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Original question:

LM3886: Bridged Amplifier with Single Supply Voltage

LM3886: HF Oscillation with single power supply

Julien Leloup
Prodigy 20 points
Part Number: LM3886

Hello,

I built up a LM3886 with single supply voltage as described in the datasheet application (Figure 3. Typical Single Supply Audio Amplifier Application Circuit).

I added a potentiometer below "Ra" to regulate the bias voltage of the input signal.

My intention is to make a bridge amplifier and adjust the offset of both chips with that potentiometer so it becomes nearly 0V.

The problem : the Chips are oscillating at a high frequency (see picture below) as soon as I turn off the mute by bridging it to ground with "S1".

My mute circuit is CM= 68µF in parallel with RM=15k. So by closing the mute switch it should turn on the amp (or in other words turn off the mute) if I got it right.

When i do so there's an oscillation on the output with a remarkable amplitude (as shown in the picture below), on both outputs the same...

https://ibb.co/jH7YGSn

I tried desoldering one chip and just having the behaviour of one isolated, absolutely the same. I tried varying by adding or deleting some filters (stability, zobel, etc. ) as mentioned in the datasheet, with no success.

  • 0
    TI__Mastermind 41730 points

    Hi Julien

       Sorry that the link is blocked by TI policy. Could you please paste your schematic and waveform to this site direcly?

    Julien Leloup said:
    the Chips are oscillating at a high frequency (see picture below) as soon as I turn off the mute by bridging it to ground with "S1".

      After turn off the Mute, is there any input signal from the input side? We could see the frequency of this oscillation first.

      And possibly it's the Ca is causing the oscillation, could consider remove it first.

  • 0 in reply to Shadow He
    Prodigy 20 points

    thanks for your reply.
    i tried different variations of input: sine wave, floating(open) and grounded. The sine wave gets through but is also "infected" with that high oscillation. In the other cases it's always there as well.

    here's the oscillation:

     

    (weird thing, it's unperiodic, maybe capacitor charge/discharge? basic freq seems to be around 1Mhz?)

    here the schematic (i already added 100nF close to supply voltage, didn't change)

  • 0 in reply to Julien Leloup
    Prodigy 20 points

    i tried touching a few points on the pcb to see when the output signal changes, so maybe i can find out where the problem comes from. The oscillation doubles in ampilitude and period time (so half frequency) when i touch the inverting input, just after the feedback resistor Rf1.

  • 0 in reply to Julien Leloup
    Prodigy 20 points

    So I figured out that I didn't have placed the Ri resistor in the feedback to ground, which actually determines with Rf1 the gain of the LM right? This made the oscillation disappear. But when I apply an input signal it doesn't appear on the output. The Chip seems to be out of mute mode though, cause when i close the mute switch (S1) the voltage on the output goes nearly to my supply voltage (+24V). No change if there's an input signal or not.

    The bias voltage at the input is nearly half of my supply (11V). The voltage at pin 7 is more though (13V).

  • 0 in reply to Julien Leloup
    TI__Mastermind 41730 points

    Hi Julien

    Julien Leloup said:
    So I figured out that I didn't have placed the Ri resistor in the feedback to ground,

    Do you mean the R17 resistor in your schematic?

    And in your schematic, the feedback resistor R12 seems in series with a capacitor C16? It combined to a integrator I'm afraid. Do we need to use in this way?

  • 0 in reply to Shadow He
    Prodigy 20 points
    Shadow He said:
    Do you mean the R17 resistor in your schematic?

    yes exactly, and in combination with the capacitor underneath (10µF) it actually made the amp work. So output Signal is now there.

    Shadow He said:
    And in your schematic, the feedback resistor R12 seems in series with a capacitor C16?

    Yes it's also in the application schematic in the datasheet, but I didn't placed it. It reduces gain for HF.

  • 0 in reply to Julien Leloup
    TI__Mastermind 41730 points

    Hi Julien

       How about the output side of your schematic? Is the 4700uF capacitor on your board? Could you please capture this charge up process? After you close S1, and not giving any input signal, what is the waveform at both input and output side?