Electronic – OP Amp not amplifying audio signal


Two-stage audio amplifier as implemented

Hi, part of my project requires me to amplify an audio signal coming from an electret condenser microphone which outputs an average of 10mVpp to approximately 2.5Vpp. I Used two cascaded non-inverting TL071 OP-amps since one Op-amp doesn't have enough GBW. The output of the mic is low-pass filtered with a cut-off frequency of around 13kHz.

It works perfectly if I input a signal from the signal generator, the output is as required, however with the signal from the microphone there is no output, just a DC offset which is coupled by the capacitor at the end. I also tried buffering the output form the mic.
Any help would be much appreciated.

Didn't realize the error in the schematic, resistor 1k should be between mic and 5V.

Best Answer

Put a large resistor from the + input of the left opamp to ground. Say, like 470kohm.

The DC bias from the microphone is getting through to the opamp. The TL071 has very high impedance inputs. The tiny bit of DC that leaks through the first capacitor is enough to push that input away from 0V. The amplification then drives the output to one of the rails.

Just noticed something else.

The microphone bias is messed up.

You need that 1k resistor between the microphone and 5V

As you've got it, the microphone is trying to pull the 5V source up and down in response to the sound.

Best case, it doesn't work.

Worst case, you've killed the microphone.