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Original question:

LM4950TA/NOPB (PTH package) v.s. LM4950TS/NOPB (SMT package)

LM4950: Audio input trimmer design

Liu Yang
Expert 8875 points
Part Number: LM4950

Hi team,

I am trying to use the LM4950 with a 12V power supply with BTL Max output 3W.
I am trying to make a gain trimmer (input volume) by attaching a semi-fixed resistor to the input, but as I look at the datasheet, there is no circuit for attaching a gain trimmer, and the input also serves as a negative feedback filter, I do not know the method to insert a trimmer and suitable trimmer resistance value.
So please give us your advice.

Is it all right to insert 1KΩ VR as shown in the attached circuit diagram?
The input source is the line output of the audio IC ALC898 of the embedded PC. 1.2Vrms output impedance is 75Ω.
If the input impedance of the amplifier is 1KΩ with respect to the output impedance of 75Ω at the input source, the input will be dropped by 7.5%.
However, if it is set to 10KΩ VR, this time it will be close to the 20KΩ of the amplifier input lowpass filter, and it seems that the frequency characteristics and gain ratio of the lowpass filter will also change.
Please tell us how to insert a suitable gain trimmer.

LM4950volume.pdf


Beat Regards,
Tom Liu

  • 0
    TI__Guru* 84915 points

    Hi, Liu,

    Our team will take a look at this and will answer as soon as possible.

    Best regards,
    Luis Fernando Rodríguez S.

  • 0
    TI__Guru** 109395 points

    Hi Liu,

    A couple comments about the schematic:

    • Usually the middle connector of the taper (gain trimmer) is the one connected to the input of the ampilifier, Currently it connects the top and the middle connector so the effect to the trimmer might not be applied to the signal.
    • R4 should be connected from VOUTB to VINB, Currently it is connected to VOUTA. Basically you need to change the connection from the bottom side of R3 to the top side.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators

  • 0 in reply to Ivan Salazar
    Intellectual 400 points

    Hi Ivan,

    Liu posted this on my behalf. I designed this circuit. Let me change from here.

    I am sorry about my mistakes in my circuit design. I corrected those 2 mistakes.

    Then please give me advice about volume insertion for LM4950 cuircuit.

    Please refer to detailed question in the original post Liu posted.

    Best Regards,

    Andy

    LM4950volume20200608.pdf

  • 0 in reply to Andy Oyamada
    TI__Guru** 109395 points

    Hi Andy,

    I see the changes implemented in the schematic, it looks good.

    Regarding the volume control, your current connection seems correct. If you need to change the signal amplitude, placing the audio taper at the input is a common practice.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators

  • 0 in reply to Ivan Salazar
    Intellectual 400 points

    Hi Ivan,

    Thank you for proofing the taper, trimmer, resister position.

    I studied LTSPICE this time and simulated suitable taper resister value and found 10K ohm taper is better than 1K ohm taper.

    1K ohm taper slides feedback magnification ratio too much under the influence of 75 ohm input source impedance.

    And 10k ohm taper keeps magnification as designed and does not affect high pass filter too much.

    Thanks alot.