nraponi

Hi
Can anyone clarify for me what is meant when an amps says:-

Continuous output power (watts):
Into 4 ohms stereo @ 12.5vdc (IASCA/USAC) = 18*4
Into 4 ohms stereo @ 14.4 vdc = 75*4

I'm sure this is an obvious noob question but surely the amp gets the 12.5volts as thats what the battery provides, or is it 'stepped up'?

Cheers
Nick

chiark

Nick,

In short, power ratings from a manufacturer mean little. They are a way of comparing an amp's output with another amp, but there's a few things to bear in mind.

First, some acronyms:
THD - total harmonic distortion - a measure of distortion at a given power output
Watts RMS - Watts "root mean square" - a way of measuring "power" in an alternating current signal like audio
IASCA - International Auto Sound Challenge Association - people who organise soundoffs (or did!)

Your car, when the alternator is working properly, will put out around 14-14.5V in its electrical system. You only see 12v when the alternator isn't working, or the car is turned off.

The reason some amps quote such wildly differing figures is because of IASCA rules (as was). Competitions are split on power classes, so you had an under 300W class, and other classes. Using an amp like you have listed, competitors would be able to claim that they only had a 72W amp, whereas in reality it's a 300W amp. Competitors are allowed to have engines running when being scored (or used to be - I think they still can)

This all so much useless information without knowing what the thd rating at that power output is. It could generate a gigawatt of useless distortion...

And besides, doubling the power into a speaker gives you a 3dB increase in output. Due to hearing perception being on logarithmic scale, a 10dB increase is perceived as a doubling of volume... However, more power does give the amp more ability to control speaker movement.

There are two schools of thought with regard to amps: the first says "a watt is a watt, buy cheap" and the second says "amps make a lot of difference to the sound quality".

IMHO, the reality is somewhere in between the two. Just as important is build quality, as a car is a harsh environment electrically and physically, and flexibility.

Get a well built amp from a well respected specialist and you cannot go wrong. Speaker choice makes a comparatively massive difference to sound quality - amps in comparison show small differences in between them providing you stick to the good stuff.

nraponi

Thanks again Nick.

Did you receive the emails I sent you a couple of days ago?