Ok guys, take it easy on me, I'm still a newb after being a member for a very long time. Anyway, here we go: I want to make a subwoofer for my home theater. I have a car amplifier for a car sound system, two subwoofers (without boxes), and an old Sparkle PSU laying around. Is there a way I could run the 12V line into the amplifier and it actually power it? The amplifier would only be doing 125 RMS for each sub, 200 Max. That's a combined total of 400 watts. The PSU is 350, but I seriously doubt I will be maxing out these subs anytime soon. Besides, I read that the RMS is normal usage or something. I would need a way to actually turn the PSU on, because it won't turn on without the case switch, right? And the case switch runs to the motherboard? I was wondering if I could just flip the power switch on the back of the PSU on, but I looked, and this model doesn't have a switch I could buy a dual subwoofer enclosure box, and mount the amplifier on it, and put the PSU inside the box somewhere, right? I'm pretty sure car amplifiers use a 12 volt line, so this is where my idea started. I've checked other forums but I need a lot of opinions on this before I start anything Then I could run the AV cables from the amp into a mono/subwoofer wire and run that straight into my receiver. Just an idea...Anyone had any experience with rigging this sort of stuff up?
I see you posted this quite some time ago. If you're still looking for answers to this let me know...I can help.
just reply with any information you have I still haven't started the project yet, I'm not confident I have enough information yet....
Car Amp specs are very optimistic (ie, lies) and for starters will be based on a running engine, puting out around 15v. So on 12v you'll only get about 60% of the full output, say 240W, 20A at 12v. But a 350W PSU can't supply all its power on just the one voltage line, plus amps aren't 100% efficient, so you'll still have to be careful with the volume. There's hardly any smoothing on the PSU output, it's all on the mobo, so you'll have to add some.
This is true, you may not get full output wattage depending on the quality of your amp and its power supply design. Very high quality amps can power the speakers to full rated power on 11.5V and sometimes lower voltages. -Firstly i would look up the available current supply on the 12V rail of your PSU and get an in-line fuse to match. Your PSU may be able to supply close to all of its power on one rail. Some psu's regulate the 12V line and derive the 5V and 3.3V supplies from the 12V line, therefore the 12V rail would have to handle the full output power (which would be great for this paticular case), but as far as a computer supply goes this way certainly isn't the best way to do it but it saves the psu manufacturer complexity and ultimately some money. This info should be on the side of the psu. -To turn on the psu without being connected to a mobo the are two wires you can short together on the mobo connector to trick the psu into turning on. I cant remember what two they are off the top of my head but that info should be easy to find. -RMS stands for root*mean*square and is basically an average (but not quite) of a complex waveform...anything other than pure dc. For sinewaves or pure tones RMS Watts is exactly half of Peak Watts. So 125Wrms = 250 Wpeak. -As for adding capacitance...I wouldn't bother, the psu output is regulated to within fractions of a volt and your supply wires will most likely be shorter than 24". Back some years ago it was considered good practise to add gobs of capacitance just before the wires hook into the amp. Most people now realize its just a waste of money...exept those trying to sell you a $100 1Farad capacitor of course. The only time it would help is if you had an extremely poor quality power supply or hadn't installed your amp properly in the first place i.e. used too small of a supply wire and the voltage drops on the wire would be too great under high current situations. The same thing happens when people ground their amps to the car chassis...which is a very bad idea in the first place. So when you wire the psu to the amp, parallel all the 12V wires together and the grounds together to get the largest conductors possible. Also be aware that there maybe a much smaller gauge wire going to the 12V pins on the mobo connector. This is a remote voltage sense wire. If your psu has this wire, you could add some resistance in this wire to make the psu regulate higher than 12V to closer simulate an alternator output. -Just a note on sub-woofer boxes.... Unless you use a sealed box (which are very forgiving for box volume and the best box there is for sound quality), being able to buy a ported, bandpass, or single-reflex bandpass box off the shelf that will match your speakers will be near impossible since these designs have to be very accurately "tuned" to the speaker characteristics to function properly, so if this is the type of box you're after you may have to have it custom made. Let me know if you need anymore help !
As for adding capacitance...I WOULD bother. Switched mode power supples are very bad for superimposing noise on the output of amplifiers if they're connected to them, as they're very poorly smoothed - this is the reason linear power supplies are the power supply of choice for most quality amplifiers.
SteveyG, Yes SMPS can impose unwanted noise but this is usually due to EMI not electrical noise on the supply lines. And a psu designed for a computer is most certainly going to have very little noise on the outputs or you would have tons of errors in your computer. Plus there are already caps on the supply rails within the psu and in the amp (in the form of a lowpass filter). The amps power supply is a SMPS too and doesn't pass any noise onto the outputs. By linear I assume you mean unregulated. Those power supply types are used as a matter of reliability and simplicity not so much of performance, the high current spikes acossiated with these supplies can just as easily impose noise on the outputs. Actually these power supplies have a higher ripple content but this is dealt with by the PSRR of the amplifier which is why they dont need the cleaner power you get from a SMPS.
I'm talking ripple, not just noise. Isn't that why there are so many large-value electrolytic capacitors on the motherboard? edit: checking a reverse-engineered schematic of an ATX psu, the 5v line has 4700uF, 1.5uH, 2200uF smoothing, the 12v line has 2200uF & a 30uH choke. Double the "normal" load on the 12v line and you'll double the ripple. Also the choke may not take the extra amps, YMMV.
Some of them don't even have capacitances that big. I use a set of 10000uF capacitors in most amplifiers because even high current power supplies will generate lots of ripple (or dip as a result of the high ripple) as the current draw increases.
Thinking about it, these mega-watt car amps can't have a 12/15v power rail at the output end, they have to convert the battery supply up a good few notches. Best you can get with a bridged amp, 4R speakers, on 15v supply is about 25W, and 20W is more usual (eg TDA2004). Power (W) = (Vs - 2Vce(sat))2 /2R according to NS. So the 12v input is feeding an invertor? Supply quality may not matter a damn.
Yes that's right. Some of the larger amplifiers use an inverter to bring the voltage over 100V in some cases. They can draw very high currents from the 12V input. Certainly more than a standard 350W PSU can supply.
Every amp i've seen uses an invertor or switchmode power supply (SMPS), even 20W models, so supply quality isn't so critical. Plus these amps are designed to work with automotive supplies which are quite noisy, but SteveyG is right, adding capacitance never hurts. Depending on the size of amp BlueMax has the 350W may just cut it. Taking into account inefficiencies in the amp's power supply and output stage that would leave about 200Wrms for the speakers... at any rate I would think it's enough to power a sub for a home theater.
