Lately i have noticed that there are several brands on the market that advertise amps to have lets say a 300w rms, but the max power rating is 3x more then this rateing at 900w. I thought that the rms rating was generally half of the max power, so why is the products being advertised in such a way ?

 

A good rule of thumb to use when looking at an amp, is its fuse rating. If an amp says it is 500 rms and it has 20 amps of fusing, multiply the fusing by 12 volts. 20 amps x 12 volts = 240 watts. So this amp wouldnt push what it they say it can. But if it had 50 amps of fusing. 50 amps x 12 volts = 600 watts, so this amp would be a much better buy. But generally an amp will do 1.5-2 times max power what its continous rated power is.

 

hey thanks for all the in site, i did not know that a amps power rating could be figured out from its fuse rating. Now i know it would never be a very good idea, but lets say if i was to replace a 20A fuse with a 25A fuse, would that mean i would increase the power rating on my amp?

 

lol that would work if power came from the fuse lol. no man that wont increase the power rating. fuses tell you how much current the amp should draw.

 

Consider that for a sine wave the peak instantaneous power is 2x the average power. (or so called RMS power)

Now as far as 'maximum' power, in my experience this generally refers to burst power output where the amp is cycled on and off... most amps will produce more 'maximum' power then they can continuously; and this may better represent the amount of power the amplifier can produce on a musical transient...

Of course a manufacturer could call maximum power whatever they want and it may be a rating with the amp seeing 18V, a reduced load, or any other test conditions they choose.....



It sounds to me like MTX is advertising maximum instananeous power on their aformentioned heatsinks under who knows what test conditions......

 

fusing is there to protect the amp in case of a fault, a larger fuse size would allow more fault current probably damaging the amp.

And would putting in a larger fuse allow it to make more power... my knee jerk response is NO NO NO ... but if it is a true input fuse and not a stage (multiple fuses may protect different stages of the amplification) fuse the you could put out more power as it slowly destroys itself.
bottom line- you put a larger fuse in the amp than the manufacturer put there you are asking for trouble. Yuli and Haunz has a good point, increasing fuse size does not give you more transistor output devices, it will also not help you sink more heat... therefore it shouldn't necessisarily increase your power, and if it does happen to allow more power through... you wont enjoy it long

 

I just did a little Goggle'ing and found this on JBL's website... http://www.jbl.com/car/product_support/AUTOMEDIA_CEA2006.pdf

This might help explain how some manufacturers rate their amplifiers.

 

When doing this simple calculation, do not forget to factor in the amplifiers efficiency.

From the sample above, 50 amps X 12 volts equals 600 watts. Just remember that the 600 watts is a combination of the wattage the amplifier is capable as well as the heat output of the amplifier.

Since most class D amplifers are approx 80% efficient we can estimate that the amp above would produce about 480 watts. A class AB amp with approx 50% effieiency would give us an amplifier that would produce approx 300 watts.