See the full discussion on the Westone Forum:
“I had more time on my hands over the weekend and pulled the “Mk1” (mid-boost) circuit out of the bass. It was okay, but having spent some time playing with it, it wasn’t quite what I was after.
“Mk2” is now a “proper” two-band EQ with around 15dB of cut and boost at two centre frequencies of 50Hz and 4KHz.
I’ve had to order a dual concentric pot for the treble and bass cut and boost because I don’t want to make an extra hole in the body. The plan is that the volume and tone controls will move to occupy the current tone and boost control positions.
What is now the volume control will become a pickup blend control. And the pickup selector switch will come out so I can drop the dual pot in there.
This will also allow me to keep the overall “look” of the control layout pretty much as it is. I’m keeping the original parallel/series “tone” switch and the active/passive switch exactly the same.
The circuit is built, tested, and mounted on the control cover in place of the original circuit ready.
I didn’t bother drawing out the mid-boost circuit. To be honest, it’s not really worth the effort.
The two-band is nothing revolutionary either. It’s a simple Baxandall circuit round a dual op-amp. There are loads of similar examples all over the interwebs.
I use pretty much the same circuit in one of the models of valve amp I build for customers.”