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Eliminating Battery Charger Noise on DIY Stereo

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I'm currently working on a DIY stereo that will be powered by a 14.4V NiMH battery pack. I've already built a previous stereo with a similar design and the same style of battery pack, which I recharge with a "smart charger" I purchased online. My problem is this: If I turn on the stereo when the battery is charging, the charger produces noise on the line. I suppose this should be expected, as there is nothing regulating the power to the amplifiers (in this case, dual TDA7240A ICs), so the charging voltage is not only going to the battery, it's going right into the power supply for the ICs, decidedly not the best design. So I'd like to fix it in the next stereo. Is the solution as simple as adding a voltage regulator? If this is a solution, I'm thinking I'd want to use a regulator rated for the voltage of my battery pack (14.4V+). Is this a sound approach?

Thanks, SCW

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Using a voltage regulator should reduce the line noise. Your best bet would be to isolate the charger from the amp, but I don't know how to do that. If you are going to use a voltage regulator, I would use a fixed regulator and if the battery pack is 14.4V, I would probably use a 15V regulator or something close to it to compensate for voltage drop.

If you don't mind me asking, what is the stereo for? Is it any better than what one could buy at a store?

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One good way of possibly doing it is to filter the supply to the amplifier with an LC filter. If you know the frequency of the charger hum, you can base the filter frequency on this but otherwise, find the largest inductor that is ok with your current required for your amplifier and use that. This kind of filtering was used on most Valve amplifier to filter out the 50/60hz line noise I think :)

Charger > Batteries > LC Filter > Amplifier

Since you are using two TDA7240A, Am I right in assuming that they're for stereo operation?

This equates to 40w of power I think, therefore equivelant to: 40/14.4 = 2.78A.

Since you will probably want to filter out power supply noise down to 50Hz (just incase you are getting Line hum through the charger) Try: 4Mh Choke (available from Rapid) and a 3300uF capacitor.

A schematic of this would as attached.

Obvilously, just use the largest choke and capacitor pair you can get hold of to ensure your horrible charger hum is filtered :)

Best of luck!


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