the additional current for the second transformer won't be much more than a few milliamps, so you can keep the same line cord.
here's what i came up with for a regulated and current limited supply for the USB. just pay attention to the stuff inside the dotted box. the "idealized" supply to the left can be anywhere from about 12-20VDC. which is what you'll get from a transformer, bridge rectifier and caps, similar to the amplifier supply. inside the dotted box is a constant current source feeding a 3 terminal regulator. below the set current (520mA, 500mA for the load, and the rest for the regulator itself), the input voltage to the regulator is well above it's dropout voltage. when you hit the current limit, the current remains constant at 520mA, but the input voltage to the 7805 begins dropping rapidly, until the dropout voltage is reached and the output shuts off. it's probably a good idea to add an LED and resistor to the output as a status indicator.