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Old 12th May 2003, 07:00 PM   #1
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Default Computer Power Supply for Gainclone ?

I've searched this site over for this but couldn't find it So i've decided to start building a gainclone. I priced it out and found that I can get alot of high quality components for not that much. The only thing that would cost the most would be the power supply. I decided to go to the local thrift store to see if I could find the right transformer I need in an older amp. Instead there were no amps around, but I did find a old computer power supply. The data sheet for the LM3875 says to build a 40 w amp you need about a 30v power supply that runs at about 3A. My questions are, I can get the right voltage, but how many amps can those little LM875 chips handle and does anybody have any advice pertaining to the suject ?
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Old 13th May 2003, 02:06 AM   #2
blip is offline blip  United States
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I've had the same thought about PC powersupplies before but there are, unfortunately, several problems:

1) Voltage is too low. Most GC are powered on at least +/- 20v after rectification. (DC volts) Most PC powersupplies provide 12v as the highest voltage... While you could combine this with the -12v to get 24v, the amperage on the -12v line is too low.
2) Changing the voltage output would require some major redesign work on the powersupply. Someone who is more experienced with them might be able to give you a better idea of exactly how much would be needed, but my guess is that you would basically have to strip the components, replace a few of them and put them on a new PCB before it would work.
3) The components in a PC powersupply are generally pretty lousy so the performance is bad.
4) Switching supplies are generally not well thought of for audio work to begin with.

I know there are some other problems, but they aren't comming to mind.

Anyway, my advice is if you want a cheap powersupply, look at a discount parts shop online. I know that www.allelectronics.com stocks some transformers that will work... They won't be great.. but they'll work.

Good luck!
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Old 13th May 2003, 12:58 PM   #3
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I can get up to 34v per channel (keep in mind this an older power supply) and on the sticker on it it said it have about 20A max. I'm worried if this is to much for the LM3875's ? and if it says max what will t normally operate at ?
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Old 13th May 2003, 02:23 PM   #4
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A basic electronics understanding will reveal that the max current ability of a PS cannot be 'too much' The 3875 will take as much of the 20A as it needs and no more. Whether your PS will be able to provide bipolar output or you'll require two power supply modules is another matter. In any case this project is no longer a 'gainclone' but a lo-fi chip amp.
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Old 13th May 2003, 03:23 PM   #5
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I've been studying basic electronics and hace built a few circuits, and yet I know the computer PS is cheap could somebody tell me why this would degrade the sound quality ?
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Old 13th May 2003, 09:27 PM   #6
blip is offline blip  United States
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34v? Is that single sided or can you get +/- 34v? That is an odd PSU indeed!

Anyway, the problem with PC PSUs is that they are switching supplies. I can't explain much of theory behind this, but basically it means that the voltages are generated by switching the power on and off rapidly. (I think it is the same principle as Pulse-Width Modulation) This switching activity produces a lot of power supply noise that will degrade the sound quality.
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Old 13th May 2003, 09:38 PM   #7
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Default Use of switched mode (i.e. PC) power suppplies

Switched mode supplies often require a minimum current to be drawn before they will work (otherwise they consider themselves to be "off-load" and shut the supply rail down).

I don't know what the quiescent current of a gainclone would be, but you should ensure that there is always at least the minimum recommended current being drawn from the supply (perhaps by loading the output with a resistor - wasteful i know).

There are not so many switched mode supplies that deliver the goods as regards hi-fi. The rails on a regular SMPSU will not be clean enough to use on a hi-fi amplifier, the switching frequency will most likely be too low as well.

You should rethink your startegy and see if you can buy something cheaply enough surplus shopping
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Old 14th May 2003, 07:27 PM   #8
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Has anybody used any of the toroids that are availible from apexjr.com ?
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Old 15th May 2003, 12:21 AM   #9
SQ Kid is offline SQ Kid  United States
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i used one of the signal brand transformers from apexjr. however, i got one of the last few. but for $10, i couldnt be more pleased
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Old 19th May 2003, 02:16 PM   #10
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I am attempting to design a SMPS for a LM3886 application. The project is proving to be a challenge. First it needs to provide about 300 watts at +/- 35 volts. That is higher than usual voltage for a common SMPS, and to many watts for a simply flyback converter, so it is difficult to find anything similar for reference.

The topology I have tentively settled upon is a two transister forward converter, with dual output windings. This will require custom magnetics. After getting this far, I realized I need to read Pullman's book on SMPS to finish the project. In any case, I welcome comments.

Cheers.
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