simple system, perplexing problem

pjjoseph

I have assembled a simple system that powers a 12 volt/ 20 watt stereo amplifier in order to drive speakers and create soundscapes in remote areas.
The panel, controller, and power to amplifier are fine. My issue comes about when installing the mp3 player in the system. I have used either a 12v to 5 volt, 750mA adapter with usb power out, or a simple car plug, 5volt, 1amp for usb cable power out. The mp3 player, a Sansa clip, takes power and plays fine, and I can listen to it with headphones ok.The problem arises when connecting the mp3 output from its headphone jack, into the amplifier, as both get power from the same battery. Before even inserting the usb mini into the player, simply by touching the outer metal shield to the player, there is caused some type of minor short. the music signal ceases and is replaced by a low volume static with what sounds like the inner chip sounds of the player.
Please send any feed back on the above.

BB.

Re: simple system, perplexing problem

Sounds like the "grounds" between the USB 5 volts and the 12 volt system are not "common"...

Two ways of fixing that... One is to connect the two grounds (5 volt and 12 volt) together. Basically "short" the USB ground with 12 volt ground (cut open cables and solder USB adapter near Sansa to ground bus near amplifier negative input--Possibly even "mount" the USB power supply to the negative chassis of the amplifier (if this a grounded/negative return common chassis amplifier for a car).

A second would be to get a little audio isolation transformer--Put the transformer between the Sansa audio output and the auto input for your 12 volt audio system. HAMs (amateur radio) frequently need audio isolation--Here is one example of a guy that built an isolating adapter:

http://www.oppcatv.com/kg4cna/psk31/

I have had those problems before (audio noise between MP3 players and any sort of audio amplifier or small FM transmitter to use with a car radio).

I ended up using my cell phone as an MP3 player and a Blue-Tooth receiver/amplifier to get around those problems (trying to charge/run the player and getting audio to the amplifier). Blue tooth itself does not use much power and, perhaps for your installation, the wireless distance of a few tens of feet may be helpful.

-Bill

RandomJoe

Re: simple system, perplexing problem

Radio Shack sells a ready-to-go audio ground-loop isolation transformer with RCA jacks:

http://www.radioshack.com/product/index.jsp?productId=2062214

If that doesn't work, I've had similar issues (in my case the audio continued to play fine but with an annoying whine on top of it) when using newer DC converters that were switching supplies. If I used an older linear voltage regulator there was no problem at all. Not sure it's possible to simply buy a 12V-5V linear supply anymore, might have to build that with an LM7805 and associated components.