Noise on speakers powered over +12vDC on ATX molex

Started by ken_cinder, April 10, 2009, 10:09:25 AM

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Here's a strange one I hope someone can help me with.

I've wired up +12vDC to a set of stripped down speakers via a 4 pin molex, and I'm getting noise. Not just any noise though, what sounds like polling and when the hard drive is accessed I could SWEAR I can hear the access noise through the speakers.

The speakers work fine powered via their normal adapter, but I'm putting something together that requires I keep outlet plugs to a minimum.

I think it's strange this is happening at all, but is there something I can do to get rid of it. Would using a separate ground from the one on the molex connector help?


2x 100uf capacitors and 1x LM7809 should work for you, man.  Just hook it like the datasheet says to.


1 cap each on input/output?

I've only got some LM340T5's here, don't think they carry 7809s locally....damn Ebay it is.

Edit: Or not! None on Ebay. Wish I had asked this before putting in my last order with Digi-Key on the weekend......fark!


The LM317 has much better noise rejection than the LM78xx regulators. But they're all useless when it comes to ripple/noise over about 300Hz or so.

There's a reason why no commercial powered speakers leach power from the PC. The noise on the power rails makes it totally unacceptable for audio stuff. It's simpler to use a seperate linear power supply..