MantisLadder Voltage Controlled Filter

Hi! Here's how to build your new MantisLadder module. It's a voltage-controlled filter, built as a low-pass filter, but can be tweaked to sort of be a high-pass filter too by using the Inversion knob. Anyway, you've already got one and don't need to hear more about why you want it, so let's build it!

That picture is of my first prototype of this filter. The resemblance to a praying mantis is um, pretty obvious LOL anyway, point-to-point circuit building is my jam. Dead-bug it's also called, but keep dead bugs away from my jam, ew gross.

You'll need the kit, which is hopefully complete. Juanito (me) is not an organized person so perhaps you got a kit from him that's missing one or more parts. a) he apologizes and b) you can probably get hold of him online and he'll send you parts.

Or maybe you just have the PCB. Or got the files and printed your own? Anyway, here's a BOM:

• PCB and faceplate
• 6.2V Zener diode, 1N4735
• 1 x 10-pin IDC shrouded pin header
• 1 x 100K box trimmer
• 1 x 1K box trimmer
• 4 x 100uF electrolytic capacitors, at least 16V, more V is okay I use 35V caps
• 4 x 10nF film capacitors (the PCB silkscreen says 22nF but that's too large)
• 1 x 4.7nF film capacitor (optional -- a half-size capacitor in the bottom rung may give your filter a more acid sound)
• 4 x 100K vertical sealed box-style spline-shaft potentiometers (I always use log)
• 1 x 10K vertical sealed box-style spline-shaft potentiometers (log or audio who cares)
• 4 x Thonkiconn 3.5mm mono audio jacks
• 5 x knobs (I always use 3 larger ones and 2 smaller ones)

Break the front panel faceplate off from the PCB. You'll also need to break off the tabs, and file or sand the rough bits smooth.

The 6.2V Zener diode is there to provide a stable reference voltage to the "top" and "bottom" rungs of this circuit. This particular voltage inhabits the mysterious crossover point of thermal variability, making it the ideal voltage for this purpose. I don't really understand it, but I do understand that ladder filters are vulnerable to power-supply noise if the "top" and "bottom" of the ladders are tied to the power supply. In real life, this filter isn't completely free of noise, but what can you do... it's an analog filter with lots of potential for noise. Call it character :D

Place the IDC header. You don't have to use a shrouded header, but I always do. Pay attention to the silkscreen markings to place the 1K and 100K trimmers in the correct place.

Here's the back of the PCB so far. You probably know how to solder already, but if you're using solder with no-clean flux, your finished project will have flux on it like this. You can clean your PCB if you choose, which will make it look nicer.

Anyway, trim those leads.

Buckle in, folx

Here's the four 100uF capacitors placed. Be aware of the polarity of these capacitors, and put them in the right way.

The silkscreen on the PCB calls for 22nF capacitors, but that's too large of a value. 10nF is great, and there's a really fun thing to try with this module -- you can use different strange values and types of capacitor here to get some interesting sonic qualities out of this. I have one copy that's got two ceramic disk capacitors, two film capacitors, and none of them are the same value. That particular filter seems to have a breathiness unlike the others I've built, it's less willing to self-oscillate, and hilariously, touching the knobs creates an actual signal because of the use of ceramic disk capacitors. Ceramic disk capacitors ACTUALLY ARE microphonic, so yeah, don't use them in signal paths unless that's a quality you want.

I've chosen to build this version with a half-value capacitor in the "bottom" rung of the diode ladder. This copies the topology of the world's most influential diode-ladder filter, the one included in the Roland TB-303.

Make sure the potentiometer legs are straight. Pop them into the PCB where they go, making sure to put the 10K potentiometer where it goes. Also, break the little locator lug off if your potentiometers have them. They'll make the faceplate sit wrong.

Hmm, I forgot to take a picture of the jacks. Well, it's clear where they go. Put them there. If you have the earlier Thonkiconn jacks with a short tab for ground instead of the longer leg, make sure to use a component lead to connect them to the 3rd pad on the PCB. You know what you're doing.

Once these parts are added, and before soldering anything, put the faceplate on to make sure the pots and jacks are placed properly. A nut on one or two potentiometers will keep the faceplate and 3.5mm jacks in place while you turn the project over to solder them all.

This is how I put nuts and knobs on. Make sure they're tight, and if you want, put a tiny dab of cyanoacrylate glue on the 3.5mm nuts (that would be "superglue on the knurlies") because they sometimes work loose.

Three bigger knobs on the upper pots, two smaller knobs on the lower pots... that's how I roll

The 100K potentiometer labelled Cutoff Trim sets the initial cutoff frequency. In this version, the trimmer rotation is wrong -- clockwise "closes" the filter. The schematic and more recent printings of this module will have the trimmer rotation correct, with clockwise "opening" the filter.

Power your module, connect a signal into the In jack. Make sure the Inversion knob is all the way down. Set the Drive knob to about noon. Set the volume so you can hear the output. Resonance can be set wherever you like. Put the cutoff knob all the way down, and listen to the output.

Adjust the Cutoff Trim trimmer until the filter has silenced the signal completely. You may have to twist it the wrong way to get it there :D

If your MantisLadder filter will self-oscillate with the resonance knob turned high enough, you'll be able to trim this filter so it responds properly to a 1V/octave control voltage. It doesn't have a NTC temperature-compensating resistor anywhere, so it's not going to be perfectly thermally stable. You'll get a bit of pitch drift with temperature changes.

Anyway, getting the filter to respond to a 1V/octave CV is beyond this build guide... I always do it by ear, but there's other ways.

Here's the answer to the question I just wrote.

The Resonance knob increases the gain of the filter right at the cutoff point. With enough resonance the filter will amplify that cutoff point so much it'll oscillate, creating a tone!

The Cutoff knob sets the frequency where the signal gets cut off.

The Inversion knob feeds an inverted copy of the audio going into the filter. You can coax a high-pass type response out of this filter by tweaking the Inversion knob in conjunction with the Drive knob -- they kinda interact.

The Drive knob attenuates or amplifies the signal coming into the In jack. There's warmth, saturation, and distortion that will happen by overdriving the filter, and with "hotter" signals, higher resonance settings will be less likely to cause self-oscillation.

The Vol knob adjusts the volume of the outgoing signal. From about 2 o'clock and higher, this control adds diode-saturation to the outgoing signal. You can hear this best by turning the Drive knob all the way down, and turn the Inversion knob all the way up. The signal going through will then be just the incoming audio plus saturation.

The In and Out jacks are self-explanatory. The Cutoff jack accepts a CV to control the cutoff. The Res jack accepts a CV that controls the resonance of the filter -- using this as a modulation source can be interesting.

Great! You've done it! Enjoy your new filter.