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LM4880: Noise/instability on headphone amplifier design

Adam DeMuri
Prodigy 10 points
Part Number: LM4880
Other Parts Discussed in Thread: TPA6111A2

I'm working on a product which includes a headphone amplifier, using an LM4880M. I'm producing a low-frequency signal using a DAC, which feeds into the LM4880M. I'm seeing significant noise/instability on the output of the amp.

As you can see, I added in-the-loop compensation. The first revision did not have this compensation. Adding the compensation did not reduce the noise. Here's what the noise looks like:

If instead of connecting headphones, I connect a dummy load of 47R before the output capacitor, then the signal is clean.

With the DAC at a fixed output, I still see similar noise in the output. If I put the amp into shutdown, then the output is clean.

  • 0
    TI__Guru** 103185 points

    Hi Adam,

    As part of the debug process, can you please try to use the same dummy 47R load but use a long cable to connect it? I'd like to know if the cable on the headphones could be causing the oscillation.

    If the results points to that, I would suggest to move R27/R29 after the feedback resistors, basically put these in series with the load to isolate the capacitance, also try changing the value of these resistors as they seem to be 2-Ohm?

    Also have you tried to remove feedback capacitors C29/C30?

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    I used 2ft wires when connecting the 47R dummy load, which is about the same length as one of the headphones that I tested with.

    The first revision I did did not have the feedback capacitors or the 2-Ohm isolating resistors. It had the same issue.

  • +1 in reply to Adam DeMuri
    TI__Guru** 103185 points

    Adam,

    Have you tried to reduce either input or output capacitors? This should reduce the low frequency response, loosing bass for better stability.

    You may also try with TPA6111A2 to check if it's more stable.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    This appears to have fixed it, partially by accident.

    I'm using through-hole film capacitors for C9/C11. I replaced them with 0.1uF ceramic disk caps, and the noise went away. I tried adding 1uF electrolytics in parallel, and it was still clean. The problem appears to be with the film caps (part number B32529C0105K189). They don't list an ESR.