Hallo Matt
MI> Why not just get one of the many 12VDC-120VAC converters available and 
MI> put a couple professional, 120VAC-powered amps in?  The high-current 
MI> converters tend to get pretty expensive, but keep in mind that even in a 
MI> perfect world (no losses), your converter will still draw 10 times the 
MI> current that it outputs: expect to be upgrading your battery and 
MI> alternator to match.
I think it's not a good way to make 120VAC from 12VDC - then transform it to 
-for example- 2x40VAC and then rectify it to 2x55VDC (that is what a normal 
amp with AC-Input does).
You have 3 voltage conversions with significant power losses.
Additionaly the amps you mentioned are linear ones. That means their 
efficiency is max. 60-70%.
When you multiply all those efficienies you get an over all value of hardly 
more than 50%.
So your battery has to deliver 400 watts when you need 200 watts on the 
output.
My class-D amp has an efficiency of 94% at 200W output power. When we assume 
90% (minimum) for the DC-DC converter we get a total efficiency of 85%. This 
results in only 235W input-power.
I'm developing switch-mode powersupplies at work and we spend most of our 
time in reducing power losses in our designs. So -in time- I have got a deep 
aversion to such "oven-constructions" (perhaps not only my person).
many thanks for your reply - Wolfgang
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