i have a problam.... i have a car amp and i want...


Disabled Account
2002-07-01 12:45 pm
i have a problam.... i have a car amp and i want to us in my home... i want to buy transformer 12V but i dont know how match amper (- A -) i need for this amplifier...
someone can help ?

my amp is 800W RMS total


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2002-04-28 2:11 pm

If the amp is rated at 800W, allowing for inefficiencies in its switch mode converter, you need roughly a 1kW power supply. This works out at a massive 83A at 12V! You would be better off buying a lead acid car battery (and charger) to run the amp off, than building such a huge 83A supply.

IMHO, it is in fact probably more worthwile to just buy a mains amp for use inside. Unless you buy a very large (and expensive!) battery, the car amp will drain it quickly at full power; a 100Ah type could cost over $200US, and will only last just over an hour. Remember, this cost does not include a charger. If you already have a battery you can use, then go for it, but in the long run I should just buy a good quality PA amp - this will be far more reliable for DJ use anyway. Yamaha do some good value entry level units, or you could get a more prestigious brand second hand.

Hope this helps,
my hebrew is a little rusty but from the specs on the unit
it is 4 channels @ 200 watts per channel into 1 ohm @ 2%thd.
which is 100 watts per channel into 2 ohms, 50 watts
per channel into 4 ohms, and 25 watts per channel into
8 ohms.

The switching power supply in the unit is 80% efficient absolute
maximum. The amplifiers are either A/B or B which means
that they are 50% efficient max which results in an amplifier
that is 40% efficient.

So 800 watts turns into 2000 watts, which at the nominal
battery voltage of 13.2 volts is 151 amperes!

But it is not designed to be used at home and would be
horrible for any home use. Speakers for home use are either
4 or 8 ohms resulting in far less power than you would

Even the cheapest of japanese receivers would be better.
I do this for a living.... I am MECP. An 800w 4ch amp would be much bigger than the one shown in the photo. This amp is crap. I say this because only low-end amp manufactures claim such outragous specs. This amp just might produce 800w--with a 16v power suplly on a test bench at 1000Hz with 25%THD. Like I said, cheap. Another example would be the Rockford 700s 2ch amp. its twice as large as the amp in the photo. and it will produce 700w at 14v @2 ohms mono all channels driiven. RF test each amp and includes a test report of the amp in the box. You may get 20w with this thing off a 13.8v regulated power supply..but it will be very distorted.

At the retail location I work at we use a very large 12v ps and a 1F cap and we still dont get half of what this amp will do in a car...

Have fun playing around with it though.. just dont expect a whole lot...
Kevin's scientific analysis proceeds with validity; however, from a marketing standpoint: on these amplifiers the 800W is used as a marketing tool. The absolute maximum power the power supply can deliver instantaneously for brief instances is 800W. The amplifier is really only about 100-175W total. The power supply can be constructed from a 10VAC RMS (500VA) secondary transformer, some beefy discrete diode packages, and some huge filter caps. However, it would be cheaper and more effective to build or buy an amplifier designed for the indoor use intended...

I did try to be nice without directly calling the amp
"cheap" which it certainly is
and "crap" which it is too.

If you notice from the picture you can see the 12 volt
power input connectors. Do you think that this can handle
100+ amp peaks???

No free lunch here. Power in * efficiency === power out.

Very few automobiles have alternators of sufficient strength
to power such a device if it actually met spec.

If you actually ran this thing at even half power all channels
driven it would go into thermal meltdown in less than 10

In reality this amp is at most 25 watts per channel all
channels driven into 8 ohms. And likely at really lousy
thd numbers too.

This is part of the reason the Federal Trade Commission has
specs on how things like this have to be measured.

I'll post a picture on tuesday of an Oxford Instrument
Magnet power supply. It is 0 to 20 volts @ 0 to 125 amperes.
It takes two people to move it. The output cables are
.75 inch diameter fine strand pure copper.


2002-04-28 2:11 pm
"soo what u say that i need to the amp a car battery 12VAh ( i have one) and connect charger to the battery.... all in once...
and is work good?
i have a charger of 15.5 A 13.8V is good?"

No, you need a 12V lead acid battery with a high Ah rating, say 50Ah or more. You charge the battery and then connect it to the amp.

You will by now have read the negative comments posted about your amp - Kevin et al are quite right, take the power rating of this amp with a pinch of salt. Don't expect HI-FI sound quality, and don't expect the reliability of a real PA amp (I assume you're thinking of using the amp in a live situation given your handle "DJ").

See ya,
The easy way to determine the needed power supply: Look at the rating of the fuse that the amplifier uses. Add oh- about 10-25% to that, depends the size of filter caps in the PS. That is the size of power supply you need.



Aren't you the guy that wanted to put a home amp in your car? Just swap the fish out of water with the the cat in a lake. (insert sly grin here...)

Well you are close. All the caps + leads go to the red wires,
all the caps - leads go to the black wires.
12 volts in this manner will generate 16.8 volts no load.

And better yet you will be able to practice arc welding in
your spare time. :bigeyes:

and my amp is class AB 65% efficiency....

Hate to tell you this, but a pure class B amp is 50% efficient
not counting the power supply. AB is less than 50%.
The picture is of a standard unregulated power supply which is what you need; however, a standard bridge rectifier is going to give you "fits" in an application such as this due to the high currents. The large bridges with the .250 tabs are usually rated for 30A. You will need to go to huge diode packages which are in a two or three tab package and discretely mount them. Also using two transformers here is unnecessary because it wont really need that much (600VA). I believe this may have already been addressed here but it would be more beneficial to bypass the power supply in the amplifier which is a simple DC/DC converter and replace it with an offline AC-DC (dual rail) power supply like you drawing (but modified of course). Also to the efficiency issues of class AB car amps. Many of them using output MOSFETS achieve efficiencies in the 60-70% range. The channel resistance is much less and contributes to a smaller loss that a Vce junction of a bipolar transistor. I have a couple of good Phoenix Gold amplifiers with birth certificates (end-of-line tested specs) with efficiencies in this range.

In summary to DJ,
It is undesirable to go from AC-to DC to AC to DC. You can tell just by saying that, that there is actually two supplies at work here. Eliminate the middle and just make a supply to go from the original AC (wall voltage) to the needed DC (amplifier power rails)

What I do with my car amps is disconnect the DC-DC supply inside and just make a power supply that works right of my outlet. That's more efficient if you can do it. However, almost every retail car amplifier out there is crap. They are designed exclusively for BOOM BOOM BOOM, no fidelity whasoever required here! Furthermore, they never reach the BS power rating listed on them (without at least 25% THD) and they aren't efficient at all. I have a Harman/Kardon HK A402 amp rated at 65 watts a channel into 8 ohms. This amplifier easily outdoes any 200 watt per channel rated car amp and sounds way way way better. Also, the HK has such a good design, I can load it to 2 ohms and it won't break a sweat, it takes lot's to really work it out even though it's rated at 8 ohms. This shows how badly lots of car amps are designed and the cheap parts they use. I can put 4 ohm speakers on a 4 ohm rated car amp and fry it in an instant, something I haven't yet come close to doing with any REAL quality amp like the HK ever.
I would advise care be taken with regard to phasing on the power supplies. Having the AC inputs to the diode bridges not hooked up correctly, could cause some cancellation. If you make it as designed, (along with kevin glimore's cap corrections) measure the output and then swap ONE of the AC inputs to ONE bridge and observe the results. Use the configuration that creates the highest output.

If this were my design: I would build it as 2 power supplies, using 3 filter caps on each diode bridge. I would then "sum" the supplies with isolation diodes.



PS1 + ------[>|-------
PS2 + ------[>|------- PS1+ & PS2+ connected here to output

PS1 - tied to PS2 -

Again, this may not be needed. One 600 VA transformer might be enough. Check the fuse rating on the amp, that will tell you how much power you need.