Modular for the Masses BadSidechain

Hi! Today you probably will be building a module called BadSidechain. It's a module that takes an audio signal, feeds it through a state-variable resonant filter, and then uses the result to sidechain, or "duck", a different signal. You can use the module as a plain old voltage controlled filter, with highpass, bandpass, and lowpass responses available.

Here's all the parts you'll need:

1. The PCB (which already has almost all the SMD parts put on at the factory)
2. The front panel
3. 2x red LED, SMD 1206
4. 2x green LED, SMD 1206
5. 3x blue LED, SMD 1206
6. 1x green LED, SMD, 3528
7. 1x bipolar LED, 3mm
8. 1x 2x5 IDC pin header
9. 7x Thonkiconn-style mono jacks with nut
10. 1x on-off-on micro toggle switch with nuts
11. 3x 100K RK097 style sealed potentiometers, linear
12. 2x JuanitoPots (clear-shafted pots with the same footprint as RK097 pots
13. 1x DIY Vactrol

You'll also need a soldering iron, solder, snips, and all the other paraphernalia anybody would use to construct a project like this.

This is an entirely optional step. You don't have to put LEDs under your jacks, they're just to make your module more pretty, and of course there's a whole philosophy of what colors mean what function in Modular for the Masses modules. Using the colors of the rainbow: red comes first so that's primary signal inputs. Green is in the middle, so CV inputs or other input signals that are more secondary, those are green. Blue is at the end of ROYGBIV, (okay fine, I guess violet is at the END but stay with me here) so blue jack LEDs indicate outputs.

The color for each jack is written right next to each square jack hole, so if you wanna conform to the standard, stick with that color. BUT you can do any color you want, totally up to you.

The silkscreen for each jack also has little images to show you which way the LED should go.

Oh, obviously put the LEDs facing "down" into the square hole. You want the light to shine through the jack.

Put solder on one pad, put the LEDs in, then solder the other pad.

This square LED goes on the non-parts side of the PCB. You can choose whatever color you want, but green makes the most sense since it's showing what the green LED inside the Vactrol is doing. You gotta put solder on one of the LED pads, then use tweezers to hold the part (mind the polarity! the LEDs have a little corner notch which matches a corner notch on the PCB silkscreen) and get it stuck in place and solder the other side.

A bipolar LED will light up however the polarity of the current is, so you can install it either way. In M4TM modules, the square pad ALWAYS ALWAYS is for the longer LED leg. So I'll always put the longer leg of the bipolar LED in there. Bend it over before installing it, so that the lightey part of the LED will poke through the hole and shine on the bottom of the clear potentiometer shaft we'll be installing later.

Get those legs soldered in, and snip them off!

The VCA part of this module is controlled by a Vactrol. That's a light-dependent resistor (LDR, photocell, electronic eye??) that reduces its resistance when light shines on it. Glued face-to-face with the LDR is a green LED.

It's super important to get the longer leg of the LED half of the Vactrol into the square pad of the LED symbol, otherwise the module won't turn down your audio.

So bend the Vactrol leads so that the long leg of the LED goes in the square pad, solder, and trim. Oh, also install the 2x5 power header because why not.

It's time to slap all the parts into the PCB! It's mostly self-explanatory, but here's some tips for what might be confusing:

1. JuanitoPots are the clear-shafted pots. They only go where there's an LED under them.
2. One of the non-Juanito-pots needs to have one of its legs bent up in order to fit in the PCB
3. I like to put a nut on the micro toggle switch under and above the pane
4. Where the Thonkiconn style jacks are very close to each other, it's easiest to kinda squish the ground connection against the body of the jack.

When all the parts are installed into the PCB but BEFORE you solder them, put the front panel on the parts, and install the nuts so the solder joints won't be stressed by having the nuts torqued onto them after being soldered.

Squoosh some knobs onto those metal-shaft potentiometers, and you're probably finished!

Good job! You did it! It's time to plug it in and see if it works.

As long as everything works, here's a user manual:

Filter

This uses a state-variable filter with well-behaved resonance which won't scream at you. The "CV" input controls the cutoff of the filter, the switch selects the response, and the resonance is, well, the resonance. The "in" input is for audio that gets fed to the filter directly. If you want to use just the filter portion of this module, here's where you plug in a signal for that. This input does normal down to the "audio in" input. Take the output from the main "aud out" jack, and you'll be able to sidechain your audio signal with a filtered copy of itself. A sort of slow, frequency-dependent compressor.

The top "out" output is where you get the signal that's being filtered.

Sidechain

This section of the module is where the VCA action happens. It's a Vactrol-based VCA, so the response is slow and smooth, allowing for click-less operation, but making it no good for amplitude modulation or if you expect the response to track precisely to a voltage.

There is an envelope follower here, which either takes the signal from the output of the filter, or takes an unfiltered signal that you might choose to plug in to the "envelope in" jack. Below that is the "audio in" jack, which is where you'll usually want to plug in the signal you want to be sidechained by the voltage being created by the envelope follower.

The envelope follower has two controls, depth and decay. Depth adjusts how much envelope voltage is being created. There's a green (or other color if you chose a different color) LED under that pot shaft, which mirrors what the LED inside the Vactrol is doing. Brighter potentiometer shaft, more "turn down" effect on the signal. The decay knob adjusts how slowly the peaks of the envelope fade.

The "envelope out" jack is where you can get the voltage being generated by the envelope follower. Ooh, you could cross-patch this output to the filter CV and get a auto-wah effect, something I just now thought of right now while writing this.

Okay, hope you enjoyed this project!