Car amp and PSU advice about my plans, please :)

[captainpotato]

I've spent the last hours trawling through every page in this sub-forum and more widely elsewhere to work out exactly what I need to do, but it's now time to check with people who know something...

I'm planning to hook up one of these amps in my cabinet (only link I could find):
http://www.digitalhome.com.au/Car_Sound_and_GPS/Ads/JM2002_400_WATT_AMP/23109

It's a cheap, 2x200W, 2-channel amp that has two 15-Amp fuses in it. I'm going to be connecting it to 2x100W 4", 4 Ohm speakers, and because of the proximity of the speakers to the user (for a pic of the cab, see here: http://forum.arcadecontrols.com/index.php?topic=38343.msg924966#msg924966 - the speakers point downwards towards the screen, and it's a mini-cab, not full-size), I doubt that the the volume will ever approach 11 (maximum, for the non-Spinal Tap fans out there).

From what I've worked out so far, I plan to use a separate PSU to drive the amp, so as to avoid stressing the PC (Which most likely won't have a powerful enough PSU regardless). Assuming that I have understood correctly everything that I've read, driving the speakers at their full 100W will draw a little under 17 Amps (around 8.5 per channel). Therefore, allowing for a bit of leeway, I'd need a 20-Amp PSU to drive them at full volume.

Not having actually plugged it all in yet, I don't know just how loud this would be, but I suspect that I'll want to turn it all down somewhat, in which case the PSU wouldn't necessarily need to be the full 20 Amps, but I may want to go with that anyway, just in case I choose to install some jukebox software on it (unlikely, to be honest - the building bug has got me, so I'm likely to build a jukebox one day, after the other projects are done...).

At this point I'm thinking that I could plug the setup into an older PSU (let's say around 10 Amps maximum) to see just how (un)impressive the sound is at fairly modest levels and to check temperatures. If everything seems fine, maybe I'll stick with the PSU, otherwise I am looking at something like this 450-Watt Vantec ION2+, which seems to be able to handle 20 Amps per 12-Volt line (here: http://www.oz-digital.com.au/index.php?main_page=product_info&cPath=17_343&products_id=939). If you read through the bad layout on the page, it seems that each line can handle up to 180 Watts, which would also be enough for the speakers.

Does this all sound correct, or have I completely got it wrong?

Also, would having two PSUs (this one and the one in the PC) and a 14" monitor likely put too much strain on a (decent) power board? I'm guessing not, given some of the other cabinet setups I've seen on BYOAC.

[captainpotato]

Anyone?

[AussieJuke]

From what I understand of your question, a PC PSU with a  20A 12V line should work. You will need to link a couple of pins on the ATX connector to make it run though. The only real issue is the PC PSU may not like not having any load on the 3v3 or 5v lines.

[Blanka]

Why does no-one consider just building a simple 12V PSU from scratch?
It's about car electronics, not computer electronics. A car battery does from 11-14 V so 12.0V stable is not needed.
Get a 9V transformer like this (200VA, but it will be enough I guess):
http://nl.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0503940
Some caps and 2 bridge rectifiers like this
http://nl.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6296695
or built with 2x4 seperate rectifiers like this:
http://nl.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=5450696

And built a circuit like this:


It will be pretty efficient from small currents to 15 ampere or so, more with a heavier transformer, but as I said, a 200VA PSU will do well.

[captainpotato]

From what I understand of your question, a PC PSU with a  20A 12V line should work. You will need to link a couple of pins on the ATX connector to make it run though. The only real issue is the PC PSU may not like not having any load on the 3v3 or 5v lines.

I'm no electrician by any stretch of the imagination, but why do you think that this could be an issue?

[captainpotato]

Why does no-one consider just building a simple 12V PSU from scratch?

In my case, because your hieroglyphics may as well have been the genuine thing from ancient Egypt. The reason that I'm looking at a PC PSU is not for stability, but rather for ease of use. I may not be able to decipher your diagram, but I can follow instructions that tell me to connect the 12V and GRD lines up to an amp

[AussieJuke]

With all due respect Blanka;
A PC PSU should be cheaper to buy than the parts for a linear supply.
A PC PSU will be smaller.
A PC PSU has much less chance of killing the OP or catching fire due to build errors.
The OP doesn't have the skills to make any PSU from scratch. (no offence OP)

Now, PC PSU's are designed to delivery a fair amount of current on the 3v3 and/or 5v outputs.
These units are usually designed to regulate via one of those outputs, not the 12V one.

They do this as those lower voltage outputs are required to be closer tolerance.
If you don't put a small load on either of those outputs, the PSU may refuse to start.
If it does start, it may only run for a few seconds. I know this from experience.

You need to link two pins on the ATX connector, as that's how the PC mainboard signals
the PC PSU to start up when you press the PC's power button. I don't recall which two
pins they are off the top of my head, but it's well documented on the Internet.

[Blanka]

What's an OP? You mean TS?
If you have to hack a PC PSU to boot up, and make a dummy load to draw extra 5V current, I don't know what is easier. And a linear PSU is fanless, so it can be put anywhere in the cab. BTW, why are there never ever switching powersupplies in Audio Amps?

[Blanka]

BTW, 400 watts out of 12V is ---steaming pile of meadow muffin---. It would mean 16 amps current, so that makes a 0,75 ohm speaker! That would either mean the car-amp uses a dc/dc converter to get a higher rail voltage, and if that is a switching one to you could get enormous EMC mess around your amp, or the 400 watt is PMPO (Peak booM Prior to an explOsion) and ---That which is odiferous and causeth plants to grow--- anyway.
You better find a nice pre-built gainclone.

[MonMotha]

Might be easier, better, and possibly cheaper to just buy one of the standard formfactor 12V SMPS boxes off the internet.  My common place for these is http://www.mpja.com/ but they seem to be out of most of their single output "SMPS in a box" units at the moment.  These big ones (100-200W output) tend to go for ~$40-50, which is what a somewhat dodgy PC supply costs (though if you've got one laying around, what the heck).

To force an ATX PC PSU to turn on, link the (usually) green PS_ON wire at pin 14 (note tha the connector is numbered a bit oddly compared with some connectors) to any (almost always) black ground line.  Many supplies will run with a load on just 12V, but some do need a load on +5, +3.3, or both in order to maintain regulation and actually work.  Unlike some "SMPS in a box" arcade supplies, every PC supply I've found will just shut off rather than sustain damage if it is run with an unacceptably light load on an output, so you shouldn't have to worry about that at least.

Note that many PC supplies are not rated for full output on a single rail.  Good supplies will have each rail and its output rating on the sticker, but if that isn't present, figure you can get about 1/3 to 1/2 of the rated output on the 12V rail.  That means that a 300W PC supply can probably do 100-150W on the 12V rail without trouble (possibly more, but let's play it safe).  You'll be pulling upwards of 10A to get your desired power here, so use large gauge (small AWG number - at smallest #18) wire, and use several of them hooked up to several lines coming from the supply.  The ATX and CPU power connectors are good sources for 3-4 12V lines, and PCIe auxiliary power connectors will have 4 12V lines available.  Also try to keep wire length as short as reasonably possible, and remember that your ground lines need to be as beefy as the 12V lines.

I would not recommend that a person unfamiliar with AC line power take on the task of building something even as simple as an unregulated linear supply.  Incorrect wiring or incorrectly specified parts can easily lead to death, fire, etc.

Blanka says that a linear supply is fanless, but those bridges might get a little toastier than you think, and certainly not ALL linear supplies (especially regulated ones) are fanless.  They'll dissipate about 7-10W each just from the forward drop at full load (assuming they end up sharing equally, which isn't guaranteed), which is a lot more heat to get out of those things than it sounds like.  A heatsink might be necessary, though a fan would probably still be overkill.  Note that the "SMPS in a box" type supplies I suggested at the beginning of this post should be fanless, as well.

[AussieJuke]

What's an OP? You mean TS?

OP means Original Poster. (the person that asked the question)

BTW, why are there never ever switching power supplies in Audio Amps?

There are. I own a repair company and see them all the time. Not much in domestic hi-fi, but more in professional PA and very common in car audio.

BTW, 400 watts out of 12V is ---Cleveland steamer---. It would mean 16 amps current, so that makes a 0,75 ohm speaker! That would either mean the car-amp uses a dc/dc converter to get a higher rail voltage, and if that is a switching one to you could get enormous EMC mess around your amp

Open up any high power car amplifier in the last 15-20 years. They have a DC:DC converter in them. Any amplifier over about 25W (of real power) per channel does - it has to!

[captainpotato]

Okay, thanks all for the input. So, as I understand it, a PC PSU will be fine, and should I have cutting out issues, I need to put some load on the 5V line.

Now that the painting aspect is approaching completion, I'll start working on this soon - will keep you all informed as to how it goes. First step will be an older PSU at low output (on the speakers) to see how the amp works in principle.

[captainpotato]

Okay, time for an update. I'm writing this listening to my iPod playing through the speakers wired up to the amp, which is powered by a 13A max. PSU (with the gain on the amp turned down quite a bit). As the speakers aren't encased, it doesn't sound brilliant, but it is all working fine. The PSU is still plugged into a motherboard (but nothing else is), so that it powers up.

I'm letting it run for a while to check wire temperatures, and then packing it away until the cab installation time comes (next couple of days - the final coat of paint (I hope) is drying at the moment). In the meantime, time to get a better PSU and get to work on other wiring issues, artwork, the CP, bezel, marquee...

Anyway, thanks everyone for your advice.

[richms]

Speakers not in a box never sound good, and cant take much power so things will be under a lot more stress once you get them boxed up and crank the thing to 11..

The thing with PC power supplies is that they are all split rails now, usually at least 2 since the atx spec had a max current of 20A per rail for saftey reasons. There are a few that ignore that and have a single massive 12v rail, but most stick to it.

So you can use diodes to combine them but then you lose another 0.7v which when I tried it was enough to make the red protect light on my amp come on with the bass since it was going into low battery mode.

Running on a single 12v rail (I chose the P4 connector) was more sucessful at getting louder but the PSU would shut down when I really taxed it. I now have got 2 smaller 2 channel amps that I will be using instead but the garage looks like a warzone at the moment and the cab is on hold till I get it lined since the neighbours dont like the sounds of power tools in the evening and call noise control.

[Blanka]

With a car-amp on a PSU you probably better include some fat capacitors. A car battery can deliver 60 amp peaks with ease, your PSU will freak at those pulses. A big capacitor can help dealing with strong peaks.

[richms]

There is already plenty of capacitance in the amplifiers themselves, thats half the battle in getting the pc power supply to turn on is that the capacitance makes it take too long to get to a stable voltage so the PSU turns back off again till you try 2-3 times to get it to turn on.

Mains powered amps have a soft start on them for that reason, in a car its not a problem since the only protection is a fuse that takes ages to blow.

I also had problems with the amp having the remote wire tied to 12v so it was powering on straight away, to solve that I connected it to the PC so that the amp came on with the computer which was after the cab was powered up, but again that would cause the PSU to shut down sometimes because of voltage dipping as the PWM started up. More capacitance may have helped there, but that would just make the initial turn on harder for the PSU unless I got some relays to sequance the whole thing.

The new power supply I tried (acbel) doesnt seem to have any problems powering up into the amp even with it jumpered to be on straight away, will see how it goes once I get into redoing the cab with a woofer in place of a coindoor.