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LM4667 with DC coupled inputs

GerryP
Prodigy 160 points
Other Parts Discussed in Thread: LM4667, LM4671

I am using an LM4667 in an application which drives the LM4667 in single ended mode with the -ve input AC coupled to an opamp drive and the +ve input AC coupled to ground, as per one of the suggested configurations in the datasheet. I'm finding that there is a low level whistling/rasping noise when the amp is idling with no audio input which doesn't seem to be relating to supply decoupling (operating at 5V) and there doesn't seem to be any instability in the driving audio chain. If I DC couple the +ve input, the noise goes away but there's then a large speaker click when the speaker enable input is changed. If I DC couple both inputs, the speaker click goes away but a constant low level whistle is present (maybe about 10kHz). Has anyone experienced these sorts of issues with the LM4667?

 

 

  • 0
    TI__Expert 3350 points

    Gerry,

    This is a known issue with the LM4667.  The LM4667 is a second order sigma-delta class D, which has output tones when no input is present.  The tone frequency and amplitude vaires from part to part, you may have one device with tones in the audio band, and one device that is completely silent.  There are two ways to get rid of the tone, one is to add a DC offset, which you've done.  That method does have the draw back of click and pop due to increased DC offset at the output.  The other method is to introduce low level noise at the input, basically dither the amp's input.

    Traditional PWM class D's do not have this issue.  Take a look at any of the TPA20XX series, or LM4671 or later amplifiers and see if any fit your needs.

    Regards,

    royce