Any idea how many amps a modest car amp draws? (1 Viewer)

[Kevin_W]

I have a amp used for car audio laying around that is rated at 300x2w max that I want to use to drive a pair of passive subs for the basement system. Looking through the radio shack catalog they sell several 12vdc power supplies ranging from 3amps to 23amps... any idea what an amp like this would draw? Oh yea, I also have to add my electronic low pass EQ though I doubt it draws much.
Kevin
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rumblewood
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[Chris Hoppe]

It's generally not worth it to run a car amp off a DC power supply.
If the amp you have really puts out 600W, then assuming it is about 33% efficient (typical of most amps) then that would mean you need to provide 1800W to the power supply. Divide that by 12v and you would need a 150 Amp power supply!
Needless to say, Rat Shack has no such thing at any price!

 

[Kevin_W]

{groan} Thanks Chris... I was hoping for the cheap way out, doesn't sound like its going to work...
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rumblewood
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[Dustin Haug]

Look at the fuses on the actual amp. For example if it has 2 35 amp fuses then 70 amps is a pretty good guess for how much current it will draw. This is the general rule of thumb for determining if you need to upgrade an alternator due to a large stereo, it should work the same way to determine how large a power supply you'd need. But I doubt RS would have one big enough.
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My DIY sub page

 

[MattE]

The problem with looking at the fuse ratings is it tells you nothing about how much the amp will draw playing music or HT. Music has a duty cycle of 10% so it won't draw much current even cranked way up. There isn't really any way of knowing how much the amp will draw. I got a 60 amp 12V power supply off ebay for $70.

 

[Brett_A]

My car system draw around 300 amps at near peak levels. I am only using 60 amp fuses for each amp though so it doesn't corrulate.
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Brett

 

[AjayM]

Well, out of that 300 amps, what is going to each amp? Fuses don't blow right away when they go past their rating, it does take a little bit of time. A 30 amp fuse my work fine for a minute at 40 amps or 35 amps..etc. The fuse will also take a huge peak for a few milli-seconds if needed. This is where the Rat Shack DC power supply won't cut it, especially for a sub. Unless you have a very effecient amp, I've seen amps that don't draw much and would probably work fine with the Rat Shack piece, it all depends.
I agree looking at the fuse on the unit is a good place to start, but that's just a starting point.
Andrew
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[Tim Morton]

Lots of missinformation being offered up here in regards to amp draw. First off considering that in a car all the current is being supplied by the alternator and most alternators proved less than a hundred amps and that needs to be used by the entire car it would be laughable to think that this amp you have laying around would require a power supply capable of producing 150 amps. Why not buy a power supply from a company that will allow you to return it if it does not do what you expect it to do? Radio Shack sells some pretty crappy power supplies and the truth is that the cost of a decent power supply might outpace the cost of a new amp. I think in order to use this amp you have now you might want to look at nothing below 75amp supply.

 

[AjayM]

This isn't entirely true, during large power draws, the alternator can't keep up and the power comes out of the battery. These large power draws come almost entirely from subwoofer amps, and the amount of time that the current is being drawn that is high is measured in milli-seconds. This is why you will see headlights flickering when the bass is pumping, this is because the alternator is past it's limits and power is coming from the battery and can't keep up with the transients.
Now there are a lot of conditions to have this happen, you need to have some power hungry amps, the Class A variety are bad when it comes to this, the volume needs to be cranked up, and of course these need to be hooked up to some subs.
Knowing this, having a car amp hooked up to a power supply to power a sub is probably not going to work, as when you turn up the volume you are going to start running out of power. Not to mention the quality of power that usually comes out of those DC power supplies. But as Tim said, find out what the return policy is and give it a try.
Andrew
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[Kevin_W]

Thanks for the tips guys.
Well its pretty obvious that the top of the line Rat Shack power supply isn't going to work - and even those are over a $100. I can only imagine what a high grade 75-100 amp will cost. At this point I think the most economical path is to just get an inexpensive PRO amp like a Samson.. In the end it may cost a little more, but its still probably better than even a nice DC power supply/car amp combo - cleaner looking too.
Kevin
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rumblewood
[email protected]

 

[Philip_G]

without doing the math I can tell you I'm running a 300 watt soundstream reference amp in my car on a 35amp fuse

 

[Blake N]

I have run car amps in home serveral times. For parties and such and have never had a problem. I have 2 power suplies a 7.5 amp and a 20 amp. The largest amp I have run on the 7.5 was a 200 watt amp. The 20 amp however has powered a 600 watt amp and 8 decks simultaniously. I didnt have to pay for the 20 amp but I have seen it in supply catalogs for $180. This power suply is ideal for car audio situations however. It will handle 20 amps continous 35 amps 50% of run time and 60 amps for 1 cycle. It is made by astron. You might also look into geting a realy big capacitor .5 to 1 farad. Put this in line with the power going to the amp. This will decrease the strain on you powersuply for larg spikes. DO NOT go to a car audio place and get a stinger cap or somthing like that. Look for a welding supply or industrial supply shop. They sell the same thing for about 90% less.
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[AjayM]

I know the series very well, I'm running a Ref 500 down around 1/2 Ohm load (read: crazy amounts of power), but it's all being sent through a 40amp fuse (I haven't gotten off my butt to re-wire it all yet) and even with a moderate volume with a good bass note I will fry the fuse very quickly. I really should have a good 60-80 Amp fuse for it, or basically around a 100A or so for the whole system. But Soundstream Reference and Class A amps are known for being very greedy with the power, it's part of the reason they run well at ridiculous loads (like 1/2 Ohm) and create so much power at those loads.
Andrew

 

[Philip_G]

I'm only pushing a 2 ohm load with the ref. 300.
outstanding amps. WOuldn't own anything but right now, especially since a 300 can be had for about 75$ and a 500 no more than 100$
I'm pushing a pair of the reference series 10's (well one now, the magnet shifted on the other and locked the voice coil, so I'm looking for a magnet structure to recone it)

 

[AjayM]

Sounds like a good setup, my subs are still leftover from my "need more bass" days, a couple of SPL10's, incredible woofers, not very good on the SQ though, but they sure can play loud...although I've pretty much torn them apart, the foam surround on one of them is about half gone on one of them, and it's not from rot. I'm about to do a full make over of the system anyways, I'll probably keep the Ref500 to drive the subs, but I'm going to move over to an Alpine or something along those lines to drive the front end instead of the Soundstream 4 channel I have now. New speakers for the front, move the fronts to the back, new wiring, a good active x-over and of course some new subs and I should be set.
Andrew

 

[Philip_G]

Hrm the SPL's must not use the rubber surrounds like the refernce series? I'm seriously thinking of replacing both subs with a pair of the competition 12 car subs from http://www.audioc.com,
I go in and out of my MORE BASS phase. Just when I think I've grown up I go right back into it