Why is the audio input from the microphone connected to the inverting input on the TPA6111A2?

Question

As far as I know, when we connect a signal to the inverting terminal of an op amp, we get an amplified output which is 180 deg out of phase from the input. Going through the datasheet of TPA611A2, the input from the microphone goes to the inverting input. However, the LM386 has the input given to the non-inverting input. Can someone help me out regarding this?

The way I'm interpreting this is that giving the microphone input to the inverting terminal will shift the phase of the sound we want to hear by 180 deg, making it a different waveform from the original signal, changing the sound we'll hear altogether.

Answer 1

In some cases it will make no difference, as the microphone signal is an AC signal, and in many cases the inversion does very little, if anything, to the audio (there are cases, like sending differently phased signals to different speakers, can do bad things to sound).

In other cases, it can make a big difference, often depending on the type of microphone. Piezo-based microphones, for example, require an amplifier with a very high input impedance. Generally, non-inverting amps can be build with higher input impedance than inverting amps.

Answer 2

Any sound picked up by a microphone can be assumed to come from some point distant to the microphone (maybe mm to several metres or more). Take a sound at 1 kHz at 10 metres distant to the microphone. 1 kHz has a wavelength in air of around 0.34 metres and this means that between the source of the sound and the microphone diaphragm, the signal has undergone 2*10/0.34 (= 59) phase inversions.

If you moved the source 0.17 metres closer it will have undergone 58 phase inversions.

I'll leave it to the reader to decide if it's at all important to worry about the phase change effects of a mono source on the listener or any amplification stage after a microphone.

Answer 3

(EDIT: Complete rewrite)

The TPA6111A2 is a stereo speaker driver with an inverting input. To use it correctly, the source would need to have negative polarity. The datasheet shows that, labeling the inputs IN1- and IN2-.

The LM386 is a single channel with internal biasing that accepts a positive input.

Neither chip is really a mic amplifier: they don't have enough gain, and a mic input should have some AGC function.

That all said, by convention a mic input should not invert: positive voltage from the mic results in positive voltage at the final driver.

Answer 4

The TPA6111A2 headphones amplifier is not designed for electret microphones as inputs because with enough gain then its input impedance is too low and the external compensation capacitor required will cut high audio frequencies. An LM386 power amplifier works fine with an electret mic since its inputs are a higher impedance than the other inverting amplifier and its frequency response is fine for all audio frequencies. But it produces hiss because it does not have a low noise preamp.

Answer 5

To solve my question, I did a simple experiment on Audacity. I took an audio sample and loaded it on Audacity and I did a phase inversion by 180degrees. Sure enough, as per the answers, there was absolutely no difference in audio quality, proving that phase of an audio sample doesn't change the sound when we're listening to it.