There are plenty good receivers and amps with iPod 30pin dock. Of course there are Bluetooth adapters cheaply available, some amps need pe-amplified models. I have not come upon a version that properly supports remote control commands, so let's create our own. Then a replacement PCB for Yamaha R-840 can be created easily.

For modification of iPod Dock on Yamaha R840 see: ipod-30pin_dock

Some older players (e.g. car radios) only provide 12V via firewire lines (pins 19+20) instead of 5V (pin 23). Use an LDO or switching regulator to get desired voltage.

#TODO insert iPod links

https://pinoutguide.com/PortableDevices/ipod_pinout.shtml
AdrianGame : iPod Accessory Serial Protocol
https://web.archive.org/web/20080622202730/http://stud3.tuwien.ac.at/~e0026607/ipod_remote/ipod_ap.html
http://www.ipodlinux.org/Apple_Accessory_Protocol/
xtensa/PodEmu: Android application that emulates iPod to use together with old 30-pins iPod serial interface
PodEmu on XDA
xtensa/oap: Implementation of iPod serial communication protocol used in old 30-pin iPod interfaces
oandrew/ipod-gadget: iPod usb gadget for audio playback

#TODO insert communication protocol