In previous experiments I've made circuits to transfer sound from electret microphone to a powered speaker. Now I'm doing my first experience with an Op-Amp to make the sound louder and to be able to hear it in a headphone. The Op-Amp I'm using is LM358N. Here is my circuit:
I'm using the formula below, that I copied from the Internet for the non-inverting operation amplifier:
$$Gain = 1 + \frac{R2}{R1}$$
Given the resistor values of R1= 4.7 ohm , R2 = 150 ohm, my gain is about 32.9 times.
So here is the summary of information about my circuit:
- V+ = 5v from USB
- Capacitor : 0.1uF
- R next to V+ : 4.7 Kohm
- R1= 4.7 ohm
- R2 = 150 ohm
But my speaker doesn't work when the op-amp IC is placed on the circuit. If the op-amp IC is inserted on it's socket, I can hear only "rett, rett" when I try to blow to the electret microphone. What is wrong here?
Best Answer
Your problem may be that the LM358 is not designed to drive a speaker as a load. You will need to add an output stage with a low output impedance of ~4 to 32 ohms or less, depending on your speaker's impedance. Here is an example of a typical configuration using BJTs as the output stage:
Note how the feedback loop has been modified to incorporate the output stage. A trimpot is used in this schematic to adjust the volume.
My second hypothesis, is in the event that you have a high impedance set of headphones (around 600 ohms), is that your gain is too high and your are railing out. If that's the case, then you can either reduce your gain or replace your gain resistors with a trimpot and try adjusting the volume until it sounds better. If the "rett rett" sound you described is a loud sound, then I would recommend skipping my first suggestion and try this first instead.