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Se o sinal de saída do circuito é lido sem problemas por um osciloscópio, então o problema é o seu telefone. Teste em outros.
You can measure up to 50v. The circuitry reads this voltage and converts it to lower voltage (+-1.8v), because you can't feed high voltages to the phone's mic input.
Audio input via bluetooth would probably have worse quality, because of low sample rate. Also, I'm not sure if the app can read the audio input from BT, or only from the jack.
It will be about one volt less than the voltage you're feeding the op amp. Example: if you feed the op amp with +-15v, your output will be between +-14v.
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