From: csmith@plains.NoDak.edu (Carl Smith)
Newsgroups: sci.electronics,rec.audio,rec.audio.car,rec.audio.pro
Subject: Re: 12V Power Amplifier Design
Date: 28 Jan 93 02:42:29 GMT
Article-I.D.: ns1.C1JM6t.H8u
Organization: North Dakota Higher Education Computing Network

In article <1993Jan27.231045.3093@scott.skidmore.edu> jreiser@scott.skidmore.edu (Jason Reiser... Asleep) writes:
>csmith@plains.NoDak.edu (Carl Smith) writes:
>> In article <C1It67.5E5@world.std.com> DPierce@world.std.com (Richard D Pierce) writes:

>> >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).  

Yeah.  I was just doing simple calculations.  In reality you would
need to know the on resistance of the power output devices.  Just for
the sake of argument, lets say they are one ohm.  With 8 ohm speakers
you have a 1 ohm/8 ohm voltage divider connected to the supply 
voltage, and the speaker gets 8/9's of the voltage, or 88%.  If you
go to 4 ohms you have a 1 ohm/4 ohm voltage divider, and the speaker
gets 4/5 of the voltage, or 80%.  So you loose 8%.  But 80% seems
rather high, and in reality I think they average amp is even less
efficient.

>> >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. 

Well, The original poster said something about kilowatt amps, not 100
watt amps.  I can see running a 100W amp at half volume and not having
any electrical problems.

>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. 

Well, again, the original poster mentioned "multi-kilowatt" amps.  I
assume that if you have a 1000 watt amp that you don't run it at
30 watts all the time.  If you run it at 1/3 or half volume, you
still have to be drawing a few hundred watts average, which the 
average cars electrical system can't supply indefinitely without
battery discharge...

>
>> 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!?!  

No, I said I have PLANS for a 300W/channel amp.  I meant that I have
instructions explaning how to build one.  I don't think I will build it
though. (BTW, it turns out that it is only 200W/channel, not 300 like
I remember.  But 400W is till plenty loud, and I have seen amps for
sale that claim much more than 300W, so my questions still apply.)

>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). 

Yeah, that's another thing that makes me wonder about the people with
massive amps.  A kilowatt of sound, even at 1/3 volume, should be
!@#@!$ loud.  Not to mention the fact that when you crank it up
you probably won't have windows in your car for long.  My 200W
home stereo makes the glass in my bedroom window rattle at about 
half volume...

>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.

I don't know which is worse, buying one HUGE amp or MANY little ones.
But I will agree that a separate amp for each speaker would work
far better than trying to wire up a 300W crossover network.

>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).

Exactly.  10 amps at 12 volts give you a maximum of 120 watts 
available.  Or 60 per channel.  And that's assuming 100% efficiency.
Using the 80% and 43% someone mentioned above, that comes to 
about 20 watts per channel maximum that can be output indefinitely
without draining the battery.

>	To save yourself the headaches of these kinds of problems,
>build a system that plays as loud as you plan to use it.  

Well, like you said yourself above, you need some headroom for
loud transients, so buy one twice as loud as you plan to use it,
and keep it at half volume or less.   :)

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