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I am trying to build an audio amplifier which will get input from an electret microphone and music signals from mobile using aux cable. I found a schematic diagram in the Circuit Digest website which amplifies the audio signal from an electret mic. What modifications should I do so that I can add the aux cable input and the LM386 will amplify both the signals simultaneously?

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  • \$\begingroup\$ The mic signal level is about 10mV and the AUX signal level is about 350mV. Then make an attenuator to reduce the 350mV down to 10mV at an output impedance of more than 10k ohms so that it does not reduce the mic signal level much. an lM386 with a 12V supply nd an 8 ohms speaker GETS HOT. When its output power into the speaker is almost clipping at 0.5W then the IC heating is also about 0.5W \$\endgroup\$
    – Audioguru
    Commented Oct 16, 2023 at 13:29

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Mixing in high-level AUX audio along with low-level electret microphone.

Can be done a few different ways:

  • Add two sources in series
  • Apply one source to LM386 "+" input, other source to LM386 "-" input.

The schematics below are incomplete, not showing Zobel RC network, and gain-boosting capacitor.

In series addition, electret, with its 10k bias resistor is high-impedance signal source, loaded down by ~50k LM386 input resistance. Since AUX input is high-level, it needs an attenuator (@Audioguru suggests roughly 35:1). This attenuator adds an equivalent series resistance to the electret that can be made << 10k:

schematic

simulate this circuit – Schematic created using CircuitLab


Differential inputs of LM386 allows the un-unused "+" input to accept the attenuated AUX input. Linear mixing of both microphone and AUX signal occurs inside the input stage of LM386 chip:

schematic

simulate this circuit

It is assumed that AUX_in is an AC signal that has average DC voltage of 0V. If there exists a DC component, add a series coupling capacitor to R3.

Since LM386 gain is very high due to the gain-boosting capacitor between pin #1 and pin #8, grounding becomes vitally important. The +12V source is assumed to be well-regulated. Even so, a capacitor should be added between pin #6 and pin #4 with short leads to prevent oscillations. If the +12V source is not well-regulated, a large electrolytic capacitor should be added to help smooth any variations & noise.

You can adjust the ratio of R2/R3 to balance microphone level with AUX_in level. Perhaps make R2 a variable resistor.
Do be aware that audio feedback between microphone and speaker is a possibility.

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