The OP-HPF High Pass Filter
I wanted to have an Isolator-style high pass filter device to play with.
I found this design at https://www.eeweb.com/blog/circuit_projects/20hz-to-200hz-variable-high-pass-filter that uses a very common op-amp, the TL072.
And there I found the design for a 555-driven negative voltage generator, needed for the TL072: https://www.allaboutcircuits.com/projects/build-your-own-negative-voltage-generator/
The only case I got is that Walkers biscuits box. Well English. I punched two holes to mount a rotary switch and a 50k pot, and added big buttons.
The rotary switch will select between 3 different max-cutoff frequency, which will be controlled by the right pot. I chose 220nF, 150nF, and 66nF. The smaller the cap, the higher the Cf.
Here I soldered the 4 wires of the rotary switch and the 2 wires of the 50k pot.
You can see the breadboard with the TL072 for the filter and the 555 used to get a -12V negative rail.
Got all that stuff inside the box.
Here it is. A very British design indeed.
Quick demo. Plug your PC in a PA.