From: DPierce@world.std.com (Richard D Pierce)
Subject: Re: 12V Power Amplifier Design
Organization: The World Public Access UNIX, Brookline, MA
Date: Wed, 27 Jan 1993 16:15:42 GMT

In article <1993Jan27.000515.19195@parc.xerox.com> tapscott@parc.xerox.com (Peter Tapscott) writes:

In demonstrating how much power you can derive from an amplifier based on
a 12 volt rail can supply, Peter Tapscott says:
>
>You have to be cautious about accepting wattage ratings
>from car amps. The manufacturers frequently exceed the laws of physics
>with their ratings.
>
>P = 16/R for a 12 volt DC supply. For an 8 ohm speaker, the max
>output is only about 2 watts. Figure 4 watts for a 4 ohm speaker.
>
> . . .
>
>I would be suspicious of claims of driving loads much below 4 ohms, except
>for very expensive audiophile car amps, like Orion. 
>

Well, Peter, there are two possibilities here. 

   1) your assumption are correct and the manufacturers are attempting 
      to violate physics, or are just plain lying or 

   2), your assumptions are just plain wrong

Let's guess which one it is .... BZZZT! Number 2! Your Assumptions Are
incorrect, Sorry, you loose!

It's now a trivial excercise to design and mass produce car audio
electronics that have output amplifiers that run of supply rails that are
much in excess of 12 volts. Try, for example, supply rails of +- 50 volts.
Before we see how, let's see what this gets us. Assuming a couple of volts
drop across the output fevices when they're fully conducting (a generous
assumption but what the hey), that gives you a peak-to-peak output
capability of +- 48volts, which translates, by your math (which I will
accept as essentially correct) to an output power 143.9 watts into 8 ohms.
I would hope that you agree that this is a bit more than the couple of
what figures you came up with.

Now, where does this +- 50 volts come from. Simple. Take 12 volts, turn it
on and off at some frequency (let's say 200 kHz, making it AC) run it
through an appropriately sized transformer (which need not be physically
very big for these sorts of power levels at this frequency) that steps it
up to an appropriate voltage, the rectify it so you have +-50 volts DC. 

Poof! like magic, you have a perfectly realizable 150 watt amplifier
running off a 12 volt car battery. (The downside, of course, is that when
you realize the the switching converter runs probably 80% efficiency, and
the am, if biased class AB runs at best about 43%, then we have to have
about 420 watts running into the beast, which implies an input current at
13.8 volts of about 30 amps. And there are people who drive vehicles with
systems having capabilities in the multi-killowatt range, implying many
hundreds of amps of current draw. Gulp!)

It might not be the most common way of doing it (let's see, there are only
about 100 manufacturers making hundreds of models, and they have only sold
several million examples over the last 10 years or so) :-)

It's cheap, it's easy, it's common, and it's what you missed in your
discussion.

-- 
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