Designing an audio amplifier to replace a failed one in a TV

Discussion in 'General Electronics Chat' started by tom66, Sep 9, 2011.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I'm working on repairing a 32" Lexsor LCD TV. This TV has had a rather spectacular failure of the class D audio amplifier; see picture. (The chip has several holes blown in it.)

    The failure is probably due to the fact that the design runs it off 18.5V, when its absolute maximum is 15V. It seems inevitable. It's also possible the supply capacitors were dodgy, causing a loss of regulation on the output, exposing the chip to higher voltages than it is rated for.

    I'm looking to replace the current audio amplifier with a new amplifier, probably a class AB type. I've never designed an audio amplifier before, so I'll need some help.

    Requirements;
    - Two channel stereo amplifier, with independent channels into an 8 ohm load.
    - Output power around 7-9W per channel, maximum.
    - Takes the audio signals coming in from the preamp circuit (believed functional) based around MSP4330. Only needs to be an amplifier.
    - Decent freq response 100 Hz - 18 kHz is fine.
    - Distortion better than 10% at full load. Remember the casing is plastic and the speakers are cheap. It's not worth building a 0.01% distortion amplifier.
    - Operates from 18V supply, drawing no more than 2 amps with full output power.
    - Small enough and low profile enough to fit in 5 cm cabinet, including heatsinks.
    - Can handle the typical noise on the 18V supply (which is switch mode) - approximately 100mVp-p, without producing annoying buzzing or squeaking.
    - Stripboard design okay, PCB also good (I can get PCBs fabbed.) Through hole is easiest. I can design a board around a schematic or even just a chip recommendation. But I will need some clues on how best to do it, e.g. is it okay to route power traces between signal pins... probably not?
    - Low cost to implement as I am on a student budget ;).

    Any help would be much appreciated!:)
     
  2. abc_user

    New Member

    Aug 24, 2011
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    i have an lg tv i had a problem with and going through the schematics i have found a very detailed schematic for the audio amplifier circuit...perhaps that would get u started...
     
  3. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I have a schematic for the audio section. However replacing the chip will be very difficult as it has a thermal pad.
     
  4. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    If you look through the completed projects forum, cumesoftware has quite a few, maybe one of those will fit the space.
     
  5. tom66

    Thread Starter Senior Member

    May 9, 2009
    2,613
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    I've been looking at his designs and they are very good. I might decide to go with one of those if I can make it fit in the required space.
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A class-AB IC amplifier with a supply of only 18V will have an output at clipping of only about 3W into 8 ohms.
    For an output of 8W into 8 ohms then the supply must be about 28V to 30V. Look at trhe datasheet for an LM1875 or TDA2030A.
     
  7. tom66

    Thread Starter Senior Member

    May 9, 2009
    2,613
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    How do car amplifiers manage 10W per channel then? (with hideous distortion of course, but still...)

    I figured peak output would be (18^2) / 8 = 40.5W of course ignoring any losses.

    I *do* have a 24V inverter bus I could tap off, but it might pick up the noise from the inverter circuit.
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    The car amplifiers manage the higher output by "bridging" two amplifiers.
    One is in phase and one is 180° out of phase, the speaker is mounted between both outputs.
    This way the output swing is double in comparison with a single amplifier.

    Bertus
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A bridged car amplifier produces 15W into a 4 ohm speaker or 20W with hideous distortion. Some produce 25W into a 2 ohm speaker or 33W with hideous distortion.

    High power car amplifiers use a DC to DC converter to make a 50V or more power supply for an amplifier.
     
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