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Old 25th June 2006, 03:10 PM   #1
ardo is offline ardo  Canada
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Default PSU Help Needed

This is a bit of a repost, I asked the question in another thread, but still am looking for an answer.

I used two cheap PC PSUs in series to power my son's Class-T MC8X100 amp. I get 24V at 11A, and the amp works great at my place. However, there's a lot of noise when the amp is connected at my son's place. The noise goes away if the amp inputs are disconnected. The PSUs also disrupt my son's wireless keyboard and mouse.

No modifications were done to the PSUs, except for the ground disconnected on the second unit, like this:

(GND)[PSU1: grounded](12V)-(GND)[PSU2: floating ground](12V)

I saw someone mention that it should be the other way around: PSU1 with floating ground, PSU2 grounded. Does it make a difference?

Another question: the box I used to house the amp is not grounded, I only have GND & 24V connected to the board. Is this right?

Lastly, if I cannot get this PSU going, I'd probably buy a 300VA toroid. Does it make a diff. if the toroid is shielded? I was thinking of getting one from Plitron (they're 10min away from my place), but I can't get a shielded one unless I order 10 units.

TIA - ardo.
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Old 25th June 2006, 05:17 PM   #2
Netlist is offline Netlist  Belgium
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I personaly never tried but I read many times here on the forum that computer PSU's are not the ideal source to feed amplifiers.
Unless someone can come up with a reasonable solution (could be grounding issues) I'd go for the Plitron; even unshielded.

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Old 25th June 2006, 06:57 PM   #3
Bakmeel is offline Bakmeel  Netherlands
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Computer PSU's are far from ideal sources for amps, and they are not happy when floating.

I suggest you go for the plytron, but if you really want to stick to the PSU's try adding 1 or 2 100nF X-capacitors between the "floating" ground and the real ground.

Your son's keyboard and mouse aren't working because the two PSU's are acting like a radio broadcasting their switching frequency's. The cap might help but is not guaranteed.
More Power Igor! More Power!
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Old 26th June 2006, 07:25 AM   #4
AndrewT is offline AndrewT  Scotland
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is there a saety issue with the case of one of the PSUs floating.

What happens to the internal safety earth in the floating unit?
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 26th June 2006, 05:15 PM   #5
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Default PSU Help


The keyboard is being disabled because of the BEAT frequency generated by using two computer PSUs operating at same, but not exact switching frequencies. A beat frequency is generated by the difference between two such oscillators operating in close proximity to each other. Both SMPS PSUs would have to have their clocks synchronized to eliminate this problem.

As for floating one of the PSUs, it might be better (and possibly easier) to just modify one of the SMPSs to give you a symmetrical (+/-) output. Depending on the model used, and there are literally thousands of different designs, rewinding the main power transformer's secondaries to produce the desired voltages, modifying the output voltage feedback, and replacing output caps with appropriate voltage units, this can be done. If you choose this path, it would be helpful to have a schematic so you know where you are in the circuit. More ofetn than not, none are available, but at the QRP4U website, there is a link to an AT power supply schematic.

Just do a search on this forum for all the threads about doing this.

Hope his helps.

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