Introduction: Diode Ladder VCF With NO PCB!

About: I love dancing, building circuits, and making electronic music. My wife and kids are pretty cool too.

Hey yo what is going on?

Welcome to a BONKERS complicated project which, if done right, will result in you having a very nice diode ladder low pass voltage controlled filter. This is based on a Electronics For Musicians design, with a couple important mods, and one mistake fixed. And of course, this is done without a PCB!


Here's what you need to build this!

  • 1 LM13700
  • 3 2N3904 NPN transistors
  • 2 2N3906 PNP transistors
  • 12 1N4148 diodes
  • 2 100K potentiometers
  • 1 100K trimmer
  • 1 100nF ceramic disc capacitor
  • 1 47nF film capacitor
  • 3 100nF film capacitors
  • 2 10uF electrolytic capacitors
  • 1 100uF electrolytic capacitor
  • 1 220uF electrolytic capacitor
  • 1 220R resistor
  • 5 1K resistor
  • 5 10K resistors
  • 1 47K resistor
  • 5 100K resistors
  • 1 220K resistor
  • 1 330K resistor
  • 1 1M resistor

Step 1: Grab a Bug! Kill It!

Here's a LM13700. This chip's killer app is as a voltage-controlled amplifier, a way to amplify signals based on another signal. We're only BARELY using it like this in this project, and that's because it also features extremely sensitive inputs that are perfect for extracting the filtered audio out of the ladder.

If you're attempting this circuit, you probably already know about the way chip pins are counted, starting at pin 1 to the left of the notch or mark on the chip, going down that side, across, and up. I'll be referring to pin numbers so your circuit looks exactly like mine!

Okay. Trim the skinny parts off of pins 1, 8, 9, 14 and 16. You don't HAVE to do this, I do it to make the chip easier to handle.

Rip off pins 2 and 15. These pins are sometimes used, they basically clip the signal from the inputs if the voltage gets too high. We aren't gonna use them.

Bend out pins 3 and 4. These are the input pins we're going to use to get the signal out of the diode ladder.

Pins 5, 7, 10, and 12 get bent straight up and over so they touch each other just like in the picture.

Pins 6 and 11 get the skinny parts bent out. These two pins are where power enters the chip.

Pin 13 gets bent under the chip -- it's going to be grounded. Maybe next time it will be home before curfew.

Basically, make your chip look like that chip!


Here's our first soldering job!

Pins 6 and 11 get power, so they need a capacitor like this across them. You know, to keep noise out and also keep noise in!

Step 3: This Is an Important Resistor

This is a 330K resistor going from pin 1 to pin 13. It doesn't need to go to pin 13, it just needs to go to ground, but pin 13 also needs to be grounded, so let's put all our grounds in one place.

This resistor sets the gain of the top bit of the circuit in the schematic. The original specification was 470K. Lowering the resistor to 330K increases the resonance possible in a very pleasing way. You could lower it further, but you risk clipping and more distortion, but hey, experiment away!

We're going to need a nicely accessible chunk of metal that's ground, so let's try to make the ground half of the resistor look like that.

Oh... and I've started buying 1/8th watt resistors because they're smaller. You totally don't need small resistors for any of this build, it's just what I prefer.

Step 4: One One Hundred Kay Resistor

Here is the 100K resistor that takes the signal from the output of the first half of the LM13700 to the other half.

It goes from pin 5 (and pin 7, they're soldered together) to pin 14.

Step 5: Our Least Valued Resistor

Here's a 220R resistor going from pin 14 to ground. Remember how the inputs of this chip are incredibly sensitive? The signal from the other half of this chip is going through a 100K resistor, which is 100,000 ohms. The signal is then shunted to ground through a 220 ohm resistor.

Step 6: Training for a Couple of 10Ks

Couch to ten K am I right?

Take a couple 10K resistors and twist them together. We'll solder the twisted-together bit to pin 6, which will be where the negative power goes in.

Step 7: Making Outputs Just a Bit More Negative

The other ends of the pair of 10K resistors will go to the two outputs of the, of the... the Darlington pair that's on the LM13700. Don't let the fancy name confuse you... just solder the two resistor ends to pins 8 and 9.

Step 8: A Cute Little 47K Resistor

For some reason we need to attach a 47K resistor from pin 10 (and 12) to ground. Do it like this!

Step 9: The Other Gain Setting Resistor and a Current Sinking Transistor!

This 10K resistor is going to attach to the bit of circuitry that we'll be able to adjust the resonance of this filter. Hook it up like this!

Then we're going to take a PNP transistor, bend the legs like in the second picture, and solder the two not-bent-legs like that. The middle leg will go to the mess of resistor leads that is ground in our project. The other leg (if you're looking at the schematic, the leg without the arrow) goes to the bent-over end of that 10K resistor that's soldered to pin 16.

When it's nice and securely in place, snip the free leg off. Poor little guy.

Step 10: The Rest of the Resonance Setting Circuitry!

Let's put a 1M resistor from the snipped-off free leg of the PNP transistor to pin 11, which is where the positive voltage goes into the LM13700.

Also we'll add a 220K resistor to that same leg of the PNP.

Check it out! If you want voltage-control over the resonance of this circuit, attach more than one 220K resistor to this point! You can do very interesting sorts of modulation by controlling the resonance of a filter with an audio signal.

Step 11: A Final Touch for This Part

Reach into the void with your trans-dimensional Gauntlet Of Mystery and grab four 1N4148 diodes. That's what I do, at least, you might just have them in a small bag in your parts bin.

Diodes have polarity, with electricity flowing only one way through them. Let's twist together the non-striped-legs of a pair, trim the legs that have the stripe, and solder non-striped-legs to the striped legs.

Confusing to explain, easy to copy, so just copy the picture!

Step 12: Wow, This Looks Messy!

The four diodes we just hooked together are the "top" of the diode ladder. The twisted-together ends connect to pin 10 of the LM13700. Pin 10 is where the positive voltage will enter the chip!

The two free ends of the diodes go to the two inputs on the other side of the LM13700. Those are pins 3 and 4.

I included a couple more pictures so you can be sure to get this part right.

It's really tight in there. This kind of diode is made of glass, so it's no biggie if the glass bit of the diodes touches other parts of the circuit, but please examine stuff very carefully to make sure there's no metal-to-metal contact, and even keep your leads away from the bodies of resistors -- there's metal right under a thin layer of paint!


This part is the FUN PART! It's going to go quickly, so enjoy it while it lasts!

Collect all your film capacitors and all your diodes. These parts are going to make the ladder!

Step 14: Start Like This.

Everybody* knows that diodes let electricity flow only one direction through them. The black stripe "stops" electricity. It's super vital, important, and critical that the polarity of the diodes in this build all go the same direction. Just one backwards diode will completely break your filter.

We need to work quickly with the diodes and let them cool off between solder joints. Too much heat for too long can break them.

Go ahead and build the ladder with the first three 100nF capacitors with all the diodes pointing one way. Once it's time to add the 47nF capacitor, you'll have to get it right.

*Everybody doesn't actually know that...

Step 15: It's a Ladder!!!

Look! The 100nF capacitor "rungs" are "upstream" of the direction of electricity flow from the 47nF capacitor.

The reason we're using one mismatched capacitor is that the most mind-bendingly cool diode ladder filter in the world is the one in the Roland TB-303. The designers of the filter in the 303 probably used a half-value resistor as the "bottom" rung by accident, or they were way too high on cocaine to coherently explain their space-trippy idea. Seriously. Play with a 303 (or a clone thereof) and try to explain how in the world that thing got made. It's a complete mess, but a completely amazing mess.

Right, anyway, the smaller capacitor goes on the "bottom" rung.

The "bottom" of the ladder gets another pair of diodes, the "top" does not.

Step 16: That Was Fun. Now Comes the Most Fiddly Part

There's just no good way to build this next part. It will wind up as a ridiculous chunk of resistors and transistors and capacitors, there's just no way to avoid it.

But follow along carefully, step by step, and we'll make it happen!

This is our first step. Conjure up a pair of NPN transistors, 2N3904s, and bend those pins like that. Looking at the schematic, you'll see that the pins we're bending are the ones with the arrows.

These two little transistors will hug each other now, and bend the legs together like that. Cute, huh?

Once the transistors are securely hugging each other, take the other side legs and bend them like that. Really you can bend them either way, at this point, the circuit is sort of symmetrical.

Step 17: Focus!

Take a pair of 1K resistors and twist the ends together.

And then, let's take the free legs, wrap them around the middle pins of the hugging transistors. Let's try to get your project look just like this, so have the hugging-legs pointing up, and have the 1K resistors toward you, matching this picture.

Step 18: Look! You Built a Tiny Little Man!

He's so so cute!

Step 19: Another Bit

Oooh, a 220uF capacitor!

Take one of those little guys and hook it up to a 1K resistor just like this!

Step 20: Another Pair of Transistors!

These, however, are different from each other.

Take the 2N3904 and bend the middle leg toward the flat side.

Take the 2N3906 and bend the side leg toward the flat side, the leg to the left, looking at the flat side.

When you've bent the legs like that, bend them even further, while making the transistors hug flat-to-flat, and solder them like that.

Step 21: The 2N3904 Does the Splits

We can't look at the flat bits of these parts anymore, but that's okay. Take the one with the middle leg bent, and make the side legs do the splits. Wow, flexible!

Step 22: Making a Diamond

All those three bits we just built get hooked together like this. Notice how I layed out the first picture, and notice that I was planning to mess up. Whoops! But I did build it the right way. Make your build look like this.

Pay close attention to the polarity of the electrolytic capacitor. All capacitors like this are polarized, meaning they can only really handle it when one of their legs has a higher voltage than the other. The "more negative" side is always marked with a stripe with minus signs printed into it.

........see, they make capacitors like this with two very thin sheets of aluminum foil wrapped up like a veggie wrap or a Little Debbie Swiss Roll or a cinnamon roll. There's this electrolyte gunk that can conduct electricity that's smeared on the aluminum foil and somehow they keep the aluminum foil sheets from touching each other. Then what they do is pass a current from one of the aluminum sheets to the other. This current causes one of the surfaces to collect aluminum oxide. Aluminum oxide is a dielectric, meaning it's an insulator. That insulation barrier is the most important part of capacitors, which are two plates of conductive material with a non-conductive material in between. Film capacitors have a layer of mylar or polyester or propylene or even waxed or oiled paper between the metal "plates" (sheets of foil). Ceramic capacitors have a little ceramic wafer between the plates (which actually look like tiny little plates in this case LOL). Aaaaanyway, if you try to put too much voltage into the negative side of an electrolytic capacitor, the dielectric coating of aluminum oxide will try to leap off the foil and follow the voltage to the other place, which will cause the capacitor to fail. Sometimes explosively.......

Step 23: Adding the Little Man

The little man's head from step 18 gets soldered to the joint between the + side of the electrolytic capacitor and the 10K resistor. Whew.

One of the ways I check my work with this kind of build is to count the components at a joint, and compare that to the schematic. I'm going to do that right now, you should do it too...

Hmm... 1, 2, 3, 4 resistors... one electrolytic capacitor... yup, that's five components, and that checks out with the schematic! That also means nothing else is going to connect to this spot. You can forget about it now!

Step 24: ANOTHER 1K Resistor

I hope you get lucky and cast a summoning spell with a +6 productivity bonus and get lots and lots of 1K resistors, because this build uses a lot of them

This 1K resistor goes between the free side leg of that one transistor that did the splits and the two transistor legs that are holding the pair in a hug.

Step 25: Get Ready for Heat, Middle Leg!

Our project at this point has only one transistor with nothing connected to its middle leg. Now's the time to solder a 1K resistor to that lonely middle leg. The other end of that resistor goes to the spot that includes the - side of the electrolytic capacitor.

This point of the build is where the voltage to control the filter's cutoff point goes. We'll deal with that in the next step. Don't worry, it's easy.

Step 26: Triplets!!!

Three 100K resistors converged in a wood, and I... wait, nevermind. Just hook up three resistors like that.

Then, we'll attach them to that point I was talking about in the last step. The 1K resistor and the middle leg of the transistor. The free end of those three resistors will be all the things we're going to use to adjust and control the cutoff of this filter!

I don't know why there's a nearly identical picture but there is. Just for reference, I guess.

Step 27: Oh! It's a Cute Blue Box!

A multiturn trimmer!

This little guy will go between the + power rail and the - power rail. By "rail" I don't mean literally the wires, I mean any point of the circuit that get that power. Actually the power wires DO attach here in my build.

To make our builds match most perfectly, bend the legs of your trimmer like that. To make our builds match even MORE perfectly, pull a trimmer out of some different project that eventually quit working correctly like a VCO based on a 4046 PLL chip.

Step 28: The Blue Box Finds a Home

Okay. The pair of 10K resistors are twisted together at the point where the + electricity will enter this circuit. The side leg of the transistor whose middle leg has the triplet of 100K resistors from a couple steps ago. Step 26. Good grief. We're more than halfway done, have courage!

The middle leg of the blue box trimmer gets connected to one of the 100K resistors. When you power on your completed filter and no sound comes out, you may have to adjust this trimmer to get the cutoff at a proper point.

And there's a couple reference pictures. Make it look the same!!!

Step 29: Time to Electrify! or at Least Attach the Electrifying Wires

You will notice (because I drew all over the photo) that my ground wire is in the wrong place.

Make sure to connect your ground wire (in this picture, it's white with a green stripe) to the - side of the electrolytic capacitor. Not like in that picture. I made a horrible error.

Luckily, I caught it before powering on my circuit.

The negative wire (green in this build) goes to where the side leg of the trimmer connects to the transistor leg.

The positive wire (orange in my build) goes to the other side leg of the trimmer, the leg that connects to the two 10K resistors.

Step 30: The Project Bits Unite!

The "bottom" of the ladder should have the diodes still hanging free. Those diodes attach to the side legs of the two transistors that were the Cute Little Man. Remember that guy?

At this point, the Cute Little Man is still symmetrical, it doesn't really matter which diode connects to which of the guys legs. But it will matter soon, and will be super confusing to explain if you don't do it just like this. Let's make our projects match each other's!

Step 31: All Together Again for the First Time!

Here is the step where the symmetry of the ladder and the Cute Little Guy gets destroyed! I'm not a physicist, so I'm not sure if additional symmetry increases or reduces chaos, since to my mind, a symmetrical object is orderly, but on the other hand, a universe with zero order at all is perfectly symmetrical in all ways.


Anyway, here's two views of how the "top" of the diode ladder attaches to the LM13700. Looking at the schematic, you'll see that the "right" upright of the ladder connects to the + input of the LM13700, while the "left" upright connects to the - input of the LM13700.

Look at the physical ladder with the capacitors pointing up at you. The upright on the right connects to pin 3 of the LM13700. The other upright connects to pin 4.

For some reason I didn't take a picture of the power wires going in to the chip. The positive power wire connects to pin 10, the negative wire goes to pin 6. You can barely see the connections in the pictures in the next step.

Step 32: Oooh, the Input Capacitor!

Here's the capacitor that the incoming audio signal will go through!

It's an electrolytic, so be sure to hook it up with the + side connecting to the middle leg of the transistor that connects to the "left" side of the diode ladder.

Next, we'll connect a 100K resistor to the - side of the capacitor.

Step 33: The Resonance Feedback Resistor!

This little guy is the same size as the 10uF capacitor, but it's higher capacity, at 100uF. Your 100uF capacitor will probably be bigger.

Connect the + side of the capacitor to the middle leg of the transistor that connects to the "right" side of the diode ladder.

Connect the - side of the capacitor to a piece of random wire you pulled out of the PS2 controller cable your sister's guinea pig chewed through. Or whatever.

The other side of that guinea-pig-mutilated wire goes to pin 9 of the LM13700, but while I have two pictures of the wire connecting to the capacitor, I don't have a single picture showing the other side of the wire. So check out the picture I did include. See? Pin 9, the corner pin...? OH MY WORD I just realized you can create notes on photos. I'm going to do that.

Step 34: Just a Couple Potentiometers

Here's two 100K potentiometers. I like this type of pot because they're seriously cheap, and they can be turned quickly very easily. They don't feel precise and they will wear out more quickly than fancier pots, but hey, tradeoffs, am I right?

You can use any type of potentiometer you like, sealed, expensive, recycled or repurposed, and even different values will work okay with this circuit, from 10K to 1M. The only difference will be in how the circuit parameters respond to the "action" of twisting the knobs.

Step 35: Our Pots Get Voltage

I think about potentiometers as having a "high" side and a "low" side. There's a wiper inside potentiometers that follow the knob, dragging against a 3/4ths circle of resistive stuff. When we turn the volume "up" all the way, we're carrying the middle pin's connection to the "high" leg of the potentiometer.

In this build, both potentiometers get + electricity to the "high" leg. Both get ground at their "low" leg.

Step 36: Resonance Under Control!

There's a 220K resistor connected to the middle leg of a transistor that's hanging off the LM13700 chip. That resistor gets connected to the middle leg of one of the potentiometers. Either one! We just need to remember so we can mount it in the right spot.

Also, remember the thing I talked about way back when we were dealing with this part of the circuit. If you want CV-controllable resonance, this is the place to do that.

Step 37: Cutoff Under Control!

Check out the blue wire!

Let's connect the middle leg of the other potentiometer to one of the 100K resistors in that triplet of resistors.

There's one more resistor there that's still unconnected to anything. That is where the CV to control the filter goes!

Step 38: Our Final Part!!! Also, Calibration.

This here is the VERY LAST PART! So so so exciting.

It's a 10uF capacitor. The + leg gets connected to pin 8 of the LM13700. That is where the audio of this circuit leaves! And now you're ready to connect all the bits to power and signals and turn it on and hope nothing explodes! Probably nothing will explode.

Once your is plugged in to power, an audio signal, and with an amplifier connected to the output capacitor, you may have sound coming through. Turn the resonance knob all the way down. Turn the cutoff knob to maximum. Then adjust the trimmer until the filter is letting the whole signal through. Then, turn the cutoff knob to minimum to see if all the audio signal is cut off. Fiddle with the trimmer until you're happy with how the cutoff knob works.

Turning up the resonance knob should create a frequency peak at the cutoff frequency. Turn the resonance knob higher than about 3 o'clock, and the filter will self-resonate. This signal is a breathy smooth almost-sine-wave. It sounds wonderful.

Let me know if you built this! It's very complicated, but the resulting project (if it works) is really good. I have over a dozen VCFs in my modular, and this one is my favorite.