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LM4766 Mute Function Question

Christopher Cuccia
Prodigy 150 points
Other Parts Discussed in Thread: LM4766

Dear Engineers,

I have a design using the LM4766 Mute function where I am taking the mute current off of a -12V supply vs. -Vee of -25V. I am using the mute function for headphone output. Problem is when I input into the power amp a signal above a certain voltage I am getting heavily distorted output. I am using the mute function to attenuate  the output signal approximately 20DB. It dawned on me that this distortion might be caused because I am drawing current for the mute function from -12V vs. -Vee of -25V. I do not know specifically (only can infer generally) how the mute function works from the drawings. Might switching the current source from -12V to -Vee at -25V solve the problem? I find it hard to believe that the function would be designed so that it wouldn't work with a smooth mute like the graphs imply.  

Note: There are no distortion problems under normal audio play with unattentuated output at high voltage input levels (full volume).

Chris

  • 0
    TI__Guru** 104335 points
    Hi Chris,

    You could try to use the same Vee reference for the mute current. Perhaps the mute is drawing too low current so it is attenuating more than expected.
    The distortion is generated from the signal clipping?
    However is there a chance that you could use a different way to attenuate the signal? The soft mute is intended to eliminate pops and clicks, not attenuation control. Perhaps adjusting the feedback resistor?

    Best regards,
    -Ivan Salazar
    Texas Instruments
  • 0 in reply to Ivan Salazar
    Prodigy 150 points
    Yes, the distortion is heard signal clipping on loud passages. So specifically, when the voltage in is increased, it gets to a point where it starts to clip on loud passages much like the same as you would have if output voltage were to reach above rail voltage. I have tried adjusting the feedback resistor with a potentiometer. You are welcome to call at 646-248 2914. As per the attenuation graph, I am attenuating the output by approximately 20 db.

    Chris