Driving a real speaker - what do I need?

I've been building up my knowledge towards this, starting with comperators (understanding signal generation), pre-amps, power-amps, resistor networks etc...

Right now, I'm at a state, where I have an power amplifier stage where the output I can characterize as

  • 4V peak-to-peak swing
  • 125mA output

Something like this:

The speakers themselves are very lame, the sound quality is not important, what's important is that I learn how to drive them.


And yet, if I connect things the way I showed you, cap->speaker->ground, nothing happens.


  1. What am I missing? Is there some piece of the schematics that is implied, customary to connect but is usually omitted when creating circuit diagrams?
  2. If not.. well, any pointer is appreciated why the speaker won't...speak.


The schematic is incomplete, and you forgot to mention whether a signal is applied to the base of the transistor.

The best way to drive a speaker is to use an amplifier designed for the purpose. The design process is not at all trivial. Sparkfun, Adafruit, etc. all have very inexpensive audio amplifier modules designed for use with Arduino and other MCUs.

Indeed, the rest of the schematic does contain all the previous steps I think I need.

Signal to base is coming from either a

  1. electret mic with a 20mV peak to peak signal -> LM358 pre-amp
  2. or a signal generator I built @ 440 Hz

So post the rest of it. You forgot to specify a value for the emitter resistor, and depending on the emitter resistor value and input signal voltage, you probably need a base resistor.

Something like this will work with an Arduino (R3 and C2 are not necessary)


There is nothing to it. The circuits work, there is nothing more to do.

Why it didn't work for me was because of a faulty transistor. Open collector-emitter. YAY!

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