These devices and accessories from various, mostly independent DIY-type engineers/creators address special needs and can make life easier in the studio. All work well in/with Linux.
MIDI hub
MIDI/USB interface, very flexible and configurable, programmed with control app (MIDIhub Editor) available as a Linux AppImage.
https://blokas.io/midihub/
MIDI Expression
MIDI/USB interface for expression pedals & other potentiometer (continuous) or switch (on/off) pedals (TS or TRS jacks)
note: firmware initial update needs to happen in MS Windows; subsequent updates easy using Linux version of the app
https://www.audiofront.net/MIDIExpression.php
DoreMIDI MIDI Pedal Converter-2 MPC-20
Perhaps an alternative to the MIDI Expression device above, although apparently not nearly as configurable.
https://www.doremidi.shop/products/midi ... r-2-mpc-20
https://www.amazon.com/DOREMIDI-Express ... B0CDWQ7T14
Nuances
MIDI/USB controller for CC (control change, continuous controller) messages using high-quality, smooth faders. Each of the 3 faders is easily programmed to a particular CC number using the device itself, no companion app necessary.
https://nuancescontroller.fr/
MusioTech Fader Pro
MIDI/USB controller for CC messages using high-quality, smooth faders, alternative to Nuances above, several different models/sizes. Programmed using the device itself, no companion app necessary.
https://musiotech.com/
Yamaha FC7 Expression Pedal
Just an expression pedal (potentiometer based), but built like a tank and very dependable, very smooth movement. Beats the heck out of most other popular brand expression pedals.
https://www.amazon.com/Yamaha-FC7-Expre ... B000UJHIS8
