Quieting a Switching PSU? or easier to replace it with a linear PSU? - Page 3 - diyAudio
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Old 5th March 2010, 05:09 PM   #21
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Join Date: Aug 2008
Originally Posted by at77
it's powering only low power stuff - +/- 12v for opamps and a 48v circuit (for phantom mic power), + 5v and + 3.3v for digital and logic components.
Originally Posted by at77
First is a 'lower noise' adjustable voltage boost/regulator with an LM350 or 338 for the positive rail because the psu is rated at 3.5amps on the positive rail.

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Old 5th March 2010, 06:26 PM   #22
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Join Date: Jul 2003
Location: Bath, UK
Originally Posted by p.o'neil View Post
Ok, this probably has a simple explanation but I can't see it! I'm trying to get about 200v dc for a valve preamp. I have 2 small transformers back to back giving me 177vac. This goes accross a diode rectifier bridge and should give me about 238vdc (177x1.4). So why do I end up with 160vdc. I've never seen this before!
So how small are the transformers? Used like this the AC source to the bridge is pretty high impedance, so when you try to draw much current the voltage sags badly. 'Small' transformers often have poor regulation of around 15-20%; two in series is suddenly nearly 40% meaning that at the rated load current you'll effectively lose up to 40% of the off-load voltage. Guess what, it appears you do!

You need to either use grossly-overrated VA transformers, or two with much lower step-down ratios. If you don't have such things to hand in the bits box, I'd expect one approriate new valve transformer will be cheaper.
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Old 6th March 2010, 12:27 PM   #23
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Join Date: Jan 2010
Ah! that explains it! I had a feeling my theory was flawed. I'd better buy a proper tramsformer. I had hoped to build the whole thing out of stuff from the bits box.
Thanks for the help
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Old 18th March 2010, 05:46 AM   #24
Kuro is offline Kuro  Hong Kong
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Join Date: Jun 2008
Hi guys,

I've got a question. What if the diodes in the SMPS are replaced by ultra soft recovery HexFRED diodes? Would this help remove noise from the SMPS to begin with?
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