PC PSU To Power Car Amplifier

Hey I am brand new to the forum and this is my first post. I am a student studying Electronics at a senior school as one of my subjects so my knowledge is not very extensive with a lot of situations.

Here is a little information on my current situation.

I am currently working on a school project were I am making a stereo system some what like you would see in a car. I plan on having two 'towers' with two 6" 3-way speakers in them each. I also am going to have another piece to the system called the main hub. This hub will be a medium sized box that will contain all the electronics (power supplies, amplifiers and a fan or two). This hub will also contain a 10" sub woofer. I will seal the sub woofer off from the rest of the electronics so it doesn't shake them to pieces and I can ensure it has all its needed air volume.

On the electronics side of things. For the speakers, I will be powering them of two 50 watt amplifier kits I am building. These kits run off a 39V power rail (or something close to that) which will be supplied from Transformer circuit that will give me 240V AC to 39V DC. For the sub woofer I will be powering it off a old amplifier I have (Clarion APX 200.2). To power the amplifier I am using a 350 watt computer PSU ( http://www.skycomp.com.au/item/Huntkey-CP-350--350W-80mm-Fan-Power-Supply.aspx ) that have a 12V rail at 19A.

So far with the PSU I have opened it up and trimmed all the unessential wires and heat shrinked them. I also connected the green wire to the ground so it turns on. With all that done I wire up the PSU, amplifier and sub woofer.

And now to the problem ... almost. I was unsure how the power supply would do when the amplifier was at full power and according to the specifications the amplifier draws 28A at full power and produces 235 watts. So I didn't know if the amplifier would draw only up to 19A and go no louder or over draw the PSU and destroy it. And from that I put a 10A fuse in the main power line into the amplifier. The intention behind this was to see how far the volume meter would go and how loud 10A of power would be before it blew.

I used my multimeter and tested the amps which revealed about 9A to 10A both at no volume and full volume. At first the was unsure about it and thought it should drawing way more current than that. But then I tested the voltage and notice that as I turn the volume up the voltage drops to about 10V. Why is it doing that?

I played the sub woofer for a while changing the volume testing it often but got the same 9 to 10 amp reading.

I thought that as the volume increased the amp drew more current and increase the output, is that correct?

After about an hour of playing different song and testing the amp I unwired it all and tested just the PSU by itself. Testing the +12V rail and ground I got a reading of about 10A, why is the PSU not running at 19A like it says on the specifications?

I have also tried it with out the fuse in line. I still barely draws over 10A of power.

Some things I might note:
- It says Switching Power Supply on the PSU, is that a problem?
- If the you follow the link the description there says it have some protective circuitry
-I read this post
http://www.diyaudio.com/forums/class-d/137757-my-powersupply-giving-enough-power.html

And although it was of some help I don't understand the formula, the thing about the voltage and load; and the adding a capacitor in the voltage rail.

Any help would greatly appreciated and I apologise for the wall of text.
 
Does your PSU have any pots on its pc board? If so, these are most likely output voltage adjust and current-limit adjust. Start by removing all parts in the +5V, -5V (if any), +3.3V, and the -12V sections- resistors, transistors, capacitors, everything. This will leave you with just the +12V section(s) Now, as the voltage adjust pot is mostly for the +5V section, it also does affect the +12V section. Measuring the +12V output with a DMM, slowly adjust the voltage until it reaches anywhere from +13.8V to +14.2V. Your Amps and other audio electronics will run much happier than from straight 12V.

Hope this helps! And keep those questions coming- they're all good ones, and it sounds like you're off to a good start. Don't let any negative-sounding advide dissuade you from completing it. Please keep us posted of your progress. Pictures are always encouraged, too.
Best of luck,

Steve
 
Thanks for the reply, but I don't understand you entirely. What do you mean by pots?

And how do I know what parts are for the each section?
If I am able to remove all these parts how would I adjust the Voltage?
It sound like this much modification leave a lot of room for error as I am not that experienced.

Sam
 

FoMoCo

Member
2012-12-04 10:04 pm
Don't Do It!

Does your PSU have any pots on its pc board? If so, these are most likely output voltage adjust and current-limit adjust. Start by removing all parts in the +5V, -5V (if any), +3.3V, and the -12V sections- resistors, transistors, capacitors, everything. This will leave you with just the +12V section(s) Now, as the voltage adjust pot is mostly for the +5V section, it also does affect the +12V section. Measuring the +12V output with a DMM, slowly adjust the voltage until it reaches anywhere from +13.8V to +14.2V. Your Amps and other audio electronics will run much happier than from straight 12V.
That's not how a PC power supply works. Most of them are flyback power supplies and removing all that stuff will not do much at all. Except break things.

If the feedback is tied to the 5 volt line, removing it causes an open feedback loop. Depending on the supply this could result in failure or shutdown (if it has overvoltage protection for the other voltage rails.)

Your suggestions don't address the primary cause of his problem. He has only 19A available and needs 28A. Modifying the power supply doesn't solve the problem. Even if he successfully defeats current limit, he'll have a trashed power supply when it fails.
 
Well thats not my problem exactly.

The main problem at the moment is that I used my digital multi-meter and tested the +12 rail and it was running at about 10A. I also tested it under load with a car amplifier running a sub woofer at different volumes but it still said 10A. The only thing that changed was that the voltage dropped at the volume got higher for 12V to about 10V.

Sam
 

powerbob

Member
2010-08-22 12:39 pm
“So far with the PSU I have opened it up and trimmed all the unessential wires and heat shrinked them.”

“ I put a 10A fuse in the main power line into the amplifier.”

“The only thing that changed was that the voltage dropped at the volume got higher for 12V to about 10V.”

The computer power supply has many wires in parallel to the power connector to carry the high currents. Cutting all of them off except one wire will cause voltage drops.

A 10 amp fuse is a resistor. If you measure across the fuse at full power you will see how much voltage it is dropping.

And lastly, 12 volt car amplifiers have there wattage ratings measured with 14.7 or 14.9vdc input at the amplifier terminals.
 

FoMoCo

Member
2012-12-04 10:04 pm
Well thats not my problem exactly.

The main problem at the moment is that I used my digital multi-meter and tested the +12 rail and it was running at about 10A. I also tested it under load with a car amplifier running a sub woofer at different volumes but it still said 10A. The only thing that changed was that the voltage dropped at the volume got higher for 12V to about 10V.

Sam
That's because the power supply isn't designed to do what you're asking. A multi-output power supply is not designed to deliver a ton of current into one output while the others sit idle.

Get yourself the correct power supply and save this one for something it'd be good for.
 
“So far with the PSU I have opened it up and trimmed all the unessential wires and heat shrinked them.”

By that I mean that I have trimmed all of the wires except the yellow and black and green wire. I kept all the yellow because I knew only one would not be able to handle the current and black.
 
Hmmm.......

This thread contains loads of contradictory statements
and of course that means some of them are nonsense.

10A idle current seems a lot, and an absolute disaster
area for a car amplifier if that is its constant current
drain on the battery. It should be wrong and likely is.

I don't know what your measuring. A big cap would
help as would upping the output voltage to around
14V, 12V car batteries are 13.8V at low loads.

I can't see where you have got the spec
for the 12V outputs current capability.

rgds, sreten.
 

wwenze

Member
2008-03-07 12:46 pm
Shouldn't you guys first worry where did the 120W of power go?

The content of this thread is so cryptic...

according to the specifications the amplifier draws 28A at full power and produces 235 watts. So I didn't know if the amplifier would draw only up to 19A and go no louder or over draw the PSU and destroy it.

The amplifier can only draw 28A @ a certain power supply voltage and with a certain load impedance. With 12V supply the most you can get into 4 ohm is 20~30W.
Almost all computer PSUs will auto shutdown when too much current is drawn from them. Look for OCP (over current protection) somewhere on the box.

I used my multimeter and tested the amps which revealed about 9A to 10A both at no volume and full volume. At first the was unsure about it and thought it should drawing way more current than that. But then I tested the voltage and notice that as I turn the volume up the voltage drops to about 10V. Why is it doing that?

Can you illustrate how you took the current measurements? Any dummy load connected to amp output when you varied the volume?
Voltage across what drops to 10V?
Voltage from PSU drops to 10V = your power supply is bad. Since it's a Huntkey I'm not too surprised.
Voltage across amp drops to 10V while PSU stays at 12V = somehow 2V @ 10A = 20W is being dissipated by your fuse.
 
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FoMoCo

Member
2012-12-04 10:04 pm
With 12V supply the most you can get into 4 ohm is 20~30W.
Most untrue. Some of the ICs use flying capacitors to deliver peaks in the 70W range. But, most decent car amps use a switching power supply to increase voltage. The rail voltage is most likely not 12V.

Voltage across amp drops to 10V while PSU stays at 12V = somehow 2V @ 10A = 20W is being dissipated by your fuse.
The fuse isn't dropping 2V without opening, it's the supply sagging because it's not made for this.
 
Just for clarification.

There will be two pairs of 6" 3-way speakers. Each pair will be running off a 50 watt amplifier kit. These kits run at about 39 volts.

The sub woofer will be running off a Clarion APX 200.2 200 watt amplifier which is powered off the 350 watt PSU that has a 12v 19a rail.
 
.......... With 12V supply the most you can get into 4 ohm is 20~30W.

Most untrue. Some of the ICs use flying capacitors to deliver peaks in the 70W range. But, most decent car amps use a switching power supply to increase voltage. The rail voltage is most likely not 12V.........
Agreed, it is Most untrue. The correct figure is ~12W into 4r0 from a pair of bridged amplifiers running on a 12Vdc supply.

If the "chip" has some switching circuitry to achieve a higher rail voltage then the amplifiers are not running on 12Vdc. They are using a switching technology !!!!!!
There is another Thread discussing this as well.
Understand the science and you won't get conned by these exaggerated power claims.
 
So my PSU puts in about 12v DC into the amplifier and then the amplifier steps up the voltage again to something a lot higher?

Also I looked at my PSU and it does have a single potentiometer on it. When I turned it up to about 13.8v it still started up fine, but then I connected up the amplifier to it and then turned it on but the light only comes on for a second and then the PSU shuts down.
 
Computer PSU and car amp gives a buzz!!

Hello everyone. I've read about using a car's amp and sub in the house using the PSU from a PC to power the amp. I get most of the amperage and other things, but I'm facing a problem. I hope the experienced ones here will be able to help.

Here's the scenario:

I joined all the YELLOW (+12V) and the BLACK (-12V/GND) from the PSU to form a bunch and connected the amp to the source accordingly as mentioned. The amp turns on and all is running great (the powering part is done). Now when I connected a small speaker (not the sub as of now) just to test if signals are propagated properly, I hear a BUZZ-ing noise in the speaker. The songs can be heard but the strong BUZZ is still there. It's worthy of mentioning that there are no loose parts in the amp (I checked inside). I guess the earthing is causing the problem because when I touch any metallic point on the PSU or the amp, I get a shock. I'm from Bangladesh and there's a chance that my mains aren't grounded/earthed, hence the problem.

Any help to get rid of that shock/buzz is appreciated.

Thanks
 

Arty

Member
2011-02-24 5:04 pm
whoooo...
look, get a darn sealed lead acid battery, and a battery charger with auto-shut down.
then, wire up a switch and some relays, what to achieve is:
-> when the amp is off, the battery charger should be on, charging the battery, and hopefully the auto-shut down will kick in once the battery is fully charged.
-> when the amp is on, the charger should be shut down, and disconnected from the battery.

tadaa, you got it, working car amp in the house, even if main's power is down.