I've done this before manually to convert an Arturia Minilab MkII into a wireless MIDI controller. Turns out there's enough space inside the Arturia to fit a Pi Zero W and a flat Lithium-Ion battery.
I can now use GarageBand on my iPad, MacBook and iPhone without always having to connect a USB-C hub or camera adapter.
I've been trying to find a mobile handheld console to install Debain on, I'm not sure how well it would work, but it could be cool.
Something like this should be enough to keep it running for a few hours: https://www.amazon.com/Ultra-Slim-Ultra-Portable-Ultra-Safe-...
My own use case for this is that my main synth tool is my 2nd gen 12" iPad Pro, but I hate having to deal with the Camera Connector and the irritating fact that it won't recognize my higher wattage USB-C power adapter and USB-C to Lightning cable. So if I want to use it for any length of time I have to use the factory power wart, which actually loses power if I'm running the iPad at full brightness and using a lot of CPU on synthesis. Which makes it really annoying to work on stuff for long periods, and I can't plug in my iRig Pro and a synth unless I'm using a powered USB hub.
But this is great. I can just keep the Pi running and hooked up to the synth, even power it off a USB power pack for doing stuff on the go. Thanks for sorting this out!
All credit due to https://neuma.studio/ who blessed me taking their guide and wrapping it up into a small Python script.
My next step is to see what other devices I can run this on. Looking for the smallest device with USB host capabilities, Bluetooth, WIFI and Debian !
You don't mention your level of MIDI expertise, but what you should probably know, if you don't, is that analog synths with MIDI outputs are almost always MIDI-out only, unless they're mad expensive and have motorized controls (or use digital pots to control analog hardware, which is vanishingly rare).
Of course, you might be able to send notes to the synth via MIDI if it's got onboard MIDI to CV conversion, but you probably won't be able to control your synthesis parameters with MIDI. Your mileage may vary and I could be wrong, and if it's a newer analog synth you might be able to do it. But mostly you'll be able to sequence notes from another device with it. No matter what, you'll probably have to spend some time with the synth's manual just to get it running. It's good nerdy fun though.
First, skip this project entirely and buy a direct midi to Bluetooth adapter. https://www.sweetwater.com/store/detail/MDBT01--yamaha-md-bt...
Second, use a MIDI to USB cable, https://www.sweetwater.com/store/detail/Uno--m-audio-uno , and then plug into your Raspberry PI. You can also buy USB MIDI to Bluetooth adapter for about 50$.
Finally, you could buy a MIDI hat for your Raspberry PI. I don't think this will work out of the box, but I can't imagine it would be too hard.
You could hook the wires from a MIDI cable to some ESP32 pins, read the MIDI messages and send them instantly through BLE to an iPad/MacBook.
An ESP32 board has the advantage of booting instantly, can be powered from a battery for a lot more time, can't cause SD card corruption, and is less susceptible to delays caused by the kernel scheduler.
I usually choose the Lolin32 Lite because it's very compact and has a built-in Lithium battery connector which can also charge the battery: https://diyprojects.io/wemos-lolin32-lite-compact-revision-l...
