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Original question:

LM4755: LM4755 Mute pin functioning

LM4755: Output Noise when connected to external pre-amp through RCA

Richard Ma1
Prodigy 150 points
Part Number: LM4755
Other Parts Discussed in Thread: LM1877

Hi Team,

We found there is output noise from LM4755 when connected to external pre-amp and larger speaker through RCA cable.

Usually LM4755 is connected to Speaker (~2W). My customer is designing a multifunction product, so it has to support RCA output (input resistor of RCA is 47k ohm)  

Please imagine this use case is similar to "mobile phone supports internal speaker and also AUX OUT to external speaker"

Is there any suggestion to solve output noise issue?

Due to input voltage is 9V, only LM4755 and LM1877 can support this design requirement. In order to avoid output pop noise, it requires MUTE function. Therefore LM1877 can't meet. So my second question is : what if LM1877 is better solution than LM4755? will there be pop noise when connected to external speaker? Will there be output noise?

  • 0
    TI__Guru* 84915 points

    Hi, Richard,

    Do you have a capture of the noise issue when there's no audio at the input? Could you provide a circuit schematic, please? This is to verify if there should be an external filter added to reduce the noise level.

    The pop issues are commonly seen in the audio devices due to the common-mode voltage and the inputs capacitors. There should be a charging time that must be respected before apply an input signal. Otherwise, it may result in a pop event. Do you also have a capture of the input capacitor charging level?  I mean, could you monitor the input level when the device is powered up?

    Please let me know if you have questions on this.

    Best regards,
    Luis Fernando Rodríguez S.