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TPA3122D2 (or other TPA31xx): use power supply center tap as speaker ground?

achoox4
Intellectual 730 points
Other Parts Discussed in Thread: TPA3122D2

Not requiring a center-tapped power supply is a great feature of the TPA3122D2 and related devices.

But if the VCC/2 supply voltage is available anyway (e.g. a -12/0/12V supply), then, since the speaker output voltage is centered around VCC/2 (and in fact "rests", via 50% duty cycle, at VCC/2 when Mute):

Would it be helpful to use it as the ground for single-ended speakers, such that the voltage they see varies from (e.g.) -12 to +12? Or do the external filter Rs and Ls already correct the output voltage from 0to24V to -12to+12V at the speakers?

thanks.

  • 0
    TI__Expert 8505 points

    Hi Achoox4,

     

    Yes, the external LC filter centers the 0 - 24V PWM signal at ground. With a 50% duty cycle signal, the DC average is constant at the center of the swing, thus you can say signal rests at the ground level. The output signal would follow this DC average, so the higher the duty cycle the higher the output goes above the ground, and vice versa for lower duty cycle (like diagram below - output filtering). In a sense, the output is like a virtual ground where the rails are centered, but is ultimately not the real ground. Please let me know if I understood you correctly and if you have any other thoughts. Thank!

     

     

    Regards,

     

    Brian Wang

  • 0 in reply to Brian Wang
    Intellectual 730 points

    Yes, that exactly answers my question - so there is no benefit to using a center ground on the speakers, and with the 0V ground, there is (nearly) 0 potential across the speaker when the input level (at the input cap) is stable at 0V.

    Thank you. Could you have a look at my TPA3122D2 question here: http://e2e.ti.com/support/amplifiers/audio_amplifiers/f/6/t/259090.aspx

    as well? It's really 3 questions in all, of which the one about paralleling outputs is probably least important (though I most depend on advice there, as doing something 'illegal' there might blow the device), while the one about the pop (though I can safely experiment) is most important.

  • 0 in reply to Brian Wang
    Prodigy 210 points

     

    Brian,

    I have some TPA3130 amp chips in a product demo that we've built.  The amps run fine, but distort due to the output quiescent voltage being about 4VDC on each output (VCC is 24V).  As you mention in the above post, I would expect the output idle voltage to be half the rail voltage (VCC/2).  My load is 8 ohms.

    What causes the output quiescent point to be off by so much?  It doesn't appear to be the chip, as we have two cases working approximately the same.

     

    Phil