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LM1876 Mute/Standby POP

József Hermann
Expert 1010 points
Other Parts Discussed in Thread: LM1876, PCF8574, LM4766

Dear Forum Members,

I am using LM1876 chip conjunction with LM1036 tone/volume IC (with capacitive coupling). Both MUTE and STDBY

functions are utilized. When the '1876 goes to Standby mode or mute activated no any click/pop but if '1876 leaves standby

or mute switched off (output activated) it produces a pop. Its audio inputs are unchanged during these procedures. It should also be noted when supply applied to the amplifier no any pop (on-chip undervoltage protection also works fine), so there is a problem with only logic control and only in "one direction" (activation). Logic control provided by a PCF8574 I/O expander using 2K pullups on MUTE and STDBY lines.

It would be grateful if somebody - who already has experience with this IC - could give me advise to fix the problem.

Thanks vm in advance!

BR,

Joseph

  • 0
    TI__Guru** 102275 points
    Hi Joseph,

    Are you using LM1876 in a single-supply application? In some cases there are pop issues at this kind of supply due to the charging of the capacitors that need to reach half supply or discharge from half supply.
    I also want to remind you that LM1876 and LM1036 are being EOL'd, as they are shown as lifebuy at ti.com
    For LM1876 the replacement part is LM4766. Unfortunately there is no direct replacement part for LM1036.

    Best regards,
    -Ivan Salazar
    Texas Instruments
  • 0 in reply to Ivan Salazar
    Expert 1010 points

    Hi Ivan,

    thanks for the response! I am surprised that both '1036 and '1876 will be discontinued..since they are

    quite professional components.

    I use split supply +/-20V for '1876 and single supply 12V for '1036 (capacitive (foil) coupling between them).

    I do not understand why click occures when MUTE inactivated. I have discovered something:

    Ci capacitor (using Fig5 designator) is electrolythic type in my circuit however signal swings to negative as well!

    I should change it to foil 4,7uF (WIMA MKS). At this time I do not have other idea.

    Best regards,

    József

  • 0 in reply to József Hermann
    Expert 1010 points
    Hi Ivan,

    I would like to inform You, I could solve the problem.
    If you see the Mute atten as function of control pin voltage graph it can be observed
    the device does not have a sharp attenuation charachtersitic (from max gain to full mute ca. 0,2-0,25V changes needed). My idea was to utilize this feature by slowing the Mute control voltage avoiding transient pop (smooth transition).
    To perform this, I did:
    - add 10K series resistor and a common collector NPN transistor between PCF8574 out and 1876 (transistor acts as an unity gain buffer: C to Vcc, E to Mute pin).
    -add 47uF cap between transistor base and GND forming a low pass filter
    -add 2K pullup for PCF8574 output pin. It is for lowering 8574 H level out impedance to equalize Mute On/Off time constant, becasue the 8574 has initailly a very week pullup. Thus, the Mute off (8574 output goes to L with low impedance) transistion delay=10K*47uF=0,47sec, Mute On time (H) constant=(10K+2K)47uF=0,56sec. They have been almost the same.
    Without this 2K the Mute On time would be much longer since 8574 can deliver only 100uA H level current.

    Best regards,
    József