I've got two nice Orion amps running off my stock alternator and haven't had any major issues. When it's REALLY BLARING, the lights might dim slightly, but nothing that causes me concern. Like hotsauce mentioned, the key is in the wiring. Run some 1/0 or 4 gauge between your alternator and battery, from the battery to the block, and from the block to the frame. Unless you're running more than two amps, that should provide sufficient power flow. I also suggest a really good battery. I personally use an Optima blue top. (I know, they call it a marine battery, but it's nearly identical to the red top and it matches my setup better).