OctaPass Eurorack Module - Eight Channel Hi Fidelity Low-cut Filter

This is a Eurorack module designed by Modular for the Masses. It's an eight channel low-cut filter, switchable, with the frequencies selectable at build time. It's got 4-pole filters (24dB/octave) and the output is phase-coherent with the input.

It's designed for use with signals that may have lower frequency content that will interfere with kick drums or basslines. Such a low-cut filter is a very common feature used in DAWs to make room in the mix for other bass-heavy audio material, but clean high-pass filters are uncommon in the Eurorack world, especially one with eight channels.

It uses 15nF C0G capacitors, for excellent matching between the filter stages.

6HP

30mA current draw more or less depending on your choices

You'll need all the usual things to assemble a PCB based project, soldering station, pliers, side cutters, poster putty...

OctaPass BOM

• 1 x OctaPass PCB set plus front panel (SMD-assembled)
• 12 x Dual op amp, SOIC 8, standard pinout *see notes* SOIC-8
• 4 x two-by-six IDC two-row male pin header or equivalent, 2.54mm pitch, 2x6
• 4 x two-by-six IDC two-row female pin header or equivalent, 2.54mm pitch, 2x6
• 1 x two-by-five IDC two-row male pin header (Eurorack pin header, non-boxed) 2.54mm pitch, 2x5
• 16 x LDR, 5mm, I always choose 5537 LDRs but it’s not critical
• 24 x LED, 3mm, green, 8 of them filed flat *see notes*
• 16 x LED, 3mm, any color. For jack hole lights, optional
• 16 x Cut into 2cm pieces of 3/16th inch heatshrink tubing (able to fit LDR and LED inside)
• 48 x through-hole resistors, 1/8 watt, *see notes*
• 16 x Thonkiconn style mono 3.5mm phone jack with nuts, normal (with ground pin)
• 8 x sub-miniature 2 position toggle switch SMTS102, 3-pin

*NOTES*

Op amp selection is important with this module. Standard TL071 amps or similar will work great. NE5532 op amps will be somewhat lower-noise, but will use a lot more current. This module has 12 op amps, and the NE5532 draws 5mA, for a total of 60mA just for the chips. Don't use LM358 amps even though they're cheap and low-power. They have too much distortion. I built one with expensive OPA1656 amps, which are "ultra low-noise" and that module sounded great, and doesn't use a lot of current, but I wouldn't be able to pick it out from the NE5532 or TL072 copies.

Green LEDs -- 8 of these will need to be filed flat on one side so they'll squeeze into the very tight space between the Thonkiconn jacks and the toggle switches. Hold the LED up, with the longest leg farthest from you. The right side is the one that needs to be filed. There will be pictures. The other green LEDs go in the Vactrols (the LED and LDR thing)

Resistor choice!!! This is when you select the cutoff value of the filter. You can calculate values on your own using complex math, or by using the (Windows) application developed by Rod Elliott which you can download from the link on the bottom of this page (the capacitors already installed are 15nF). For this copy, I'm using 75K resistors for the top half for a cutoff value of 100Hz. The bottom half of the module gets 39K resistors for 192.38Hz.

Some common resistor values and the resulting cutoff:

• 100K - 75Hz
• 81K - 93Hz
• 75K - 100Hz
• 68K - 110Hz
• 51K - 147Hz
• 47K - 160Hz
• 33K - 227Hz
• 22K - 341Hz
• 18K - 469Hz

...and so on

There’s plenty of valid ways to make home-made Vactrols, but this is how I do it:

Take the length of heatshrink tubing, put the LDR into one end.

Insert the LED into the other end of the heatshrink tubing. Hold the LED with pliers, and the LDR legs with your fingers. Make sure the pins are level with each other. Hold the assembly over a heat source (be so careful if using an open flame, since they’re dangerous). I use a hot air soldering machine.

When the heatshrink tubing is nice and shrunk, and before it cools, crimp each end to make a light-tight seal.

Make sixteen of these.

Cheap PCB assembler factories can't put SMT parts on both sides of the PCB, and I get my PCBs made at the cheap place!!! That means you'll have to put some SMD op amps on the "other" side of the PCB.

Check the notes up there to help you decide which op amps you want to use. I recommend TL071 for normal uses. If you want even lower noise, use OPA1615 amps (they're expensive). If you want slightly lower noise but are fine with using lots of current, you can choose NE5532 amps.

Soldering SMD chips to PCBs isn't gonna be covered in this Instructable, but make sure to get the amps positioned the right way, and don't get any solder bridges or unsoldered legs... and clean it off if you want.

Next, we’ll be putting all the resistors where they go. The Linkwitz-Riley filter we’re building use one resistor in one spot, and a pair of identical resistors in another spot. You can see these spots on the PCB as pairs of pads with a circle-line, or two circles. The circle represents where the resistor body goes, so the pairs of pads with TWO circles means there’s a pair of resistors there, with the tops twisted and soldered together.

Select your resistor values per-channel using the handy app linked above, the table I wrote, or just use the resistor I sent you. Don’t mix-n-match resistors in the same channel unless you want a weird cutoff slope.

Twist pairs of resistors together and solder and trim them first, before putting them in the board.

Populate the resistor pads with your little 1/8th watt resistors (and resistor pairs), solder them, and trim the leads flush.

Next up, let’s put the pin headers where they go.

I designed this PCB with slightly asymmetrical 2x6 IDC header pads, to make it extra difficult to accidentally put the PCB sandwich together upside down and burn up a bunch of components. It might still be possible though, so mix up the male-female genders of the pins to make it truly impossible to get the boards assembled wrong.

Slap that little 2x5 Eurorack power header where it goes.

Next, we’re going to put the Vactrols you made where they go.

The long leg of LEDs is the + leg. In my designs, the rectangular pad is always the + LED leg, so figure out how to bend the LED and LDR legs in such a way that the long LED leg goes in the rectangular pad.

It’s pretty cramped in there, but the closest LED and LDR legs are usually connected to each other on the PCB anyway. But still, be careful.

Be aware that the tops of all the LEDs is metal, and if that metal touches the metal of either the LDR or LED leads, the sound will be messed up. Leave room between the resistors and the Vactrols, even where they’re covered in heatshrink tubing – the insulation may fail at some point.

Once you’ve got all sixteen Vactrols installed, it’s time to move on to the other PCB.

First, put all the toggle switches where they go. You can line up the slots on the threaded shaft so they all face the same way, but it makes absolutely no difference.

Place them all the way into the PCB, and put the panel over them all to hold them straight and fully inserted. You won’t be using the nuts or washers that came with these switches, since they’re a different height from the jacks, and besides, would cover the indicator LEDs.

When the toggle switches are installed, take the faceplate off, and install all the Thonkiconn jacks. I prefer to install the panel again to hold them in place, but be sure to also get the jacks fully inserted into the PCB so the base sits tight to the PCB.

Once that is done, it’s time to squeeze the indicator LEDs where they go (they’re the ones that have one side filed flat).

The rectangular pad you’ll barely be able to see hidden between the toggle switch and the jack is where you’ll carefully insert the long leg of each flat-sided LED. The other leg obviously goes in the other hole. Push the LED down as far as it will go, the base of the LED will hit the top of the toggle swtich, and the legs may almost touch the metal body of the toggle switch but I don’t think it will

Once all the indicator LEDs are in place, and the leads are trimmed from the other side, you can put the faceplate on. You may need to wiggle the LEDs around to let them fit in the LED holes, and you may need to use the jack nuts to press the faceplate on, since the spacing is so tight.

That little cute Shai-Hulud made of poster putty with a mouth of a Thonkiconn knurlie nut is there just to show how I align those little pesky nuts to the jacks. A convenient tool, I tell you what.

Now it’s time to (optionally) install all the jack LEDs.

Each jack will have a hole in the PCB right under it. Next to each hole is a pair of SMD-style pads. One is rectangular, one is an oval, and there’s a little symbol of an LED there too. That’ll help you figure out which way to bend the legs of the LEDs so they long leg will go on the rectangular pad.

Bend the legs of the LEDs like in the picture, and carefully solder them in place. It can help to get a blob of solder on the pad, hold the LED from rocking around with the solder wire while you wick the solder up the LED leg. Once one leg is held down, the other one will be so easy.

Once all the jack LEDs are in place, you can put the PCB sandwich together, and hopefully have a working module! Plug it in to a Eurorack power cable and see if it works!

It can be fun to test the module by plugging in a sine wave signal into the top input. Choose a frequency below the cutoff values you chose. Plug in an output in the lowest output, and listen and watch what happens to the sine wave when the filter stages are switched in and out. The sine wave should simply get lower in amplitude until it's almost completely gone, depending on how low it is.