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scwhiteley 17th August 2010 08:55 PM

Eliminating Battery Charger Noise in DIY Stereo

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

audiomik 17th August 2010 09:51 PM

If you want to regulate your DC supply from batteries to remove the charger rip[ple, you'll need to have sufficient voltage difference between the battery supply voltage and the rail voltage(s) for series regulators not to 'drop out'.
For 78xx series regulators this is about 2.5volts minimum, however this can be less for a shunt regulator but this will reduce battery life....

hope this assists

BZed 17th August 2010 10:33 PM

It looks to me like you have basicly 2 options. You can regulate the power going to the batteries or the power going to the amp. I don't think I would regulate the charging voltage. NiMH batteries have special charging needs and if you have a charger that does the job of charging correctly don't mess with it. That leaves regulating the power to the amp. I would set up the amp to work on 12 VDC and regulate the output of the battery/charger. I think you should look for a LDO (low drop out) series regulator that need less than the 2.5 v that a 78XX needs. Or you could lower your operating voltage to 10.5 or so and use a LM317. That way as the battery ran down the amp would not be affected until you dropped out of regulation and any noise that the charging circuit makes is filtered out by the regulator.

Good luck BZ

scwhiteley 17th August 2010 10:55 PM

Awesome, Thanks, this is what I wanted to hear.

I read you, BZed, I definitely don't want to mess with the charging voltage. That's why I bought a "smart" charger in the first place, NiMH batteries seem pretty finicky in their charge method, and I simply don't want to put the time into building the charger on my own... yet. (My charger was super pricey, $30+)

I'm going to give the LM317 approach a a shot, it seems like the way to go (I wasn't even aware that simple adjustable IC circuits like that were available, shows what I know.) I'm also thinking of trying out this new stereo with a 12V SLA (because it is one million times cheaper than the 12 AA NiMH battery pack I'm using). The LM317 will be easily adjusted for either battery... tremendous!

Thanks, SCW

col 18th August 2010 12:25 AM

If you are going to regulate the power from the battery to the amp doesn't that negate the advantages of using battery power in the first place? May as well discard the batteries and make a linear power supply.


scwhiteley 18th August 2010 04:51 PM

Wouldn't be portable. Got to be able to rock out on the go.

BZed 18th August 2010 05:18 PM

Glad the idea does you some good, have fun.


macboy 18th August 2010 05:41 PM

Before wasting precious voltage (and therefore, amplifier power headroom) by using a regulator, try filtering. A CLC or even CRC "Pi" filter might be very effective. CLC = Capacitor, Inductor, Capacitor. CRC = Cap, Resistor, Cap.

col 18th August 2010 10:41 PM


Originally Posted by scwhiteley (
Wouldn't be portable. Got to be able to rock out on the go.

Sorry, I must be misunderstanding something. Surely, when you "rock out on the go" you unplug the charger?

scwhiteley 19th August 2010 01:37 AM


Originally Posted by col (
Sorry, I must be misunderstanding something. Surely, when you "rock out on the go" you unplug the charger?

Yep. You got it. The charger is just for charging. When the battery pack is charged, I can take the stereo with me, portably, for rocking out purposes. When the battery is dead, I then plug the charger in, and it charges the battery. Repeat as necessary.

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