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LM4673 capacitors and resistors for certain speakers

Other Parts Discussed in Thread: LM4673

Hello,

We are using the LM4673, with 0.1uF capacitor and 76.8K resistor on each of the IN -/+ from several different speaker types while testing. The speakers are 6 and 8 Ohms. We are looking for a solution to drive the gain down to eliminate static and crackling from the output sound. Here are spec links to those parts:

http://www.digikey.com/product-search/en?vendor=0&keywords=2403+260+00132

http://www.digikey.com/product-search/en?vendor=0&keywords=2403+260+00177

http://www.seltech-international.com/en/Products/Acoustics/Speakers-Receivers/Knowles-micro-size-speakers/542-11x15x3-DONAU-SLIM-IPx8-waterrpoof.html

Using the attached PDF file as a reference, we would like assistance in understanding the equations in "Proper Selection of External Components" on page 12 to determine possible capacitors and / or resistors to use as a solution.lm4673.pdf

Thanks,

  • Hi Anthony,

    Welcome to E2E and thank you for your interest in our products!

    You can set the gain with the input resistor following the equation (2): AV = 2 * 150 kΩ / Ri (V/V)
    It is recommended to set it to 2 V/V for better THD+N performance.
    For example you can use Ri = 150kΩ for Av = 2 V/V. If you need even lower gain you can set Ri = 300kΩ to get Av = 1 V/V.
    Then you can use equation (3) fc = 1 / (2πRi*Ci) (Hz) to calculate the input capacitor. If you use Ri = 150kΩ and Ci = 0.1uF you will have a fc = 10.61 Hz
    You can also use the following equation to set Ri and fc to calculate the Ci: Ci = 1 / (2πRi*fc)
    I hope this helps.

    Best regards,
    Ivan Salazar
    Texas Instruments
  • Hi Ivan,

    Would you be able to help explain how these numbers relate to Figure 1 on page 2? When trying to lower the gain as you suggested, do the two resistors (Ri) to each of the -/+IN need to individually equal 300kΩ, or 150kΩ each to achieve an overall resistance of 300?

    I'm attaching a screen of the current amplifier schematic for your reference. Is it possible to understand what the current gain is based on this, so that we can determine how to lower it? As in, is Ri = 76.8kΩ, or = 153.6kΩ? Thanks in advance for your suggestion.

    Thanks,

  • Hi Anthony,

    Ri from equation (2) is each one of the input resistors from Fig. 1.

    As an example, if you need Av = 2 V/V you'll need Ri = 150 kOhm so you'll need two 150 kOhm resistors, one for each input. The circuit should look like the following picture.

    Both input resistors must be the same value since it is a differential input.

    I hope this helps.

    Best regards,

    Ivan Salazar

    Texas Instruments

  • Hi Ivan,

    So if I'm understanding correctly, with the 76.8 kOhm resistor we have now, the current gain we have is ~3.9V/V? So in order to lower the gain, we should use either 150 kOhm for 2V/V, or for more filtering for smaller speakers, 300 kOhm for V/V?

    Thanks,

  • Hi Anthony,

    That's right. The bigger the Ri the lower the gain you'll get.

    Best regards,
    Ivan Salazar
    Texas Instruments