leg of the second 100K potentiometer. Because that’s where it attaches!

The other 100K resistor is your CV (control voltage) input. Connect that through a wire to an input jack on your panel and label that sucker. If you want the option to attenuate the CV, you can do that! Connect the panel jack to the “high” side of a potentiometer (10K or 100K will work), the “low” side to ground, and the middle pin of the potentiometer can go to the 100K resistor in this picture.

See? Right there! The other end of this wire connects to one of the 100K resistors you just were working with.

Hey! This is the last resistor you’ll connect to your project!

Take the 10K resistor and solder it to pin 3 of the LM13700 (the top chip). This is where the signal will come in to your project. If you’re using a source that’s unconnected to anything else in this project (a battery-powered phone or mp3 player) you will need to attach a ground wire from the device’s ground (the sleeve or 3rd ring of an aux cable) and a signal wire (the tip (left) or first ring (right) of an aux cable). The project’s output is the - side of the 1uF electrolytic capacitor.

The input impedance of this project is 10K. If you connect a low-impedance device to the output (the 1uF capacitor) like, for instance, headphones, the capacitor and the device will form a high-pass filter that will take all the bass out of the sound. So make sure to either buffer the output with an op amp, or just be sure nothing you’ll plug it into will take the bass out.

The power draw is less than 15mA.