From the12volt.com

1. Power. I know, this is where MOST people look first, but it's the very reason you are buying an amp, right? Also knowing Ohm's Law, and knowing that I will almost always load an amplifier with the maximum specified impedance (impedance - 4 ohms or higher, NOT load - 4 ohms or lower), I need to know exactly how much power I am looking at to start with. When loading an amp with a higher impedance, the amp will make less power, so I have to keep this spec in mind the whole time. My speakers are almost always chosen before I decide to start looking for an amplifier, I know the impedance and efficiency numbers when I start. This will help me decide how much power I need or want for that particular driver compliment. I will NEVER look at "Max Ratings". They are often worthless, generally overinflated, selling tools, designed by old-school, low-end amplifier companies, originally made to sell product. (You remember how it went (and still goes today)... This 800 watt amplifier is only 200 dollars, but the Nakamichi 160 watt monoblock is 1300 bucks for the pair. Which were (are) YOU going to buy?) Ahhhh, the power of advertising... Classic Nakamichi, Alpine, Linear Power, Orion, Eclipse, etc., THEY never posted "Maximum Power" numbers, did they? Unfortunately, even the good products of today will state maximum power output, I just disregard the numbers posted.

2. Signal to noise ratio. This one I place a bit more weight on, especially when the amplifier is going to be used in a mid-bass (important), mid-range (more important) or highs (MOST important) application. The higher the better, and there is no exception to this rule for me. Amps with tube stages are typically less capable than their solid state stablemates in this area, but I do make exceptions for tube amps. It is NOT a terribly important spec for a bass amp, so when shopping for a bass amp, don't bother with this number. Dedicated bass amps, BTW, usually offer awful SNR numbers.

3. Efficiency. Here's where you will be able to tell a decent manufacturer from the crap. Let's say you are looking at an 250 watt (RMS) amplifier. The first physical thing I look at, is how big a fuse do they recommend (or what size fuse is in the end of the amp)? I know right now that 250 watts OUT will demand (and I use 50% efficiency to keep it simple) 500 watts in. 500 watts in, divided by the (car running) battery voltage (14.4v) is about 35A. If I look at the end of the amp, and see a 20A fuse, I'm walking away, as there is no way that amp can make 250 REAL watts, continuously. 14.4v times 20A, equals 280 watts in. 250 watts out, divided by 280 watts in, translates to an 89% efficiency. In a class A/B amp, this is an impossible number, and it even stretches the imagination for digital amps, as well. The theoretical MAXIMUM efficiency for class A/B is 66%, (95% for digital) so someone is lying. I don't care WHO the manufacturer is, if these numbers don't add up, I will start looking elsewhere... Now, if all the other specification look good, AND I can get in the vicinity of the RMS power I am looking for, I will buy an amp that's efficiency number might be stretching the truth a bit. My old Nak 160 watt monoblocks had 40A (that's 480 watts in - a 33% efficiency, people!) fuses in each one, and they ran hotter than a firecracker, even loaded at over 8 ohms. I liked that.

4. Terminals. A frequently overlooked part of an amp, this is a very important part of an amp to me. A car is a place frought with vibrations. A cheap or cheezy feeling connector in power or speaker terminals and most ESPECIALLY in the RCA inputs, will often cause problems in the future. Setscrews for power, quality terminal blocks for speaker outputs, and Tiffany style RCA connections (the type attached to the chassis, rather than a block of plastic soldered to the PC board.) The quality of the I/Os can be a direct indicator of the attention to detail paid to the rest of the piece. Translation? Overall Quality.

5. Heat sink. This one is simple. How does it look? I eventually chose Eclipse gear, because it looked like jewelry. Fine finish and appearance. Also, heatsink size and quality can be another indicator of quality of the amplifier as whole. A heavy heatsink will also provide better thermal stability, a nice thing in an amp.

6. Damping Factor. I used to place more emphasis on this spec, but my research recently (over the last few years, really) has, while not proven to me it is a useless spec, has not proven to me it is an extremely important spec. This is why this particular spec is a little further down the list. While I place a bit more emphasis on it for a bass or mid-bass amp, I, admittedly, place less weight here than I used to...

7. Price. Money rarely means much to me in an amp, (much like my computers - "Life's too short to build slow computers." - David Draper) it's something I am going to be keeping for a long time usually, so I will save if necessary to get EXACTLY (or close to) what I am looking for.

These are the things I look for when trying to decide on an amp. OBVIOUSLY, there are going to be VERY few amps on the market that will make me happy across the board, but then some of the physical characteristics listed above can be made to fit me and my desires with a little time, a couple extra bucks, and some careful use of a soldering iron. All of the above things will cost more, but it will ADD more to the finished product as well...

This is a great article, very interesting...only thing that isn't mentioned as far as amplifiers are concerned is something known to audio geeks as...

THD%

Or Total Harmonic Distortion.

Amplifiers are in no means 100% efficient, as mentioned earlier. This doesn't only apply to Power, it also applies to the input and output signal. No amplifier can amplify a signal and have it match the input in scale EXACTLY. Here's where THD comes in.

THD is the percentage of the signal that is distorted, (not matched to the input wave). This is similar to Guitar Distortion, like the distortion pedals. The only difference is that the Guitar Distortion is intentional. THD is not, the lower the THD, the cleaner and more exact signal amplification you have.

You may see THD in spec near the power value of an amp where the impedance is listed. for instance, in a pamphlet of a Sony reciever i own:

80W X 5ch @ 4 Ohms per channel at .01% THD.

.01% THD is almost nothing, (the percentage moves the decimal over two more places.) This means that the output of 1 channel is .0001 Times different than the input signal.

That $20 music box on the shelf at walmart may have specs like 4W X2 ch, 2% THD. Two percent is a BIG difference from .01 percent.


Geekyness aside, when you purchase an amp, consider all these things and also take a look at the THD of an amp, many electronics techs consider this the "Quality" of an amp. The lower this number, the better the amp mirrors the input waves. As this will have an effect on how the amp sounds as a whole, along with the circuitry that is build around the amp. You could say that the lower the THD, the cleaner the sound.

But there are several amp manufacturers out there that do have amps very very very close to perfect check out this http://www.genesiscaraudio.net/amps/ it is a european amp manufacturer that is slowly trickling it's way into the us phenominal product with all amps haveing a THD of 0.1% they are out there you just need to know whats good out there as well as where to look i can direct you to almost anything you what or need or at least lead you in the right direction just PM me

Hmmm. That is interesting. Id like to hear one of these amps for myself lol



Btw, if you want the cleanest sounding amplifier for anything in your home, believe it or not, tubes are the way to go. Many Amps produce THD from the switching in the transistors. Tubes dont "Switch"

Bump