Understanding a few audio basics

Hello everyone!

I'm trying to understand and learn the basics of audio "tech". For a simple test project, I wanted to connect an electret mic to an amp to a speaker to see if I can get any output.

For the electret mic preamp circuit, I used this document as the basics: https://www.ti.com/lit/pdf/tidu765

For you conveninece, I pasted the schematics here too.

I have a 0.5W, 8ohm tiny speaker.


  1. when they say it's a 8Ohm speaker, is that 8Ohm DC or at some kind of standard frequency in an AC circuit? (multimeter did show 8 Ohm) So I guess I'm asking if it is an impedance value or not?
  2. same question for the watts. If my circuit is a 5V circuit (op amp out will be oscillating around 2.5V), what kind of resistor value do I need on the output to drive this speaker?
  3. when calculating this resistor value, should I consider the peak-to-peak voltage change of the output signal or should I just go with theoretical maximum of 2.5V?
  4. I'm using an LM358 which seem to have a max output current of 20mA, am I reading the datasheet correctly?
  5. If 20mA value is correct, what is the P of the output (I don't know what V value to consider, peak-to-peak or Vcc)
  6. In any case, it seems that this opAmp cannot drive 0.5W. The speaker doesn't even quack :slight_smile: Am I correct assuming that the cause of that is the insufficient power to it?
  7. If I used to BJT on the output to buffer some extra juice, could that be a solution?

Thanks a lot for your answers and the time you put into them.

Its a very nominal figure - DC resistance is usually about 6 ohms, ac impedance is frequency dependent and will peak at resonance and gradually increase at higher frequencies.

Opamps cannot drive speakers, they normally can drive 2k loads, sometimes as low as 500 ohms. You need a power amplifier for this - perhaps an PAM8302 module?

A power amp does not use an output resistor - that would be very wasteful of power.

Yes, its not a power amp, as I said. Opamps handle signal level (milliwatts)


You'd need a pair of transistors as a push-pull current buffer.

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Yeah, I'm starting to gather what's going on :slight_smile: I need a pre-amp for the mic and then a power amp to drive the speaker.

The module you linked in not easily available where I live. I'll try to put together a class AB amp, see what happens.

There are LOTS of different power amplifier chips and it's a LOT easier to use one of those than to build one from transistors or MOSFETs. (The LM386 is very popular for low-voltage low-power applications but there are many others.)

Or, you find little complete amplifiers or amplifier boards. Or regular "powered" computer speakers will work, etc.

The basic formula for power (Watts) is Voltage x Current but it can also be calculated as Power = Voltage squared/Resistance.

That's RMS voltage which is 0.707 x peak, so with a 5V power supply you can get 1.75V and that's about 0.4W into 8-Ohms. (That's "ideal" assuming no voltage loss across the amplifier.) Obviously, a 4-Ohm speaker doubles the power (if the amplifier can drive the lower impedance load).

With a bridge amplifier you can get 10V peak-to-peak for 4 times the power. And with a bridge amplifier you can eliminate the output capacitor.

Note that the power amplifier doesn't need much voltage gain, depending on how much voltage you're actually getting out of the preamp. That depends on the loudness of the sound, the sensitivity of the microphone, and the gain of the preamp. Generally, you leave some "headroom" so you don't clip (distort) the preamp and then there is usually a volume control pot between the preamp and power amp.