how do you tell what a good amp is from numbers

[fatboyracing 10+ year member]

Ok I buy amps bassed off of power to ohms for mono blocks never really did it any other way but now I am going to buy a new 4 ch and 1 ch what do the numbers in amp ratings tell you how do you decided which amp is beter

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[dman4486 5,000+ posts]

Ok I buy amps bassed off of power to ohms for mono blocks never really did it any other way but now I am going to buy a new 4 ch and 1 ch what do the numbers in amp ratings tell you how do you decided which amp is beter

is this a serious question

 

[Rich B 5,000+ posts]

The only sure way to not get screwed is to buy a name brand with a history of not overrating their amps.

Rockford Fosgate, Zapco, Arc Audio, and some others come to mind.

Thats the easy answer.

 

[fatboyracing 10+ year member]

Yes, I guess I should have wrote this diff. understand what ohms are and thd, but not slew rate or s/n ratio ect

I know that the lower the thd the beter but thats about it.

 

[DJ Nitrous 427 10+ year member]

http://www.the12volt.com/installbay/forum_posts.asp~TID~75836~PN~1

I have seen several (ok, MANY) posts here lately, asking about how to choose an amplifier, with all sorts of "Is this good...?" types of questions.
Here is what I look at when choosing an amplifier, and in their order of preference:

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 They are often worthless, generally overinflated, selling tools, designed by old-school, low-end amplifier companies, originally made to sell product.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 bux 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 bux, 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...

 

[Enellz 10+ year member]

^^^^^I agree....That has to be the best write up on How to buy an amp Ive ever read

 

[helotaxi 5,000+ posts]

Most of the specs are garbage. Slew rate only needs to be adequate. Never seen an amp where it wasn't. With s/n, higher is better, but without a standard (which there isn't) it's pretty meaningless. Some give the s/n at full power where the signal is the highest which gives you an artificially high number. A more telling number is max gain and s/n compared to one watt output which actually tells you how bad the noise is. Damping factor only needs to be sufficient which is pretty much into the double digits. THD below 1% is generally accepted as below audibility with music. The last thing that matters is frequency response. This is the one where just a spec doesn't tell you much. A simple +/- spec doesn't tell you a whole lot without actually seeing where the peaks and dips are. A smaller tolerance is better here because if the deviation from flat is small enough, it isn't going to be audible. Slightly higher devs with a peak in the lower octaves and a slight roll off on top is what characterizes what most people call a "warm" sounding amp as an example.

Once the basic specs are adequate, power is really all that matters.

 

[Immacomputer 10+ year member]

Yea, most specs given by manufacturers are pretty useless for trying to decide which is better. The most important factor is power and that it will do its rated power. CEA compliance is something that can now help in that aspect. The other things to consider are the processing options available and how they fit your goals for your system. Build quality is another important factor as you don't want to keep buying them. The hard part about that is that there is no spec sheet that can tell you the build quality.

 

[Seth_3515 10+ year member]

i call for sticky on that

 

[helotaxi 5,000+ posts]

CEA compliance is kinda sketchy as well since there is no enforcement on "compliance." You have to take the manufacturer at their word. If we could take them at their word we really wouldn't need CEA 2006, now would we?

 

[Immacomputer 10+ year member]

CEA compliance is kinda sketchy as well since there is no enforcement on "compliance." You have to take the manufacturer at their word. If we could take them at their word we really wouldn't need CEA 2006, now would we?

Good point.

 

[simpsonbuck 10+ year member]

http://www.the12volt.com/installbay/forum_posts.asp~TID~75836~PN~1

Nice post. I'm sure that will answer many questions.

 

[snb778 5,000+ posts]

so what would you guys say about these ratings, not knowing what brand it is. I really really want it, but don't know if its a solid amp.... subs don't have the best rep, but its in great condition, and only 170 bucks!

4-Channel High Current Power Amplifier

RMS Power Rating:

160 watts x 4 chan. @ 4 ohms

290 watts x 4 chan. @ 2 ohms

490 watts x 4 chan. @ 1 ohm

500 watts x 2 chan. @ 4 ohms bridged

825 watts x 2 chan. @ 2 ohms bridged

960 watts x 2 chan. @ 1 ohm bridged

Hour Meter

Remote Mount Digital Voltmeter Included

Bass Remote Included

Clear Acrylic ANL Fuse Holder and 80 Amp Fuse Included

Heavy Duty Heat Sink

Beveled Clear Acrylic Protective Cover

1 Pair RCA Outputs

Tiffany-Style Input and Output RCA Jacks

High Speed Switching Power Supply

Double-Sided Glass Epoxy Circuit Board

Variable Low-Pass Filter: 50 - 250 Hz

Variable High-Pass Filter: 50 - 250 Hz

Variable Subsonic Filter: 15 - 60 Hz

Variable Bass Boost @ 45 Hz: 0 - 18 dB

Input Sensitivity: 200 mV - 8 V

Input Impedance: 10,000 ohms

4-Way Protection Circuitry: Thermal, Short-Circuit, Overload, and DC Offset

On-Board Hook Up Terminals

Frequency Response: 5 Hz - 50 kHz

Dimensions: 19-3/4" L x 9" W x 2-5/8" H

1-year Sonic Electronix warranty

 

[smd4life 5,000+ posts]

you can't

 

[Immacomputer 10+ year member]

so what would you guys say about these ratings, not knowing what brand it is. I really really want it, but don't know if its a solid amp.... subs don't have the best rep, but its in great condition, and only 170 bucks!

Did you not read the thread about how the specs on paper don't really mean much besides power ratings? Brand integrity is far more important than the numbers on paper as long as the amp fits into your power requirements.

Hour meter? It must be that alphasonik amp.