# How can I Improve the quality of an amplifier?

I amplify using LM358 Op-amp and 2 resistors.

I played a song on my mobile. The output of the Mobile is the input of my circuit.

The output of my circuit is connected to 8 ohm speaker.

I hear unclear drums, Low volume and I can not hear the singer.

I tried to change the gain 101 , 50 , 10 and 2.2 but It gives almost the same result.

Is it possible to make the volume higher with not too bad quality?

What about using transistors ( common emitter for example ) is it better that LM358 ?

• Others have talked about the opamp... good stuff. What was the power source? Two polarities or single sided? Circuit picture? The "standard" audio jack will have three lines coming from it, which ones did you use? (And try listening with a little ear bud.) – George Herold Sep 7 '14 at 12:21

I amplify using LM358 Op-amp and 2 resistors.

An LM358 is a really, really bad choice for driving an 8 ohm speaker. Typical output current is 40mA and into an 8 ohm load, the power delivered will be: -

$I^2R = 0.04^2\times 8 = 12.8 mW$

Also, you may have wired up your stereo inputs incorrectly and be cancelling out the mono part of the stereo signal - this tends to cancel most vocals, bass etc. i.e. anything that is centred in the stereo field.

Try using an LM386N-3 - it is capable of outputting 500 mW. You could do it with discrete transistors but I don't feel you are ready for this step just yet.

• Totally agree Andy. Op amps are not designed to deliver a power output. There are lots of small audio amp ICs that are designed to do the job. OP could also take look at the TDA2822 8 pin stereo IC - useful for low voltage supplies in bridge output mode. – JIm Dearden Sep 7 '14 at 11:33

There are two basic factors to consider when working with an amplifier: the gain, and the power.

The gain is basically increasing the voltage of the incoming waveform. The power is the limit to what the amplifier can do.

If you think of the output as a simple resistive circuit you can apply Ohm's Law to get a clue as to what is going on.

Say you have a gain of 1:10 (10x gain - 0.1V in gives 1V out). You provide it with a 0.1V signal, so you expect 1V out, yes? Of course, your supply voltage to the amplifier has to be higher than the expected peak output voltage.

Now you feed that into an 8Ω load. Your current through the load would be ($I=\frac{V}{R}$) 125mA, which is ($P=I²R$) 0.125W.

Now, you increase the gain to 100x, so 0.1V in results in 10V out. As long as your power supply is greater than 10V and the amplifier circuit can work with those kind of voltages, that's fine. Looking at the maths, the current would then of course be 10x greater, so 1.25A, and the power now would be 12.5W. A massive increase in power.

That increase in power is what you have to take note of. Pumping up the gain is all very well, but:

1. You have to have a speaker that is capable of taking the extra current
2. You have to have a power supply that is capable of providing the extra current

If your speaker can't cope with the power it is liable to blow. That means the coil will melt, or the cone will rupture and rip, or both.

If your power supply can't supply the current, then the limited current will cause a drop in voltage at the output. That won't affect the gain, but it will limit the upper voltages the amplifier is able to produce, and that means the output waveform will be clipped resulting in nasty distortion.

In general the main gain portion of an amplifier isn't in the power amplifier stage. That stage is mainly used to provide the extra current needed to drive at higher voltages into lower impedance loads. The main gain is usually provided in a pre-amplifier stage. This is a low-power amplifier with a high gain that drives a high impedance load (the input of the power amplifier) so instead of driving the power amplifier with 0.1V it drives it with 1V or 10V, or whatever, but the currents are very low, so distortion is also low.

You could use a simple op-amp as a pre-amplifier stage to add the gain to the signal separately to the power amplifier stage. They're simple to configure, again with just a couple of resistors, and google brings up many thousands of results for circuits.