I agree with sleeper on this.
Here is some more background info:
Your amp is driven from the 12 Volts your car battery provides. There is a power supply inside the amp that jacks up the 12 V to something in the 30-40ish range. This is done to reduce the electric current that has to be delivered by the amplifier's transistors. When you look at the formula used to calculate electrical power
P(Power[watts]) = U(Voltage[volts]) * I(Current[amps])
You can see that you can either increase voltage or current to produce more power.
However, transistors for high-current (many amps) applications are expensive (or not even available), so the designers of the amp use transistors that can only handle so much current.
This is, btw, the reason why the output of a HU without an external amp is so limited. The HU does have an internal amp (usually not individual transistors, but rather an integrated circuit), but it runs on 12V only, and if it were to deliver more than the peak 4x40something watts it would fry the internal amp, since the integrated circuit amp inside the HU can not handle the higher current.
Generally, the more current you run through any circuit, the more heat it will produce, until - eventually - some component in the circuit is destroywed by over-heating. This is why the availability of high-current transistos is limited; there are just none available that can physically handle very high currents. Anyways, I get carried away here.
So, the more viable way to increase output power is to increase the Voltage supplied to the Transistors of the amp. That is why you have the internal power supply.
Now lets just assume the power supply in your amp delivers 30 Volts to the transistors in the amplifier. It is quite obvious that the maximum VOLTAGE output of one amp channel is 30 Volts as well, since nowhere inside the amp are more than 30 Volts available.
Now we look at another formula, known as "Ohm's law":
R(Resistance[ohms])=U(Voltage[volts]) * I(Current[amps])
or, solved for I:
I = U/R
The impedance of the speaker you connect to the amplifier is also rated in ohms. Impedance and Resistance are closely related in electrical engineering, and for this explanation we can just say they are the same. When you hook up a 4-ohm speaker to the hypothetical 30 Volt amp described above, you will produce a current of (up to) 30V/4Ohm = 7.5 Amperes (see formula above). If we insert this into the power formula first used, you produce 30V * 7.5 Amperes = 225 watts of peak output power.
If you use two speakers in parallel, resulting in only 2 ohms resistance, you would produce
30V/2Ohms = 15 Amps
15Amps*30Volts = 450watts
and with a 1 ohm load
30V/1Ohm = 30Amps
30Amps*30Volts = 900watts
PEAK Power PER CHANNEL.
Now, doesn't that sound great? Hook up 4 speakers an quadruple the output power of your amp? Well... in theory only.
In practice the following will happen: The power supply inside the amp is designed to deliver a specific maximum current of, shall we say, around 12 Amps (the components used inside the power supply are don't really like high currents that much either, and the more current the power supply can handle,m the more expensive it will be).
For the original application with a 4 ohm load, everything is well within design limits, and the amp works just fine (Power supply supplies 7.5 Amps PEAK power, which is well within capacity).
With a 2 ohm load, 15 Amps peak power are drawn from the power supply. Much more heat is generated. Heat increases the resistance in an electrical circuit; in return creating even more heat. At the same time the increased resistance reduces the output VOLTAGE: The power supply will no longer supply the full 30 Volts, but maybe only 20-25 of them. If you follow my original argument above, it is quite clear that the amp no longer produces the peak power of 450 watts we calculated, but it will rather be in the 350ish range in reality. At the same time it will produce MUCH more heat.
With 1 ohm, the scenario is the same, but worse. Eventually so much heat is produced that either some protection circuit shuts off the amp, or if the protection circuit is not fast enough (or not there at all), some internal component is fried.
How low a load you Amp can handle is a design feature of the Amplifier. Some can handle 2 Ohms, some can handle 1. A higher load is never a problem, but obviously you don't use the amplifier to its full potential then. This may be a good idea since the sound quality certainly becomes worse when the amplifier approaches its load limits. Not so important for subwoofer applications, though.
Now, as for parallel vs. serial: This really is a matter of taste and testing. You get the most power out of your amp when you run it near the lowest impedance that it can handle (with a decent amp this should be noted in the manual). Generally most power = most noise, but not neccessarily best sound quality.
Another thing you often hear about is "bridging" the amp, where you use two channels simultaneously to drive a speaker setup. Bridging is generally not possible with the high-power output of a HU directly, but only with an external amplifier with an integrated power supply. The bridged mode doubles the output by adding the Voltage of the two bridged channels.
Basically the output looks like this:
Left+ = +30V
Left - = Ground (0V)
Right + = Ground (0V)
Right - = -30V
When you now connect a speaker to left + and right -, you have a total voltage DIFFERENCE of 60 Volts between the two terminals. Currency (Amps) is not affected that way.
Hope this helps a bit. I am not an electrical engineer, so some of the statements above are pretty general.
Joerg
