Monday, February 01, 2010

Well I'm filtered

Just over a month ago I purchased a pair of Medion Multimedia PC Speakers from Aldi and was delighted at both the quality of the sound they produced from my Elecraft K3 and the fact that they were free from any RFI. So when I observed some loud rasping noises in time with my voice while operating 80m SSB a few days ago I was a bit surprised. After a bit of experimentation I discovered that this was due to the fact that I had decided to power the speakers from my Diamond GSV3000 25A shack power supply in order to avoid an extra wall-wart. When I powered the speakers from the provided supply they were free of unwanted noises.

The ability to power the speakers from the same supply as the radio seemed worth pursuing, so I decided to investigate. Another nice aspect of these Medion speakers is that they are held together using easily accessible Phillips screws and can be taken apart in a couple of minutes. All of the other computer speakers I have had were put together in an unfathomable way so I could not even begin to attempt adding RFI suppression.

Once I had the thing apart I was very impressed with the trouble that had been taken to filter all the cables going in to the powered speaker. Both positive and negative lines of the power input are filtered using several turns of wire on opposite sides of a toroidal core. The stereo line input has capacitors across the input, and the output to the other speaker has wire links with a ferrite bead on each one.

Clearly the interference was coming in on the power cable, so I added a 10nF capacitor and a 470uF electrolytic capacitor across the output of the choke filter. To my surprise this did not make the slightest difference. I am puzzled by this, but life is too short to waste trying to find a solution when one already exists, so I am now running the Medion speakers using the unregulated 12V wall-wart the manufacturer provided.
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