It works! It works!

on a messy desk, a small USB midi keyboard is connected to a Korg NTS-1 mini synthesizer via a small micro-controller board that acts as a USB host for the Akai keyboard, converting USB MIDI to traditional MIDI for the Korg
Akai LPK25 keyboard has USB MIDI out, but the Korg NTS-1 only has regular MIDI in. The little board in the middle acts as a USB host for the Akai and MIDI source for the Korg

This is great: gdsports/midiuartusbh: MIDI DIN to MIDI USB Host Converter allows your USB MIDI instruments to act as traditional MIDI controllers. It uses a Adafruit Trinket M0 to act as the USB host and MIDI output.

I modified gdsports’ design very slightly:

  1. Instead of using a 74AHCT125 Logic level converter and driver, I used a FET-based SparkFun Logic Level Converter
  2. Instead of a 5-pin DIN socket, I used a 3.5 mm stereo socket.

And it works!

breadboard showing Trinket M0 microcontroller board, logic level shifter, audio socket breakout and two resistors
Breadboard layout for MIDI-standard 3.5 mm output (Korg). The resistors are both 220 ohm, and the boards need 5 V power