How to Choose an Amplifier.

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 They are often worthless, generally overinflated, selling tools, designed by old-school, low-end amplifier companies, originally made to sell product.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... It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Some of this stuff is a little over my head, I have one Kicker Comp VR sub, 10 in, with 2 ohm dual voice coils, im wiring it in series so it will have a 4 ohm load, what would be a good type of amp for me to run, Mono or 2 channel???

Any stereo amp stable to 2 ohms per channel will work in this configuration. A stero amp wired one voice coil per channel, or, a stero amp bridged, to the two coils wired in series (as you have stated) will still be "seeing" two ohms per channel. You could also (and while I wouldn't do it...) buy a mono amp capable of running a one ohm load, and wire the two voice coils in parallel. Several options here. Use my suggestions from above, and look around for an amplifier that will suit your needs: 1) a stereo amp stable to 2 ohms per channel or 2) a mono amp stable to 1 ohm per channelIt all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

That was exactly the reply I was hoping to get, thanks for your time.

stevdart wrote: haemphyst, valuable info there. I have a couple of questions. What's your opinion of this scenario, regarding #3 Efficiency: I have a USacoustics 4 channel amplifier that I bought last year as a discontinued model through one of the online houses. It's a USC4065 rated at 4 X 65 watts. The brand is "engineered in the USA" and the manufacturing was outsourced to China. The amp came with a nicely presented, well-written manual that specified 2 X 20 amp fuses. (On the contrary, the manual that came with the USacoustics mono amp that I got the year before was very poorly written / presented...and the build was in Taiwan. But both amps perform extremely well.) Using the guidelines you wrote of with an estimation of 50% efficiency, the fuses as described in the manual are right on. 2 X 65 watts out = 130 X 2 (50% efficiency) = 260 watts in; 260 / 14.4 volts = 18 amps.) However, the amplifier arrived with two 30 amp fuses installed! Not getting anywhere by trying to email them for the proper fuse value to use, I just left the 30's in there and chalked it up as "too many cooks spoil the broth" with all the outsourcing to this location and that one to finally arrive with a finished product. My guess was that the workers in the Chinese assembly plant decided to borrow fuses meant for a higher line of amp when they ran out of 20's for this one.

If I had seen something like that issue, I would have likely jumped to the same conclusion. (Sadly, "Made In USA" means little, if anything anymore...) If not the borrowed fuse from the wrong amp, I might have even gone to something as simple as a typo in the manual. 20 and 30 are pretty close to one another on the keyboard, so that also is a possibility. How many times have we been reading a manual or something of that ilk, and wondered if this is what is REALLY trying to be said, or is this a literal translation. I don't know any Chinese or Japanese or Korean numbers or glyphs, and if the manual was actually presented in one of those languages to distribution channels, a simple language barrier could explain this issue. 4ch X 65w X 2 = 520W in. 14.4v X 40A = 576W in. (reasonable, and would fit MY critera well...) 12.6v X 60A = 756W in. (This would also be a reasonable safety margin, IF the power supply were regulated i.e. it maintained the SAME output power, regardless of the input voltage. Lower input voltage would demand more current.) Two 25A fuses would have been a better choice, but 30's were possibly more available? or less expensive?

stevdart wrote: We all know how the industry has turned in the last few years with the outsourcing issue, and my guess is that this type of thing is more commonplace than we might care to think. Can we actually look at the fuse value on a new amp nowadays and use that as criteria for acceptance? Have you seen anything of the like with borrowed or rearranged parts?

I suppose it IS possible, but I think that the US Government powers that be try to make certain that electronics sold here are safe... An (intentional or otherwise) overfuse situation would cerainly not meet the definition of "safe".

stevdart wrote: And the second thing I want to ask: you said "I liked that" re: the "hotter than a firecracker" Nak...does the heat value on the outside of the amp relate to the heat transfer through the case, and so you like that because the heat sink was working well? Or was it because the efficiency was low and therefore more apt to be a better amplifier? ...Or neither?

I was afraid somebody would call me on that statement. Neither. And both. The mass of the heatsink on those amps was (sorry...) massive. Their thermal stability was astounding. The fins were probably smaller than they SHOULD have been, for effective transfer of energy to the atmosphere, and they stayed hot for a long time. The latent heat and the time it lasted, was to me, an indicator of the quality of the heatsink. The sheer amount of heat GENERATED, was to me, an indicator of the very Class "A" biasing... this was just for me, and nobody else ever wondered about the biasing, but they were ready to complain about the heat... Class "A" is a very inefficient amplifier class, and these amps were simply examples of such. As good as my present system sounds, it pales compared to that setup... (I should see if that guy wants to sell 'em back to me...)It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Very well written haemphyst, you've got my vote.Jeff

Velocity Custom Home Theater

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Morden, Manitoba CANADA