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I've created a little Bluetooth amp board that consists of an OPA2134 op-amp and a TDA7266 chip amp. It works great without issues up until the TDA7266 outputs are greater than ~300 mV. Both TDA7266 outputs clip when more than ~300 mV is pulled with an 8Ω load. The OPA2134 is used to convert the differential output of the BTM875-E Bluetooth module to single ended so that the signals can be used with the TDA7266 inputs. The clipping is not present with the BTM875-E outputs or the OPA2134 outputs, so I do not believe there is anything wrong there. I've even connected the outputs of the OPA2134 to another external chip amplifier (used on a different power supply) and everything worked as expected.

A 12 V, 1.5 A switching wall wart is used for VCC. I also tried a 16.8 V, 4 A switching power supply, and even a linear power supply but the issue still persists with all. Obviously, there is plenty of head room for more output power, indicating that it's not a power supply issue.

All capacitors in the audio path are acrylic film material.

I'm almost convinced that I just have a bad TDA7266. Any help for a solution otherwise is appreciated.

Key components datasheet reference:

OPA2134 output, 1000 Hz at -3dB:

enter image description here

TDA7266 output also 1000 Hz, -3dB (measured with a single-ended probe, I don't have a differential probe so please just use this for visualization):

enter image description here

Project schematic:

enter image description here

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  • \$\begingroup\$ I think I might have an answer, but to verify, what's the measured voltage on TDA mute input? \$\endgroup\$
    – Justme
    Jul 5, 2023 at 4:38
  • \$\begingroup\$ @Justme The voltage on both MUTE and ST-BY pins are 3.3 V when the connected PIO 3 and PIO 7 are pulled high. PIO 3 goes high when the BTM875 powers on and PIO 7 goes high when audio is being played through the BTM875. The BTM875 has been reprogrammed to accommodate this process (ignore the datasheet regarding the volume +/-). \$\endgroup\$
    – goose_ader
    Jul 5, 2023 at 4:44

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According to data sheet, TDA mute pin should have more than 4.1V to be guaranteed to work.

The IC has 3.3V IO so the voltage is not enough and there's a quite high 10k value series resistor which may limit the current and voltage present on mute input.

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  • \$\begingroup\$ Thank you for your answer. However, I don’t understand why this would effect the TDA7266 outputs. My understanding is that if the MUTE pin is above the minimum, then the outputs would be enabled, and if the MUTE pin is not above the minimum, then the outputs would be off (i.e. the MUTE pin is either in an on or off state, not in between). Please correct me if i’m wrong or misunderstanding something. \$\endgroup\$
    – goose_ader
    Jul 5, 2023 at 5:01
  • \$\begingroup\$ Please look at the data sheet curve fig. 13 for MUTE pin voltage vs attenuation, it is not just on and off, and please look at the MUTE pin voltage threshold value which is 2.3V min and 4.1V max, so on your chip, 3.3V can be below the threshold, and it should be well above 4.1V under normal conditions. \$\endgroup\$
    – Justme
    Jul 5, 2023 at 5:42
  • \$\begingroup\$ @goose_ader While you are probably right, you're making an assumption that your TDA7266 is typical. At least test this hypothesis first, by tying MUTE to +6V, as shown in fig. 6 of the datasheet. \$\endgroup\$ Jul 5, 2023 at 5:48
  • \$\begingroup\$ Above minimum does not guarantee it's on. Below minimum guarantees it's off. Above maximum guarantees it's on. Between minimum and maximum, it may be on or off, or since it is not a digital input, it is between mute and unmute with some 10dB attenuation. \$\endgroup\$
    – Justme
    Jul 5, 2023 at 6:05
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    \$\begingroup\$ @Justme I am agreeing with you. I understand all that. Why are you instructing me when I've simply told OP to go test things before making a wild assumption? \$\endgroup\$ Jul 5, 2023 at 6:09

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