I might as well try to explain internal ampifier resistance as simple as I possibly can....

Here is what a diagram would look like...

So the Rint is the internal resistance of the amplifier.

This comes from things like such as....

1) Quality of circuit board (gold or silver plated helps)

2) Thickness of power supply wires inside your car

3) Length of power wires inside your car

4) Size of internal amplifier power supply

5) Quality of Mosfet amplifier parts inside amplifier

6) Power disipation ability

There are many other factors but many are well beyond what most of you will care to know about an amplifier.

So lets jump into some calculations....

Expensive amps have a very low internal resistance and they use expensive components and methods such as gold or silver plating to stop the circuit board from oxidizing. (copper is a great conductor but copper oxide is a great resistor....so oxidation of copper will decrease power handling ability)

Cheap amps will use inexpensive components and are designed to simply work at one given resistance level.

Ok lets use two examples....

Amplifier #1 has an internal resistance of 0.1 ohms

Amplifier #2 has an internal resistance of 0.75 ohms

Now you might think that there isn't much difference..... just 0.65 ohms.... but when you do your calculations you will see just how much that difference can mean.

Both Amp#1 & Amp #2 are rated for 100W at 4 ohms

So with a 4 ohm speaker they both put out 20V of signal

If we have 20V accross the speaker we will get a current of 5A

The votage drop accross Rint can now be calculated for each amp...

Amplifier #1 --> Rint = 0.1 ohms

V = I X R = 5 amps X 0.1 ohms = 0.5 V

So in reality amplifier #1 is actually putting out 20.50 V but only 20V is getting to the speaker due to losses associated with internal amplifier resistance.

Amplifier #2 --> Rint =0.75 ohm

V = I X R = 5 amps X 0.75 ohms = 3.75 V

So in reality amplifier #2 is actually putting out 23.75 V but only 20V is getting to the speaker due to losses associated with internal amplifier resistance.

Here is where we see the huge difference..... lets now put two 4 ohm speakers in parallel accross the amplifier terminals.

The amplifier is still going to try and put out 20V but it's seeing 2 ohms so it's going to increase the level of current going out the terminals.

*************************************************

So amplifer #1 is still putting out 20.5V

We now need to calculate the total resistance....

Rtotal = Rint + R speaker = 0.1 ohms + 2.0 ohms

Rtotal = 2.1 ohms

I = V / R = 20.5V / 2.1 ohms

I total =9.76 amps

Now that we are putting out more current the voltage drop accross Rint will be higher.

Vdrop = I total X Rint

Vdrop = 9.76 amps X 0.1 ohms = 0.976 Volts.

So now the amp will be putting out 20.5V and 0.976V will be waisted on internal resistance leaving 19.524 Volts for the speaker.

We can calculate the power into the speakers two ways...

Method 1 -->

Power = V^2 / R =( ( 19.524 )^2) / 2 ohms

Power = 381.19 / 2 ohms

Power = 190.5 Watts

So about 95 Watts will go through each speaker for a total of 190.5 Watts

Method 2 -->

Power = V X I = 19.524 X 9.76 Amps

Power =190.5 Watts

Now if we want to see how much power is waisted internally we can calculate it as follows....

P = I X V = 9.76amps X 0.976 V =9.5 Watts

*******************************************

Here is where amplifier #2 is going to show it's true colors.....

So amplifer #2 is still putting out 23.75 Volts

We now need to calculate the total resistance....

Rtotal = Rint + R speaker = 0.75 ohms + 2.0 ohms

Rtotal = 2.75 ohms

I = V / R = 23.75/ 2.75 ohms

I total =8.636 amps

Now that we are putting out more current the voltage drop accross Rint will be higher.

Vdrop = I total X Rint

Vdrop = 8.636 amps X 0.75 ohms = 6.48 Volts !!!!!!!!!!!!!!

So now the amp will be putting out 23.75 and 6.48V will be waisted on internal resistance leaving just 17.27 Volts for the speaker.

We can calculate the power into the speakers two ways...

Power = V X I = 17.27 X 8.636 Amps

Power = 149 Watts

So about 75 Watts will go through each speaker

Now if we want to see how much power is waisted internally we can calculate it as follows....

P = I X V = 8.636 amps X 6.48 V =56 Watts !!!!!!

*****************************************

So now you can see why the more expensive amplifier will kick the ass of the cheap one when you start adding speakers.

Amp #1 put out 190.5W at 2 ohms with just 9.5 Watts waisted internally.

Amp #2 put out 149W at 2 ohms with a huge 56 Watts of power waisted.

Of course this extra 46Watts gets turned into heat so amplifer #2 will run way hotter than amplifier #1.

*******************************************

Lets take this one step further and calculate at 0.5 ohms ( eight 4 ohm speakers in parallel configuration )

I will jump right to the numbers and skip the calculations....

*************************************************

Amp #1 with 0.1 ohms internal resistance
Rtotal = 0.1 ohms + 0.5 ohms = 0.6 ohms total resistance

Current output = 34.17 amps

Internal voltage drop = 3.417 Volts

Voltage across speaker = 17.08 Volts

Power output to speakers = 583.68 Watts

Power to individual speakers = 73 Watts

Power waisted in Rint = 117 Watts

*************************************************

Amp #2 with 0.75 ohms internal resistance
Rtotal = 0.75 ohms + 0.5 ohms = 1.25 ohms total resistance

Current output = 19.0 amps

Internal voltage drop = 14.25 Volts

Voltage across speaker = 9.50 Volts

Power output to speakers = 180.5 Watts

Power to individual speakers = 22.6 Watts

Power waisted in Rint = 271 Watts

**************************************************

So at the end of the day the more expensive amplifier with less internal resistance was able to pump out 584 Watts into eight speakers while the cheap amplifier with a high internal resistance was only able to push out 181 Watts into eight speakers.

In reallity the second amplifier would overheat very quickly and then shut down until the heat levels dropped.

So hopefully everyone understands the importance of decent amplifiers that are stable to low impedance loads ( such as 0.5 ohms )

The better amplifier managed more than three times the power of the crappy amplifier.

So don't be fooled by these junk amps that claim similar power levels to those of more expensive brands.... they might put out similar power with very little load but when it come to crunch time the cheap amp will die every time.