#1501065 - 29/09/08 08:59 PM
how to pick an amplifer
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chipperoh
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Posted: April 07, 2006 at 11:16 AM - IP Logged
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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 They are often worthless, generally overinflated, selling tools, designed by old-school, low-end amplifier companies, originally made to sell product.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...
Is this still true today... does any one have any info on this subject?
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#1501111 - 29/09/08 10:39 PM
Re: how to pick an amplifer
[Re: chipperoh]
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keep_hope_alive
Snooch to the Nooch
SD I chew on Mike Tyson's ear
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yes, all still true.
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#1502597 - 01/10/08 06:01 PM
Re: how to pick an amplifer
[Re: keep_hope_alive]
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chipperoh
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is impedance the same as ohms?
I'm trying to get this, I don't toldly understand it...
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
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#1502615 - 01/10/08 07:01 PM
Re: how to pick an amplifer
[Re: chipperoh]
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Sadistic_customs
SD Ralph Wiggum
Registered: 15/09/08
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Posted: April 07, 2006 at 11:16 AM - IP Logged
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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.
Is this still true today... does any one have any info on this subject?
yes it is but there are exceptions to the rule like some alpine kenwood excelon and rockfordfosgate ans so on amps they will have a 40 amp fuse in them and will not put out 600 watt but the equivalent of what other amps in it's range will put out. it will put out for sure but at a lower use of power like the excelon x-10d 40 amp and puts out 1200 peak and 600 rms and the x1r amp puts out1200 rms and 2400 peak at 2 ohm. they do not put it out but only put out what the others equivalent out put is. so if they put out 21 volts at 3 amps that's what there equivalent is but what you are saying is true for the most part one the lower and kenwood and and some stuff like pyle amps. so in other words you get what you pay for.
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#1502617 - 01/10/08 07:02 PM
Re: how to pick an amplifer
[Re: chipperoh]
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gibby
gibby
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You would be correct.
Not sure exactly what you are asking or stating in the second part but see if this helps.
http://en.wikipedia.org/wiki/Impedance_matching
The power transfer and impedance matching seems to be what you were asking for more info
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#1502660 - 01/10/08 08:10 PM
Re: how to pick an amplifer
[Re: chipperoh]
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chipperoh
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gibby I was pretty much asking is ohm's and impedance the same thing? This $hit gets so confusing... I want to learn all of this stuff and I get so confused after a while. what you gave me for a website i dont get either, im trying. You got any other info on it?
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
This was out of the first post on this topic, I just didnt understand this part.
Edited by chipperoh (01/10/08 08:12 PM)
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#1502676 - 01/10/08 08:41 PM
Re: how to pick an amplifer
[Re: chipperoh]
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gibby
gibby
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I gotcha. What that's saying is that the amplifiers are stable down to an impedance. For example a 2Ohm stable amplifier is stable with anything at or above a 2Ohm load. For example a speaker this is 2Ohm, 4Ohm, or 8Ohm would all work with the amplifier. As you use speakers which are closer to the designed maximum load of the amplifier (2ohm in this example) then you are able to create the maximum stable output of the amplifier.
Each time double the intended impedance of the amp it typically halves the output. For example at 2Ohms it may have a maximum RMS output of 100 watts. By doubling the impedance (4Ohms) the amplifier effectively has an output of 50 watts. At 8 Ohms it may be 25 watts. Newer designs in the amps make exceptions to this. JL Audio (2 and 4CH) and Alpine PDX came to mind as exceptions.
If you would push this example amplifier to drive a load below 2Ohms such as a 1Ohm speaker would, then risks must be assumed. The amplifier is designed to tolerate aplification of a sound signal within the intended load range. By going lower, distortion will occur as the amplifier is much closer to shorting out. The amp may be able to produce as much power as it had at 2Ohms, but by taking this risk the amplifier will likely short out. Thermodynamic (Heat) problems limit this. Basically if the amp gets too hot it drastically decreases the life of the amp.
This should be a start. Hopefully it will explain enough for that wiki article
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#1503016 - 02/10/08 10:38 AM
Re: how to pick an amplifer
[Re: chipperoh]
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keep_hope_alive
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resistance does NOT equal impedance unless frequency is zero.
resistance is DC, impedance is AC.
I gotcha. What that's saying is that the amplifiers are stable down to an impedance. For example a 2Ohm stable amplifier is stable with anything at or above a 2Ohm load. For example a speaker this is 2Ohm, 4Ohm, or 8Ohm would all work with the amplifier. As you use speakers which are closer to the designed maximum load of the amplifier (2ohm in this example) then you are able to create the maximum stable output of the amplifier.
yes
Each time you double the intended impedance of the amp it typically halves the output. For example at 2Ohms it may have a maximum RMS output of 100 watts. By doubling the impedance (4Ohms) the amplifier effectively has an output of 50 watts. At 8 Ohms it may be 25 watts. Newer designs in the amps make exceptions to this. JL Audio (2 and 4CH) and Alpine PDX came to mind as exceptions.
yes again, because of Ohm's law. some newer amplifiers have intelligent circuitry that "sense" the load.
If you would push this example amplifier to drive a load below 2Ohms such as a 1Ohm speaker would, then risks must be assumed. The amplifier is designed to tolerate amplification of a sound signal within the intended load range. By going lower, distortion will occur as the amplifier is much closer to shorting out. The amp may be able to produce as much power as it had at 2Ohms, but by taking this risk the amplifier will likely short out. Thermodynamic (Heat) problems limit this. Basically if the amp gets too hot it drastically decreases the life of the amp.
the issue is current. as impedance decreases, current increases. the components in the amplifier are rated for a certain number of amps. go over that and you burn them up.
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
This was out of the first post on this topic, I just didn't understand this part.
you can load an amplifier anywhere in its range and get solid performance. I avoid 1 ohm loads but 2-8 ohm are typically fine. it really depends on how it's is wired (bridged vs. stereo vs. monoblock), the amplification design (class AB vs D) and the quality of the amplifier.
THIS may help visualize what impedance is actually doing.
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#1503267 - 02/10/08 01:56 PM
Re: how to pick an amplifer
[Re: chipperoh]
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Sadistic_customs
SD Ralph Wiggum
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i hope this come out better i threw it in word this time.
There are exceptions to the rule because of amps like the kenwood excelon has a 40 amp fuse in it and it put out. The same as another 600 watt amp in it class threw the ken woods speaker terminals. Just because it has 100 amp worth of fuses does not mean it efficient like the excelon. The excelon is a high tech amp that will compete with other amps in it size range because it is so efficient. If you look at a power acoustic amp it may have 100 -200 amps worth of fuses in it and trust me they are crappy. Then look at the kicker zr750.1 has 80 amps worth of fuses does not mean it efficient but it a good proven amp for its power. And the kenwood excelon is putting out almost the same amount of power with half the fuses. Just show the efficiency of amps when compared to each other.
Now I would not compare the kenwood or other to stuff like rock wood ,sound storm, or even pyle these are the amps known for having a rating of 10,000 and having a 40 amp fuse in it. let not get away from ken wood not the lower end ken wood is over rated but puts out good bang for the buck but there power is over rated compared to what they put out.
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#1503346 - 02/10/08 03:21 PM
Re: how to pick an amplifer
[Re: Sadistic_customs]
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keep_hope_alive
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you are still limited by the theoretical maximum efficiency of an amplifier type - 66% for Class AB and 95% for class D.
40A fuses with Class D amplification still limits you to 550W Max (at 95% efficient which is not likely). 500W is a more realistic Max output from the Kenwood mentioned.
Amplifiers are not magic.
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#1503542 - 02/10/08 07:47 PM
Re: how to pick an amplifer
[Re: SuperJay]
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gibby
gibby
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Chipperoh... Jay and KHA are both correct. I was unaware one resistance is DC while impedance is AC. My oversimplification does make that poor assumption.
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#1504090 - 03/10/08 05:03 PM
Re: how to pick an amplifer
[Re: SuperJay]
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chipperoh
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At the resonant frequency of a speaker the impedance will spike. When I run impedance curves on a 4 ohm sub, it will often spike to 45 or even 70 ohms impedance at resonant frequency. This needs to be taken into consideration when matching subs to amps.
Some of our competitors will run sub circuits down to .03 ohms DCR, but at resonant frequency (at which they play a single tone for a few seconds) the impedance will spike to 15 or 20 ohms, thus they can get a 1ohm stable amp to survive at .03 ohms DC resistance.
This is where I get messed up again, what do you mean by this?
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#1504279 - 04/10/08 10:40 AM
Re: how to pick an amplifer
[Re: chipperoh]
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gibby
gibby
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I believe that is referring to a subwoofer playing at its resonant frequency. The resonant frequency is the frequency the enclosure tunes the subwoofer to providing maximum excursion/output. The compatible resonant frequencies the enclosure can be tuned to is dependent on the properties of the transducer. At this frequency the amplifier is wired to 0.3Ohms DC resistance, but the resonance causes the AC impedance to spike to 15-20 Ohms for the few seconds of playback. During this short burp, the amplifier can probably be adjusted to provide close to its maximum output at a determined reasonable distortion level.
http://en.wikipedia.org/wiki/Resonance
http://en.wikipedia.org/wiki/Impedance
http://en.wikipedia.org/wiki/Electrical_resistance
I don't remember if all of this is covered in the car audio cookbook. If not, then you might want to pick up a physics book on electricity and magnetism. That way you can have a more solid backing on the physics than the wiki articles.
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