How to drive a sub woofer in addition to satelite speakers

[ioncore]

My Polk Audio 2.1 Speaker system broke down. The stereo speakers are in perfect condition, and so is the sub woofer. For some reason, the sub woofer is working fine, yet there is no sound from either of the stereo speakers -- but when I test those separately (by plugging in the jack into a MP3 player), they're working fine.

I opened it up thinking maybe I could spot a burnt out capacitor/resistor/etc and replace it, however the entire circuit is in original condition and nothing seems to (visually) be out of place.

Thinking maybe it is the pre-amps/power-amp that got shot internally, (and for what it's worth a good learning opportunity) I'm thinking about rebuilding the entire amplifier from the ground up -- nothing fancy, just very very crude/basic. I've been searching up on the various types of amplifier chips and found the TDA2009A at the local electronic store. The data sheet offers a good schema of a basic amplifier and I think I'll implement that.

(A cleaner version is here: **broken link removed** )

Now the problem is, the amplifier will be capable of driving the stereo speakers, but what do I do about the sub woofer? The sub woofer has a resistance of 4ohms, the stereo speakers are reading somewhere between 3 and 4. Is there a circuit I could build to drive the woofer from the same input? Or some how augment the above schematics to allow for the woofer?

Just some FYI: The original circuit was driven by a LA4485 and a bunch of C4570ha.

As you can tell I'm very new to this stuff, but have some elementary exposure to electronics. Any help or pointers would be greatly appreciated.

Thanks.

 

[Mad_Mat]

Ioncore, kudos for having a go.

couple of points to keep in mind however;
A repair is usually easier and better than a rebuild when your experience is limited. Stereo amps are relatively easy to fault find with the right gear, but can be elusive with just a multimeter.
Your speakers do not have a resistive property that can be measured in OHMs with a multimeter. Their Impedance (givin in OHMs) is an effective resistance to an amplifier at a given frequency. It has nothing to do with the resistance of the wire you're measuring with a multimeter, they're most likely 8 OHM speakers.
You can use most common stereo amplifier chips in bridge mode (both outputs connected together) to drive the 4OHM sub. Just review the datasheet for whatever amp chip you choose for further information, decent chips / manufacturers will provide sample circuits as part of their datasheet. Alternatively you could search example stereo circuits and choose an amp chip based on your findings.
Personally, without knowing your intended goal other than to have a go (which is goal enough in my books) or the properties of your speakers, I would build two amps of the same design. One stereo for the satelites and one in bridge mode for the sub.
Mat

 

[Nigel Goodwin]

Stereo amps are relatively easy to fault find with the right gear, but can be elusive with just a multimeter.

Not really - a multimeter is almost always all you need, the DC nature of modern transistor amps means faults are generally pretty catastrophic - it's fairly rare anything else is required.

 

[cachehiker]

I figure I would find two stereo amplifier chips with a two active crossovers in there. One amp driving the subwoofer in bridged mode and the other driving the satellites in stereo mode. I run my speakers through home stereo receivers, so this is based on the one 2.1 computer speaker system I've seen the inside of. That one was easy, three chips on heatsinks, one regulator and two amps. It was still working so I went no further.

I would be tracing signals back to the satellite amp and seeing if there's an input but no output. A good multimeter will tell you if you've got an AC voltage at the input. Finding a datasheet that tells you which pins to look at will likely be tougher.

 

[audioguru]

The output power of an amplifier depends on its supply voltage.

A TDA2009A stereo amplifier with an 18V supply delivers 7W to 4 ohm speakers or 4W to 8 ohm speakers with fairly low clipping distortion (1%).
With a 24V supply the typical output is 12.5W into 4 ohms or 7W into 8 ohms. The mimimum output is 10W into 4 ohms or 5W into 8 ohms.
A TDA2009A will be overloaded if it tries to drive a 4 ohm speaker with its outputs bridged.

A TDA7396 is a bridged high power amplifier for cars. With a 14.4V supply its output into 4 ohms is 22W.
With a 16V supply its output into 4 ohms is 26W.
Digikey has 4426 of them in stock for $6.32US each.