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How to drive a sub woofer in addition to satelite speakers

Discussion in 'General Electronics Discussion' started by ioncore, Jan 12, 2012.

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  1. ioncore


    Jan 12, 2012
    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: )

    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.

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The subwoofer receives only the low frequency part of the signal spectrum. Also, since there is only one subwoofer, it should be driven by the sum of both left and right signals.

    That said, I'd build a third amplifier which is driven by the sum of both inpüut signals (a pair of summing resistors should do). After the summing amplifier a low pass filter (at least second order, cut-off frequency approx. 100 Hz) should block the higher frequency signals that carry the stereo information.
    You might use the TDA2009A in a bridge amplifier configuration to boost the output power, see datasheet ( figure 16.

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