The Arduino Gnd pin, so there was a bit of a cable run. I moved it to next to the emitter, but no real change on the display.
By a coincidence I got these in the mail today from eBay:
2 Channels 3W PAM8403 Class D Audio Amplifier Board 5V
$US 2.84 each.
Wired one up to my iPhone and those old speakers. Quite nice sound out of it.