Frequency-Interchange HPF

eatkinola · Mar 14, 2026

I've built the Frequency Interchange and would like to build a variant with just the HPF in a smaller always-on enclosure, e.g., similar to how that smaller version is made by Broughton. Looking at the schematic, I think I'd just need to cut out a few sections (shaded below). The first section looks like a simple opamp buffer (?), so I thought I'd keep that and just get rid of the LPF section and gain section. I'd eliminate one opamp and use the remaining for both the buffer and HPF sections. Does it look like I'm on the right track? I plan to breadboard it, but I don't have a way of verifying the HPF works (frequency analysis). I'd appreciate advice on this.

jwin615 · Mar 14, 2026

Ditch LP1 all together
R4 can go.
I would keep an output buffer.
Need that block to keep the filter stable and set the output z. it doesn't have to be a gain stage, just a simple buffer like the first stage
FYI, this is a Sallen Key filter.

Feral Feline · 2026-04-07T15:08:23-0400

Are you planning on building it on perf/vero/self-etched PCB/gerbers>PCB foundry?

Schalltechik_04 once offered the Mini-HP-Vong.

I picked one up from Musikding, but they're out of stock and with Schalltechnik shuttered, Musikding may not get any more.

If it helps, I can trace my Mini-HP-Vong and send you the schematic for that, compare it with what you're doing with the Frequency-Interchange...

Passinwind · 2026-04-07T15:55:25-0400

If you can handle soldering relatively large format SMDs you can readily adapt the open source HPF board from here: https://github.com/Passinwind/PW3B-LPF/blob/main/V4 Pedal Schematics and BOMs.md

Or you can port that design to whatever format you may prefer. But I've already posted a board share and Mouser BOM for doing it my way, and it can fit in a 1590A enclosure no problem. It's also a higher order filter slope, if that appeals. I should have the standalone version for my board up on GitHub pretty soon, FWIW, but all that changes is upping R2 to 680k or higher as desired if using a passive instrument straight in, inserting C10 at 1uF, and R10 at 330k.

eatkinola · 2026-04-07T22:02:26-0400

Thanks, @Feral Feline and @Passinwind. I put a circuit down on vero a few days ago -- essentially what I indicated in my original post, minus the 1k8 resistor after the input buffer. I had actually looked at the Mini-HP-Vong (photos) and this one. Considering these and passinwind's design, the one I boarded looks reasonable. I'll test it once I do the off-board wiring. I don't have an oscilloscope to check the frequencies output, so I'll just test it by ear. Hopefully that's good enough. Here's the layout for the HPF (along with a LPF also ripped from the broughton which I plan to build next) -- the layout photos are not the best.