transform any usb midi controller to a normal midi-only controller

[supernova777]

This is the only MIDI interface I'm aware of that will be capable of bridging USB-only controllers to MIDI DIN without a computer

https://iconnectivity.com/products/midi
https://iconnectivity.com/products/midi/iConnectMIDI2plus
https://iconnectivity.com/products/midi/iConnectMIDI4plus

kind of pricey

also,

http://www.kentonuk.com/products/items/utilities/usb-host.shtml

so yes u can use this with an X mac, but u can also use it "Standalone" without any computer

enabling you to connect a midi usb controller (such as a akai mpd or any keyboard usb midi controller)
to a normal non-usb synth module, without a computer.

so u would connect it like this:
[usb keyboard] -----via usb cable----->[ usb-host ]-----8pin midi cable-----> [synth module]