I came up with this simple circuit to mix two small audio signals (about 0-400mV) coming from a pair of DAC output channels, and provide an headphone output with the help of the TPA6111A2. I breadboarded the circuit and it worked fine, both with headphones and when feeding a line input.

enter image description here

I started wondering if I could do without the NE5532 opamp, since I used it only to raise the gain, and I came up with this other circuit, adding gain on the headphone amplifier to reach the same levels as the first circuit.

But the results were poor. I noticed (but I didn't measure) increased distortion, but the main issue was that I got a loud oscillation (around 4kHz, but moving a bit down depending on the position of the potentiometers). The oscillation is only there when feeding a line input (I guess about 10K of input impedance), I couldn't really notice it when using headphones. I tried to raise and to lower the two caps on the feedback path (C33 and C34) but that only shifted a little bit the frequency of the oscillation, without really attenuating it.

So I'm wondering, what am I doing wrong?

enter image description here

update: following Raonoke reply below, I added a pair of 100R resistor in place of the two inductors on the output. Now the oscillation is gone when feeding a line input, but when using headphones the level is now much quieter. Should I put the 100R directly on the output of the TPA6111A2, and inside the feedback path, to not limit its drive capability? Also, what is the purpose of R72/R73?

update 2: I placed the 100R resistors on the output pin of the opamp, and inside the feedback loop (like showned here). The oscillation appeared again

update 3: I think I understand now why is oscillating only feeding a line input. When feeding a line input, since the line input is mono, the other side of the output is grounded, and probably this makes the other channel distort/oscillating. I'm wondering if there is a workaround to avoid this, when using mono cables

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    \$\begingroup\$ You are loading the TPA outputs with high value capacitors without any isolation resistor. Remove the output CLC PI-networks and the instability/oscillation should vanish. \$\endgroup\$
    – Raonoke
    Commented Jun 27 at 16:58
  • \$\begingroup\$ You may have a ground loop. If so, solving it depends greatly on circuit paths of ground return currents. You can try various gnd points for that headphone jack. Sometimes when you mount input jacks, output jacks and power jacks to a metal case, the ground-loop is the case itself. \$\endgroup\$
    – glen_geek
    Commented Jun 27 at 17:17
  • \$\begingroup\$ The datasheet you linked to for the TPA6111A2 doesn't indicate that a capacitor should be used in the feedback path - why did you find you needed to add them? \$\endgroup\$ Commented Jun 27 at 18:14
  • \$\begingroup\$ What's the purpose of the π filters anyway? C48/C49 represent ~22nF directly on the outputs when much more than ~100pF is undesirable without at least 100Ω series resistance. \$\endgroup\$ Commented Jun 27 at 18:18
  • \$\begingroup\$ thanks @Raonoke, the oscillation is gone with 100R on the output, but the level is now much quieter. I updated the question with more details. \$\endgroup\$ Commented Jun 27 at 20:45


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