The NFB-12 was one of the unit that could not upgrade to the TE8802 usb receiver board, since It has the USB receiver directly on the motherboard. The NFB-15 seems to be an NFB-12 with the new USB receiver.
But as you can see from the two pictures, there's more than that going on:
Left is 12, right is 15.
The most obvious difference is the use of more surface mount components on the 15. Most of the transistor in the amplifier are now surface mounted. They also modified the power supply. It now has bigger capacitors, with the total capacitance going from 0.01 to 0.015 farad. They added a 22 ohm resistor in there. I bet it's used to lower the noise of the power supply.
They probably took opportunity to apply a few tweaks to the circuit. As anyone contacted Audio-gd about the differences? If there's a new thread created about it, it would be nice to post the link here.
Edited by KimLaroux - 6/10/12 at 8:00pm