Electronic – Smallest audio amplifier

There are a number of things wrong here

1. There are no component designators in the schematic. This makes it hard to talk about, so I'll be more brief than otherwise.

2. You should use junction dots at all points where nets connect. The four lines crossing left of the transistor is ambiguous.

3. A quick calculation of the bias point not taking base current into account results in 1.3 mA thru the transistor, so your claim of 1.26 mA is plausible. However, this should tell you there is no way you're going to get anywhere near 1 W of output power. Even looking at the transistor as a perfect switch, the collector swing will be from 6.6 to 24 volts, for a swing of 17.4 volts. However all this is at a impedance of 8 kΩ. 17.4 Vpp sine wave is 6.15 V RMS. At the best case power transfer, the load is 8 kΩ so the voltage is half that, or 3.1 V RMS into 8 kΩ, which comes out to 1.2 mW. That's less power (although at a different voltaqe) than the MP3 player is likely putting out.

4. The frequency response is a mess. The gain of the amp only 2.7 at low frequencies. This starts going up 3 dB/octave at about 50 Hz, but the high end is unpredictable because it relies on properties of the transistor you don't know. Then the cap in series with the speaker forms another high pass filter, but this also allows the speaker to load the output heavily probably having a reverse effect for part of the range.

5. I can't even begin to guess what the 12 kΩ resistor accross the speaker is supposed to do. It makes no sense whatsoever.

One obvious problem is that you forgot to power the amp. Perhaps because of the double transistor gain stage, a little signal was making it onto the base of the first transistor. Connect the 9 V supply to pin 6, and put a 100 nF to 1 µF ceramic bypass cap immediately accross the chip.

Depending on the microphone type, it may need a weak resistor to power with the other side grounded. Electret mics work that way. Your circuit should work with a dynamic or piezo mic.

This amp doesn't have a lot of gain, so whatever you get out of the speaker will probably not be that loud. Still, even with a gain of 20 you should be able to hear something in the speaker when you brush your finger over the mic or tap on it.

Are you sure the 9 V supply is really 9 V after powering that speaker? The way you have it set up, you are putting significant DC current thru the speaker. With only 4 Ω, it could be trying to put nearly 1 A thru the speaker. If the supply can't handle that, the voltage will collapse and the amp won't work as intended.