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Old 16th August 2007, 12:40 PM   #1
mikee55 is offline mikee55  United Kingdom
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Smile Car amp, doin it all? Would you, could I?

Hello all,

I could get told off, and I expect this will get put elsewhere on the site, but the subwoofer is crucial in my setup. I have a 4 channel amp sat on my sub. 2 of its channels are idle whilst the other 2 drive the sub via Linkwitz Transform. Crudely, I have a PC power supply providing the amp's juice. I have a Sony TA-AX2, (STK chip inside), amplifier doing full range duty. I find it a typical budget Sony, neither good or bad, and plays loud, (which is irellavent) and drives my Mordaunt Short MS40 reflex 8 ohm rear ported, twin 6inch floorstanders. My Panasonic DVD-S49 does DVD-A and V as well as usual Mp3 and CD etc. Sky box does the telly bit, and thats it.

To my questions, I'm not financially well off, so I was wandering , if I stuck a Pot in a box, could I call it a Passive pre-amp, and I have a 250k Vishay, would that do? Also, the car amp doesn't have filters, but the one in my car does. I'm thinking, 2 channels run flat and bridged fed via my LT which has a input mixer and 24dblowpass on it, (40hz), and run stereo on the other 2 via the amps highpass filter,(variable between 50 and 1k), I 'm not sure of its slope, but if I change the lowpas xover to say 60 hz, they should overlap, (gain for ch1 & + 3&4 variable as well).

Would the PC psu be okay for all 4 channels, (I can't play too loud), or would you recommend building a Linear supply to install after the switchmode supply in the amp, severing the PCB tracks for the switcher if poss?

Opinions please.

Mike
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Old 16th August 2007, 06:09 PM   #2
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I have used a computer power supply myself to run car amplifiers when I lived in a small dorm room. I can certainly empathize with your situation. You have to be realistic about how much current the computer PSU can source at 12V. The ratings on the PSU itself are typically maximum ratings and should not be interpreted as recommended output levels for continuous use. That said, computer PSU's are cheap/free, and easy to use. You should definitely use one PSU per amp, but your quandary is based on whether or not the PSU you have can source power to all 4 channels of the amp you are using.

My first option would be to use your computer power supply with capacitor banks in parallel to store the energy needed for transients. You can get some pretty large caps for 16V ratings, including the so called "stiffening" caps that are marketed at car audio use. If you are not listening at loud levels and just need the occasional burst, this will probably suit your needs.

If you need more sustainable power, you might consider a battery powered system. Just run a trickle charger on it when not in use (disconnect the charger when you use the system, or use a nifty auto shutoff relay which you can find somewhere on the web for this type system).

I cannot conscientiously recommend using standard car batteries as they release hydrogen gas and could be an explosion hazard. For safety's sake use sealed lead acid batteries such as the type found in backup power supplies.

At some point, you have to reach a level where you understand that for just a little more investment, you could do a nice little chip amp system that wouldn't be fraught with all of the shortcomings of two switching power supplies in series to simply convert 120AC to 12VDC to +/- 30VDC.

I wouldn't hack up a perfectly good amplifier just to use the transistors and drivers with a linear power supply. If you reach the point where you need that much power, Ebay it, and build something that is more in line with your intent.

Best Regards,
David
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Old 16th August 2007, 10:23 PM   #3
mikee55 is offline mikee55  United Kingdom
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Hello gtforme00,

Thanks for your reply, I think for a temporary measure, the PC supply could be happy running on all four. I can't crank it too much, I live in a semi detached house. I can hear the neighbours music, so they must hear mine. Occasionaly, the odd DVD will call upon the sub to do its bizz, especialy last night when I gave it Pink Floyd's Dark Side of the Moon live Pulse DVD. HaHa, that was fun and it wasn't that loud except for the odd bit here and there.

Please clarify, I was under the assumption that you cannot add extra capacitance to a SMPS? I have a bank of Caps that I used in my car once to see what happened. It totaled to 57,000uf, and I was supprised it helped, on a scale of 1 to 10, I'd give it a 3 for improvement. It wasn't dramatic, but neither was the system anyway.

At the end of the day, it gives me a chance to hear what is possible and its there whilst I decide what to do later.

Many thanks for replying,

Mike
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Old 22nd August 2007, 07:55 AM   #4
dangus is offline dangus  Canada
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I'd suggest finding a computer UPS, since that gets you a battery and charger all in one neat box. Free UPS's will probably have nearly dead batteries, but they may have enough life left for your purposes.

On the other hand, consider using thrift-store/car-boot-sale stereo receivers as power amps (use the line ins).
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Old 24th August 2007, 02:18 PM   #5
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The risk of adding capacitance to a SMPS output is the large current draw at startup. If the capacitors draw too much current at startup, then the SMPS will auto-shutoff. I have never added as much as 57,000uF capacitance, but if you already have the capacitors (I assume that you paralleled more than one together from your quote) then you can add one at a time and see if it loads down the SMPS too much at startup.

How would a UPS work? Don't they output 120VAC? I've never experimented with one, but there's one laying around at work that I might have to play with.

Regards,
David
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Old 29th August 2007, 12:15 AM   #6
dangus is offline dangus  Canada
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UPSes consist of a battery charger, sealed lead-acid battery, and an inverter to produce AC. A typical 500 VA UPS will run off a 12V battery of 10 to 12 Ampere-hour rating (sometimes two 6V batteries in series). Units with ratings above 1000 VA generally run off 24 volts, with 12V batteries in series. Most units only run the inverter when power fails, but there are "true" UPS units which run the inverter full-time, and therefore have battery chargers that can supply enough current to run the inverter and still recharge the battery. These aren't common.

When they fail, it's almost always because the battery is worn out. You can usually find replacement batteries at a local electronics parts dealer for much less than getting the "official" replacement battery pack from the manufacturer.

So, if you ignore the inverter part that turns 12V into 120V AC, what you're looking at is a beige box from which you can draw 12V power, with a large peak capacity and which will recharge itself automatically. Poke some holes in the case and add some terminals and a fuse. To be safe, check that the 12V battery stuff is at ground potential. There may be units out there that have switching supplies for the charger with no isolation.
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