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Old 16th September 2007, 05:30 AM   #21
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Junkie-

Your friend did exactly the same thing I am doing. Get rid of all the (-) voltage stuff, turn the +12V into -12V and the +5V into +12V.

The one thing I can't understand is why does only the +5V output have L-C-L-C, while every other output, including +12V have only L-L-C? I am fortunate that my supply has space for a cap right after the big yellow cross-coupled inductor-toroid. So both outputs will have a true L-C-L-C filtering.

Steve
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Old 2nd October 2007, 08:23 PM   #22
bose is offline bose  Sweden
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Isn't a PC power supply very noicy to be used in a audio amplifier? It has a high switching frequency but often not out of the hearing frequencys. As you mentioned, it got to have a good filtering on the output if it would work properly. But buying components for it? Wouldn't it be cheaper to buy a 300VA transformer and maybe build a regulated supply (maybe from parts from the computer supply)?

Putting the amp in the same box without Really good shielding from the SMPS would also introduce a lot of noice..
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Old 3rd October 2007, 01:46 PM   #23
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Quote:
Originally posted by bose
Isn't a PC power supply very noicy to be used in a audio amplifier? It has a high switching frequency but often not out of the hearing frequencys.
Every Car amp over 25W/ch having a DC-DC converter uses a TL494 PWM IC, the same controller chip in the majority of half-bridge pc supplies. The typical switching frequency is 33-36kHz, about 10-15kHz above the highest frequencies the human ear can sense.

Quote:
[i] As you mentioned, it got to have a good filtering on the output if it would work properly.[/B]
The weak-n-wimpy cheapy output filter 470-1000mF OEM caps are replaced with much better units, typically Panasonic HF, HFU, HFC, or the newer FC series Low-ESR 105C electrolytics, usually 1500-3300 (or higher) mF, and 35V or above.

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[i] But buying components for it? Wouldn't it be cheaper to buy a 300VA transformer and maybe build a regulated supply (maybe from parts from the computer supply)? [/B]
The whole idea here is to use the pc supply and not to replace it. Since this is about modifying a Switching supply and not making a linear one, using a 300VA 50-60Hz transformer kinda' defeats the purpose. While there are many cheaply designed AT & ATX pc supplies out there, there are (were) some well-designed and well-built ones with good filtering and low noise. It's just a matter of finding one. With the glut of 250W and under supplies floating around, this shouldn't be too hard.

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[i]Putting the amp in the same box without Really good shielding from the SMPS would also introduce a lot of noice.. [/B]
Any well-designed SMPS in close proximity to the amp section (same enclosure) will have low EMI and RFI, and of course be shielded from the amp.

Car amps do not suffer this problem because their DC-DC converters use a big toroid instead of the traditional E-I core ferrite for the main power transformer. Toroids, for the most part, are self-shielding by nature of their geometry: almost all the magnetic flux is contained within the core, with very little magnetic strayfields.
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Old 3rd June 2011, 11:37 AM   #24
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how to lowering the current(let say from 20amps to 7amps)? i have atx psu and want to use fo my lm1875)
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Old 8th March 2013, 02:51 AM   #25
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Just don't draw as much. Unless you want to rewind the transformer.
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Old 8th March 2013, 03:13 AM   #26
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I find it's MANDATORY to have some sort of LC filter on any amp run from a 12V computer SMPS to keep out noise. Use a ferrite inductor as the input to your positive rail, with as least 2,200 uf or more, and it will fix that problem.
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