Another thing to point out is the rough quality of the amp and its powersupply can be seen from the power ratings given. For example:
2 channel amplifier/switchable mono
Continuous power @ 4 ohm: 50W x 2
Continuous power @ 2 ohm: 75W x 2
Max Power Rating: 300 watts
Max power into 4 ohms = 50w. To achieve this assuming the speaker was constant at 4 ohms (not possible) this would mean the current and voltage at the amp output terminals was 14.1v at around 3.5 amps. (14.1Vx3.5A=50w)
If you half the impedance you should get double the current.. so..
14.1x7 ~100w.
Holdon why is the amp rated at only 75w @ 2 ohms? What gives...
Well, the amp is rated at 75@2 ohms, so from this we can work out that to achieve this you would have 12.24V @ 6.12A into a 2 ohm load, which is some way off 14.1V @ 7A you would expect.
The reasons for this are due to resistance within the amp output stage. A high quality amp will have a very low resistance. From what we now know it would infact be possible to work out its internal resistance. so...
Total system resistance = Internal resistance of amp + resistance of speaker. This is now getting onto the Damping Factor of an amp, which is basically a measurement of the control over the speaker the amp has. (high = good). I have just done a few really rounded bits of maths so these wont be that accurate, but it seems that the rated power of that amp is at 16v. From this and the two output power ratings I get an internal resistance of around 0.5-0.6 ohms. (thats pretty poor) From this also as mentioned you can work out the damping factor. Which DF=(Zload/Zsource) Or for 4 ohm driver 4/0.55 =~7(approx)
These may be quite far out from what the manufactuer says. But then it just goes to show even more that this amp is pretty average/low quality at best.
Good amp = low internal resistance = high damping factor = double power into half impedance load.
I can tell you now that this amp probably does not even have a powersupply as such, and it pretty much would run straight off the cars battery. Infact I bet its fully intergrated as well, so if you were to buy this amp it would probably almost be empty inside, it will be rated at higher voltage than is possible to get in a car and in reality I would be surprised to see 35-40 watts from it let alone 50.
Ed