Receiver + Amp Using Same Power Source = Problem

[elloree23]

HELP (please)! I have installed in our old TBird a Boss 612UA MP3 Compatible Solid State AM/FM Receiver with USB/SD input capability, wired to 2 front and 2 rear, 4 ohm speakers. As installed the system works fine except the receiver doesn't have enough volume to hear it while driving with the top down. I bought a Lepai LP-2020A+ Tripath Class-T Hi-Fi Audio Mini Amplifier (2 channel) with power supply (120 vac wall wart output 12 vdc 2 amp). Using the Lepai amp to boost the rear speaker volume, have wired Boss rear outputs to Lapai inputs, Lapai outputs to rear speakers and left the front speakers on the Boss front outputs. With the Boss powered from the ignition/acc and the Lapai powered by the wall wart everything works! But when I eliminate the wall wart, substituting the same 12 vdc from ignition/acc all audio output stops, both front and back speakers, although power lights and operation of Boss and Lapai appear fine. By leaving the Lapia amp powered by the 12 vdc ignition/acc and substituting a different audio source (ipod not connected to the same power source) but to the same Lapai inputs and rear speakers, again all works fine. I've checked the amperage output of the ignition/acc and it should be sufficient for both units. As both the receiver and the amp are connected to the same 12 vdc + and - wires, could I have a ground loop problem? Any suggestions are welcome as what good is an oldies car without oldies music.

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