ZVHP high-pass VCF build guide!

Edit: I just realized that I've already told most of this story in my July 4, 2023 post. uh..... sorry? I finished that post up by saying "golly I hope the 2HP filter works" and it does, so here's this post:


It's the ZVHP high-pass filter! In PCB form!!!

I met Konstantin (Constantine??) at Knobcon 2017, and one of the pieces of advice he told me was "put your kick drums through a high-pass filter then a low-pass filter." I was like what are you talking about but I took his advice, first scouring the Internet for good high-pass filter designs, rejecting most of them as too harsh.

Sorry..... digression...... the most common variable-state VCFs have insane uncontrollable resonance. Thomas Henry fixed that with his excellent VCF-1 design, but I didn't find that one until later. Notice the 10pF capacitors C1 and C2 -- those are the magic capacitors that stop the filter from becoming uncontrollable. Digression done.

The other design I rejected was the VERY VERY GOOD Polyfusion high-pass filter. Ron Folkman (whom I met at Knobcon 2018)

Mr. Folkman's high-pass filter was very controllable, wonderfully designed, and his hand-drawn schematic... my stars, they are artistically gorgeous. Maybe you can tell I'm sorta crushing on him in that picture. 

Anyway, I rejected Ron's filter for kick drum filtering because it is a four-pole filter, making the cutoff too steep for my ear for that specific purpose. I took the design, cut it in half, struggled mightily with the resonance part of the circuit, and when I finally came got something that worked, I found that it worked very well indeed!

So anyway, here it is. I'm really proud of this design. It's one of the first times I felt like I really understood the functioning of each part of the circuit, and could take ownership of the design as uniquely mine.

Okay, build guide time.

First, break the PCB into pieces.

 Smooth the edges of the PCB where the breakoff bits are, especially on the panel.

Then, install the pins that the perpendicular PCB will slot into.

Jacks. Put the Thonkiconn jacks where they go. Solder them in.

Potentiometers. These are 100K linear Song Huei tall trimmers. I usually buy these from Synthcube. Don't solder these in, just place them on the PCB, and then put the faceplate on, installing the nuts on the jacks to hold the front panel where it needs to be. 

Wiggle the potentiometers around on the PCB until you're happy with how smooth and straight they are. Use some pliers to slightly squeeze the mechanical legs of the potentiometers. This way that metal part should let the module fit in your modular. Finally, solder the electrical and mechanical connections to make stuff all sturdy and tight.

Now it's time to mess with the electronics! 

You'll need three 100K box trimmers and one 10-pin shrouded header for the Eurorack power.

The notch in the power shroud goes away from the electronics side of the PCB. Pin 1 of the cable (the red wire) goes where the PCB says "RED STRIPE" anyway, be careful to get that part right.

The trimmers can lay over so they're in the same plane as the PCB.

Now you get to connect the PCB to the rest of the module!!! Never more to be parted. Forever and ever. Amen.

Now it's time for LEDs. This is optional, but who doesn't want light to shine out of their jack holes! I am not a person who doesn't want that, that's for sure! The long leg goes in the square pad. 

Be careful with the legs of the top LED so they're not touching each other or the resistor that's right under them. 

Okay, calibration time. Your trimmers are probably 25-turn pots, and they're probably from the factory set right in the middle at 12.5 turns. Assuming that's the case, start the calibration process this way:

Range trimmer: counterclockwise 8 turns

Scale trimmer: clockwise 4 turns

Resonance limit trimmer: counterclockwise 8 turns

These trimmer settings are a good starting point. Plug a signal into the In jack (something with rich harmonics, a saw, perhaps), listen to the Out jack, and adjust the trimmers more to get the filter dialed in just right.

The range trimmer sets the center frequency of the cutoff knob on the front of the module. Turn it counterclockwise to get the filter higher pitched, clockwise to get it lower pitched (in Rev. 2 this will be swapped).

The scale trimmer adjusts the exponential response of the frequency. If you're careful with this trimmer, and you've set the resonance to self-oscillate, you can use this filter as a 1V/Octave oscillator. It won't be temperature stable, and the response will change with the resonance knob, but it will probably be within a few percent over a couple octaves at least. I've never tried it myself...

The resonance trimmer sets the gain of the feedback signal -- higher gain, more oscillation. This is easy to dial in -- leave the In jack without anything in it, turn the cutoff knob to maybe 9 o'clock, turn the resonance knob all the way up, and adjust the resonance limit trimmer until you're happy with the sound of the resonance. 

You should have your ZVHP all dialed in, ready to go! I love what this filter can do to kick drums, turning them into monster low boom sounds, muting them up to little clicks, or turning them into a little bongo sound, depending on the resonance and cutoff. I hope you find yours as enjoyable.