From: jreiser@scott.skidmore.edu (Jason Reiser... Asleep)
Subject: Re: 12V Power Amplifier Design
Organization: Skidmore College, Saratoga Springs NY
Date: Wed, 27 Jan 1993 23:10:45 GMT

csmith@plains.NoDak.edu (Carl Smith) writes:
> In article <C1It67.5E5@world.std.com> DPierce@world.std.com (Richard D Pierce) writes:
> >
> >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.
> 
> And it will deliver 288W into 4 ohm speakers, assuming it can handle
> them.

MAYBE!  That's assuming that the amp is 100% efficient (actually more
than that).  Several amp manufacterers including Orion, PPI,
Hafler/Rockford Fosgate and other high end companies claim that their
amps will double the output power into lower impedances of a factor of
2.  ie.. 100x2 into 4 ohms, 200x2 into 2 ohms, etc (if stable).
That's one hell of an optimistic view, but they do increase power into
lower loads.  (and it is significant increase).  Of course, that is at
the sacrifice of damping factor, also by the same factor of 2. 

> 
> >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!)
> 
> This is where I have trouble accepting the amps that deliver many hundreds
> of watts of power.  I have no trouble accepting that it is possible to
> pump hundreds of watts into normal 4 and 8 ohm speakers by stepping the
> voltage up, I don't see how the car's electrical system can supply the
> necessary current for any length of time more than a couple minutes.

Exactly!  They don't need to.  If you have an amp that's 100x2 into 4
ohms in your car, it isn't putting out 100 watts per channel
constantly.  That's what headroom is all about.  You have power in
reserve for loud transients and such.  So, they don't need to put out
that kind of power constantly.  Sure, if you have a massive system,
and you blast it constantly, you will need to upgrade your charging
system, and battery/batteries. 

> I have plans for a stereo 300W/channel amp, and it gets that rating by
> stepping up the supply to about 50 volts, and by using 4 ohm speakers.
> When tested, it actually clips at about 290 WPC.  Now, assuming that
> the amp and supply are 100% efficient, that would take over 48 amps
> at 12 volts.  If you consider the 80% and 43% given above, that comes
> to 140 amps at 12 volts.

So, you doubt the idea... you think it will cause battery problems and
such, and yet you are going to buy a 600 watt amp!?!  That's fine, but
as I said, it's not going to be opertaing at full output at all times
(I hope for your ears sake).  Why not instead consider smaller amps
and set them up right.  A single 300x2 amp is not the way to go in
most vehicular systems... better off with dedicated amps for the
different types of drivers.  Yes, passive crossovers are fine in cars,
but with 300 watts, you're gonna spend a lot on crossover components
along... not to mention that the amp will be a fortune.

> Now you understand why people with these
> kinds of amps run power cables directly to the battery, and the cables
> are the same size as the battery cables.

Well, actually the reason to run an amp from the battery has little to
do with current draw.  Sure, if you have a large amp, you'll need that
large supply of current.  But, if you install a little 30 watt amp,
you better run it to the battery as well.  You could run it off of any
wire that will supply the needed current, but the battery will be the
least likely to introduce noise into the system... fuse boxes are
noise boxes!

> Well, batteries can deliver
> many hundreds of amps for short periods, but they must be recharged
> by the alternator.  And the alternator in my car can put out only 60
> amps, and a major portion of that goes to running the cars electrical
> system, especialy when the lights are on.   What I am wondering is if
> people with these amps have dead batteries all the time...

Actually, when the car is running, the battery is an almost
ineffective buffer in the DC supply to the amp(s).  Yes, if you have a
60 amp alternator, and with AC, lights, etc... you may have only
around 10 amps to spare (it varys greatly from one car to another).
If you exceed the available current being put out from the alternator,
you will discharge the battery as the system continues to play.  If
the battery is in this state often, it could do permanent damage to
the battery.  Not to mention starting problems.  So, then you consider
multiple batteries, upgraded alternators, or aftermarket units.
	To save yourself the headaches of these kinds of problems,
build a system that plays as loud as you plan to use it.  Or, don't
play it so loud all the time.

				- Jason

 
> -- 
> ------------------------------------------------------------------------
> Carl D. Smith                                       Inhale to the chief!
> csmith@plains.nodak.edu

-- 
---------------------------------------------------------------
\ Jason A. Reiser  \ Send E-Mail to jreiser@scott.skidmore.edu \
 \ Skidmore College \ Saratoga Springs, NY 12866 \ 518-581-6580 \  
  \ It's a new Ice Age!  Hooray!!  Sled City!  - Calvin & Hobbes \