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Old 1st February 2009, 08:40 PM   #1
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Default Is my Powersupply giving enough... Power?

Well I recently got into Audio, And as me, being 14yrs old I cant just go buy a car and get a sound system lol, Anyway Instead I used my old 400W 12V 18A Powersupply, to power a Infinity 300WRMS class A/B Amp, the problem, well I couldnt max my amp because the power supply was either to small or idk.. So I went out and bought a 750W 60A 12V Powersupply, Same thing happens, I can only max it where i could on my 18A PSU, well a tad bit less. Ive seen someone on youtube getting 500watt rms by using 60A 12V PSU (the difference, Hes using a corsair and a Class D Sony Amplifier and im using a ORYXX and a class a/b infinity amplifier)...

How likely is it that the New powersupply I bought is not giving 60A? Or does it really make a difference about the class a/b/d thing? I feel like I just got ripped off.

Other things: The powersupply is about 4 times heavier then the old one. Has a big large fan on the top, and a okay sized fan on the side, very stylish, old PSU taken out of a really old Computer. Im sure that Class A/b amps dont constantly consume power, so when a bass note hits it draws more power (ATX Powersupplies dont like a quick change in current) while a class D is consuming some power, thus when a bass note hits the PSU is very less likely to crash.

Yes and I know Powersupplies are not ment for powering car amps.. but I need help.
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Old 1st February 2009, 08:59 PM   #2
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The amount of power you can get out of any amplifier, regardless of it's classification, can output is dependant on voltage and impedance of load (read: speakers) following the very simple formula.

Watt (rms) = Volt^2/2R*effeciency

The reason you see some car amplifiers being able get more power out of a 12 Volt supply than this formula dictates is that they have a DC-DC converter that converts those 12 Volts to a much higher voltage like +/-40 volts. At such voltage you can output as much as 640 Watt into 4 Ohms.

Off course, you can only do that as long as your power supply can deliver such power but in effect you only need a power supply that can average deliver about a fourth of that since average power output on music signals are much lower than on measurement sine waves.

In the other post you mentioned the load was 2 Ohms, that means you output about 50 watt peak and 30 watt rms per channel. The amount of power the supply can deliver is completely irrelevant as long as the voltage and the load is the same. You could connect your amplifier to a 10,000 watt power supply and your amplfier will still only output the same, if the voltage and load is the same.
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Old 1st February 2009, 09:15 PM   #3
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Ok im begining to understand this better now.

But I dont get why I can only have my amplifier at Half W/ no bass boost or anything, and it starts to clip at a low volume with 12v, but when i have it in my brothers car it does not clip and I can have it at full volume.
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Old 1st February 2009, 09:21 PM   #4
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Sounds like your bother has a better amp.
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Old 1st February 2009, 09:27 PM   #5
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First of all, a car battery/alternator is not 12 volts but 13.5 volts on average, those tiny 1.5 volts extra means almost 25% more power. Second of all, a computer PSU is constructed to provide constant power and voltage and a not meant for transient load (sharp rises and falls in power demand), and since music is a 100% transient load a computer supply connected directly to an amplifier will perform very badly. You can cure most of this by adding a large capacitor across the voltage rail before the amplifier.
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Old 1st February 2009, 09:29 PM   #6
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Ok, I Understand this now.

Will 1Farad be enough?

and thanks for all the help.
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Old 1st February 2009, 09:32 PM   #7
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1 Farad is complete overkill, and the inrush start up current will almost certainly burn through the cables used in a normal computer psu. 10,000 uF (100th of a Farad) is probably more than enough.
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Old 1st February 2009, 10:27 PM   #8
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I just remembered, It also says Voltage on the cap, What voltage capacitor at 10,000 uf do i need?

Cheers.
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Old 1st February 2009, 10:39 PM   #9
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16V minimum
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Old 2nd February 2009, 03:44 AM   #10
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While playing music, Class AB wastes as heat approximately as much power as it delivers to the speaker, while class D wastes typically less than 10%.

Remember that short heavy gauge wiring is required for powering car amplifiers.

Computer power supplies of the switching type require some load in the 5V line in order to provide high current through the 12V line.

Clipping may come from your signal source rather than from the amplifier.

1 Farad is perfectly ok. These capacitors are routinely used in car systems, but they are usually kept charged all the time. Some include a header with an automatic charging circuit, volt-meter, etc.. These should be more suitable for your purpose.
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