The LM386n-1 I used last week off a 9volt didn't produce much volume from a small 8 ohm speaker. Also tried the same with several piezo discs, still wasn't what I wanted to hear. I used the data sheet circuit, but didn't play with it much.
On the datasheet, it says the LM386 produces only 400mW into 8 ohms at clipping. A factory installed 4 channel car radio produces a total of 60W at clipping into the four 4 ohm speakers with the engine running (charging the battery). The difference in power is a whopping 150 times!
A bridged car amplifier IC like a TDA1554 has two channels and gives 15W per channel at clipping into 4 ohm speakers or about 8W into 8 ohm speakers when the battery is charging at 13.8V. With a 12V battery the power into 4 ohm speakers is 11W or 6W into 8 ohm speakers. With a 9V battery the power into 4 ohm speakers is 6W or 3.5W into 8 ohms. A little 9V battery can't produce that much power.
A big speaker is much more efficient than a little speaker. 1W into a big speaker will make the same loudness as 5W into a small speaker.
I made a stereo bridged amp for the beach powered by 6 AA Ni-Cads (7.2V). It makes 5.6W per channel at clipping into 4 ohm speakers or 3W into 8 ohm speakers. At work I connected it to the two demo speakers instead of the normal 400W amp, and nobody noticed any difference! The demo speakers are extremely good and efficient.
Audioguru pretty well summed it up, but most of the to-220 amps will not work well off of single 9v. About the most i've seen is the 32W TDA2050 for a to-220 package. I used a pair of TDA2030s in a stereo amp I built a while back, connected to my Hi-Fi speakers (90db sens) it produces enough output in a bedroom for films or music and I never put it near to full.
If you want decent output from only 9v (as opposed to 44v like I used with my amp), use the TDA1554 or similar that Audioguru suggested, though I wouldn't guarantee the quality is very good.
EDIT: you posted as I did. You mention 9v batteries. They have low current outputs and will never produce much volume with power amps and will die quackly trying to. Use an LM386 if you use 9v batteries.
Your hearing has a logarithmic resonse to loudness, so you can hear the huge difference between a pin dropped and a jet plane nearby. Twice as much power into a speaker is only slightly louder. 10 times the power into a speaker is just twice as loud.
To avoid distortion, amplifiers shouldn't be used at full power. 400W was for occasional peaks in loudness, the average power was much lower. I played music with the little 5.6W amp that didn't have loud peaks.
If you need to ask this then you don't know enough about electronics to even consider building something like this.
A 9V battery is a single polarity source, also the small 9V batteries used in small transistor devices aren't big enough to give you more than a watt of so of power output.
What the heck is I^2R? Power= I squared, times R.
If you increase the R then it decreases the current and you get less power, not more.
Connect one 8 ohm speaker to an amplifier and it gets a power of so much. Then connect a 2nd speaker to the amplifier if it can drive the lower resistance (impedance) and the power is nearly the same in each speaker so the total power is nearly doubled.
If you increase the supply voltage to an amplifier then you get nearly 4 times the output power because the current also becomes doubled. A bridged amplifier uses two amplifiers as a bridge to nearly double the voltage across the speaker for nearly 4 times the power.
I know it's silly to ask that. Thanks. I'm just wondering if the volume adjuster in our headphones are using pot-resistor. It just look like a pot to me. So i'm wondering if its the resistor which changes the loudness. And i just had a headphone of mine seems to be broken at that part (adjuster) and thinking if i could use any components to survive it. Sorry if this had out of topic. I'm just curious and just trying to learn. Thanks
You can connect an adjustable resistor in series with headphones because the power is very low so the adjustable resistor won't burn like if it fed a speaker.
With the pot shorted, the headphone gets full power, with the pot at max resistance then it limits the current so the headphone gets hardly any current and its power is reduced to nearly nothing.
The pot might get burnt or just wear out.
You can connect an adjustable resistor in series with headphones because the power is very low so the adjustable resistor won't burn like if it fed a speaker.
With the pot shorted, the headphone gets full power, with the pot at max resistance then it limits the current so the headphone gets hardly any current and its power is reduced to nearly nothing.
The pot might get burnt or just wear out.