How reliable are the quoted RMS figures on amps?
Because, I am figuring if the speakers I buy are 75 watt RMS, I need at amp with at least that RMS output....right?
I will try to address some of your questions..
1. First of all, if your speakers have 2-3ohm impedance, the amplifier will provide a bit more power compared to 4ohm. In the best case scenario, an amplifier rated for 50watts at 4ohms, could provide twice as much with 2ohms.
2. The standard way of rating amplifier power at is 1%THD with 14.4 volt battery. If your car's voltage hoovers around 13V, then you will see less than rated voltage. And with the engine turned off, even less (maybe 2/3 of the rated). I know there are companies that use a more reasonable voltage or lower THD, but if you do a survey of all amplifiers sold online, you will probably find 14.4V at 1% THD. How do you know if the amplifier rating is done honestly? You don't know unless you measure the amplifier output with an oscilloscope. The other way to confirm the wattage ratings is to check if some third party has done a measurement. I really like pasmag.com and avtozvuk.com reviews because they always measure the RMS wattage. It is my belief that most amplifiers sold today do at least the rated wattage, except for some extremely cheap brands.. which means try to stay away from really cheap brands. Jensen, Boss, etc.
3. You don't have to match the speaker to an amplifier with exactly the same RMS wattage. I honestly don't know how speaker manufacturers come up with RMS ratings of their speakers. I have heard, the RMS ratings are the thermal ratings. However, mechanically speaking, some speakers will not take even half of RMS wattage unless you use a high enough high pass crossover frequency, and others can take twice their rated RMS wattage in stride. In the end, you should not worry too much if you end up powering a 50watt speaker with a 150watt RMS amplifier. Why? Do you play 0dB test tones on your speakers continuously? No. You probably play music, which is dynamic and nothing like synthetic tests. On most music records, peaks are at -8 to -10dB and are short lived. On badly recorded stuff, the peaks can come up to -5dB and again are short lived. The bottom line is that having too much power should not hurt. Having too little power could hurt.
4. Infinity speakers don't take a lot of wattage to get moving, specially considering that they're nominally 2-3ohm speakers. A decent quality 50watt amplifier should be able to drive them. People have recommended Alpine, Boston, Rockford, etc. all of those are fine. My preference is for some of newer Class D amplifiers, such as Alpine MRX & PDX, Kenwood, etc (very small, high efficiency, etc).
