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TAS2557: background noise issue : ferrite bead on audio amplifier to speaker output paths

Peter Chen(ICP_FAE)
Intellectual 2480 points
Part Number: TAS2557

amp experts,

customer reported the "background" noise when TAS2557 iorking n receiver mode. 

there no "back ground noise" issue when they try remove the ferrite bead(GREEN) and use 0-ohm instead. 

i do not see ferrite bead on audio amplifier to speaker output paths in TI's EVM.

customer use the ferrite bead is BLM15PD121SN1

questions:

1) is common case using ferrite bead on the audio amplifier to speaker output paths?

2) if yes, what type of bead we recommend(vendor and PN)? 

3) Can TI help to suggest the choke selection of bead (or inductor) before speaker and avoid this noise level issue?

here are the detail reasons/ actions why they need bead cascade and schematics show below

1) Since we found this background noise issue in receiver mode, we tried to measure the noise level at TP1,TP3 and TP2,TP4.

The noise level is around 25 uV at TP1,TP3(TAS2557 output before the FPC), and the noise level at TP2 ,TP4 is around 60uV.(after the FPC)

2) It seems the noise level is normal after the EMI filter and getting worse after the bead (L1,L2) on our speaker FPC.

3) Add L1,L2 bead are required by antenna team to choke metal in antenna keep-out area.

4) When we change L1,L2  to 0 ohm resistors, the noise level decrease to 25uV and it seems that L1,L2 bead on speaker FPC would make the output of TAS2557 unstable and increase the noise level.

5) Because antenna require a choke before speaker, 0ohm resistor is not acceptable.

Can TI help to suggest the choke selection of bead (or inductor) before speaker and avoid this noise level issue?

  • 0
    TI__Guru** 102865 points

    Hi Peter,

    Are L1902 and L1904 ferrite beads as well? If they are, then I think you should change C1913 and C1914 to 1nF, as suggested in the data sheet.

    Data sheet shows some characteristics to consider when selecting output filter components in Table 128. As you can see in the same table, it is mention that adding ferrite bead EMI filter has an impact in THD+N performance.
    On the schematic it seems like there is a ferrite bead filter formed by L1902/C1914 and L1904/C1913, and also L1 and L2, which causes two stages were THD+N performance might be degraded.

    If you need EMI filter and low noise, I think you should change to an LC filter. You must consider the cutoff frequency and the current that will flow through the inductor when selecting the components.

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

  • 0 in reply to Ivan Salazar
    TI__Guru** 102865 points

    Hi Peter,

    Do you have further comments or questions about this application?

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