Electrical – How to increase the output power for this audio amplifier

If the signal measured directly from the microphone is about 15mV and the measured at the output of the amplifier is about 350mV, then the good news is that you circuit actually works. Its gain is 350mV/15mV = 23, not bad!

Another good thing about your circuit is that you really need a simple amplifier to amplify the tiny microphone signal to something usable for a regular power amplifier. You are definitely on the right track.

The bad news however is that the speaker you are using is by far too heavy load for such a simple circuit. The amp just can't drive enough power into it and hence it will sound so softly that you will not hear it.

Try a pair of headphones instead, you probably have some more luck for several reasons:

• higher impedance, many headphones have 32 ohm or higher impedance. This is already easier to drive, but probably still a bit low. If you connect the two outer most pins of the headphone socket, both of its little speakers will be in series, resulting in both being driven and load halved.
• higher efficiency, less power required to make some sound;
• closer to your ear, so less power required to hear the sound.

What you need to drive your speaker is power amplifier.

The easiest for you to verify your microphone preamp is to test with a pair of active PC amplifiers, the kind with a power amplifier in them. You can try connecting the microphone directly and through your preamp and you should notice a huge difference.

Internet is full of simple little power amplifier designs, even with discrete components, that are great as beginner's projects.

This amplifier will not be able to drive large 8 ohm speakers very loud. I conclude this from the two 5 ohm resistors present at the emitters of the output transistors. The amplifier will be sufficient to drive a small speaker like the ones most people use with their PC.

It will be able to drive three 8 ohm speakers in parallel but it will not be able to drive these very loud. Most power will be lost through the two 5 ohm resistors !

But why build your own amplifier from discrete components ? If you want the learning expercience then EXCELLENT :-) go ahead and build. If you want something a bit easier, there are ready-to-use modules on ebay like this class D type or this TDA2030 based one. These can only drive one speaker per amplifier but they are cheap. Also, these don't require the +/- 15 V but only a single 5 or 12 V supply.

Oh, and volume control is done like this:

This uses an LM386 audio amplifier, you can just replace it with the amplifier circuit from your question. Use a LOGARITHMIC potmeter (marked in red) to get nice volume control. For fun temporarily replace it with a standard LINEAIR potmeter and notice how the volume control is "crazy" ! :-) Why is that ? Because our ears are also sort of "logarthmic".