I have an Arduino Mega 2560 with a GPRS seeedstudio shield and an USB Host CircuitsAtHome shield which work fine when powered with USB or with an adjustable voltage regulator set at 8V.
When I set this voltage regulator at 7V (minimum voltage from the spec to get the better efficiency) I am still reading 5V and 3.3V on the pins of the Arduino and all the leds of the Arduino and the GPRS shield seems fine but the USB Host CircuitsAtHome shield seems to not work properly. I cannot debug it using the console because if I plug the USB cable, everything works fine...
Why 7V fail while 8V is ok?
The adjustable voltage regulator can provide up to 5A which is plenty.