What I need the amp for: 4 speakers and 1 sub.

1. Do I get one super mutli-channel amp to handle the task or get a multiple amp system to divvy up the task? Will it make a difference?

2. I definitely want the amp to have a low and high-pass filter, right?

3. What is the math needed to figure out what wattage amp I need? Overpower or underpower?

4. What speaker specs do I need to take into account and how do they play into math for number 3? The RMS wattage or the peak wattage? How does the Impedance play a factor?

Anything I'm missing?

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http://www.anythingbox.net

 

Here's my $.02.

1. Two amps will be better, all things being equal. With two amps, each one won't have to work as hard. Low frequences take a lot of power, you don't want bass notes affecting your highs. As to whether it'll make a difference, I don't know how good your ears are. Those who go for sound quality will have multiple amps because they can hear a difference.
2. Amps with low and high pass filters come in handy. "Good" speakers--good separates anyway--will have their own passive crossovers to do the frequency filtering. All things being equal, I'd get the amp with the crossovers. (You may as well filter out the sub frequencies before the sound gets to your front speakers. It's less work for your amp, and your crossovers will have to do that much less work. Check the specs on the fronts though. Some may reccomend that you give the speakers a full signal.)
3. Math? Wattage? I'd base my amp decision on what the speakers call for. Some speakers like lots of power. Some people like really loud sound. If I'm putting in speakers that suggest they receive 30-200 watts RMS, I'd go for something in the middle. When in doubt, overpower. Getting a bigger amp will allow it to work easier. The saying is that "more speakers are blow from receiving a crappy under-powered signal than from being over-powered." I don't know if that's true, but if you're maxing out your amp, and it starts clipping, that's terrible on the speakers. Look for a good name brand amp (Alpine, PPI, JL, etc. (hint, if it costs more, it's probably better)) and watch for RMS power. Cheaper brands will probably say "Max. 200 watts!!!" or "Peak 500 watts!", and they'll probably only reach those watts if they're struck by lightning.
4. Speaker specs? Listen to a few pair and see which sound the best to you. Don't pay for more than you need. That said, you get what you pay for.
Impedance--most amps are rated pushing a 4 ohm load. Most separates are 4 ohm speakers, although there are some good 3 ohm speakers out there too. Subs vary. Both 2 and 4 ohms is common.
Generally, lower resistance speakers will allow a good amp to push more power. A four channel amp rated at 50 watts x 4 at four ohms may push 65 watts x 4 at three ohms. Not all amps can handle lower resistance, however.

Find a reputable store and a salesperson you feel comfortable with & ask a lot of questions. Don't let them push you into something that doesn't feel right.

Good luck.

Let me know how it goes.

 

1. Depends on your needs. You'll have a harder time finding a 5 channel amp with, say, a 1200 watt sub channel. If you're looking for a more conservative system, a 5 channel amp can work for you.

2. If you have a crossover elsewhere (say, an EQ), then you don't need them in your amp. Most amps have them though included "free of charge", they certainly don't hurt.

3. I prefer to overpower when it makes sense to. Distortion and clipping will likely kill a speaker more quickly than running it a little hot. I don't know if there's really a lot of math involved here, it depends on the subs you go with, price and space you want to consume. That's kind of like asking us "what is the formula for determining how much horsepower I should shop for in a new car?" It depends on your tastes. If you're looking to power a single 10 because you want to add some low end extension to improve the sound quality of your classical music, 200W may be sufficient for you. If you want to go booming hip hop music with a pair of 15"s, you're going to want lots and lots of wattage.

4. Impedance is the big one you have to watch. If you get it wrong, could either overload or under-utilize your amp (unless ou have something like some of the new JL amps which can apparently deal with different impedances). Many of the big sub amps tend to want to see 1 ohm, and you usually need DVC subs to get there. If you've got a pair of 8 ohm subs, running them in parallel gives you 4 ohms, which is what most bridged 2 channel amps like to see.

Give us a better idea of what you're looking to do and we can start to steer you in the right direction.

 

since you guys says distortion and clipping can destroy speakers i have a question, when i had my install done i was listening to my music pretty loud but the volume on my headunit was very low, i was wondering how can it get so loud at low volumes, i was listening to it this way for like maybe 15-45 minutes, and when i got home to check, my amp gain was maxed out! i was like wtf, did this short period of listening to my music loud ruin my speakers?

Of course i lowered the gain down when i found out, my speakers still seem to work, i dont really have sensitive audiophile ears so i cant really tell a difference...

 

Go for quality, not quantity in wattage. To increase sound by 3db, you have to multiply the amp wattage by 10. So there's only a 3db difference between 50 watts and 500 watts. Most amps nowadays are way over-rated. I recommend JL Audio amps right now for 1st rate quality. You need 6 channels total (doesn't matter how many amps), 2 of which should be bridged into one for your subwoofer, which should be no less than 4ohms since the bridged channel will bring the subwoofer down to 2ohms. Most speakers blow from being underpowered, rather than over-powered. But remember quality is what you want to keep in mind. Hope this helps...

 

Go for quality, not quantity in wattage. To increase sound by 3db, you have to multiply the amp wattage by 10. So there's only a 3db difference between 50 watts and 500 watts. Most amps nowadays are way over-rated. I recommend JL Audio amps right now for 1st rate quality. You need 6 channels total (doesn't matter how many amps), 2 of which should be bridged into one for your subwoofer, which should be no less than 4ohms since the bridged channel will bring the subwoofer down to 2ohms. Most speakers blow from being underpowered, rather than over-powered. But remember quality is what you want to keep in mind. Hope this helps...

 

Anonymous said "to increase sound by 3db, you have to multiply the amp wattage by 10."

I think he's remembering wrong. To up the volume by 3db you have to double the power.

Also, bridging an amp doesn't do anything to the sub resistance. The sub is set, at 2ohms, 4ohms, or whatever. Bridging will give you more power, but that doesn't have anything to do with speaker resistance.

 

Wrong on both counts. You have to multiply the wattage by 10 or double your speakers. The resistance DOES in fact change on a bridged channel, it is actually cut in half.

 

Wrong on both counts. You have to multiply the wattage by 10 or double your speakers. The resistance DOES in fact change on a bridged channel, it is actually cut in half.

 

Sorry, but you're wrong.
I'll quote from the FAQ at Rec.Audio.Car:
"3.5.4 Does bridging an amp would halve the impedance of the speakers?
----------------------------------------------------------------------

Impedance is a characteristic of the speakers. The speakers don't give
a flip how the amp is configured: they have a given impedance curve,
and that's that. It should be clear that when you bridge an amp, you
are changing *the amp*. The speaker's impedance is *not* a function of
the amp, but the amp's tolerance to a given impedance depends
completely on the way the amp is configured. If you'll remember from
section 4, an amp bridged into a given impedance draws twice as much
current as it would if it were driving two separate channels, each at
that impedance. So, a four ohm speaker stays a four ohm speaker, if
it's hooked to one channel, a bridged channel, a toaster, or the wall
socket. But, it is more stressful for the amp to drive any impedance
bridged than unbridged.

So, why do people talk about the impedance halving? Well, it's a
simple model that isn't correct but is easy to explain to people who
don't know what's really going on. It goes like this: When you bridge
the amp, each channel is "seeing" half the load presented to the amp.
So, if you bridge an amp to 4 ohms, each channel "sees" 2 ohms.
Therefore, each channel puts out twice as much power, and the combined
output is quadruple a single channel at 4 ohms.

Why is that still wrong? Because each channel isn't really used as a
single channel. You've used part of one channel, and an inverted part
of another channel to create a totally new channel, the bridged
channel. Also, there's no way for a channel to "see" only part of a
circuit. If it's "seeing" half the speaker, it's "seeing" it all.

Second, it makes it awkward if people believe that the impedance is
really, literally, changing. If you use that model, is it safe to run
a 4 ohm mono stable amp into a 4 ohm speaker? It should be, but we
just said the impedance halves, so that's now a 2 ohm speaker, and you
can't use it. That's wrong, and confusing, and it makes people think
they can't do things they really can."

 

And again, quoting from the Rec.Audio.Car FAQ:
"`dB' is for "decibel", and is a measurement for power ratios. To
measure dB, you must always measure with respect to something else.
The formula for determining these ratios is P=10^(dB/10), which can be
rewritten as dB=10log(P). For example, to gain 3dB of output compared
to your current output, you must change your current power by a factor
of 10^(3/10) = 10^0.3 = 2.00 (that is, double your power). The other
way around, if you triple your power (say, from 20W to 60W) and want to
know the corresponding change in dB, it is dB=10log(60/20)=4.77 (that
is, an increase of 4.77dB). If you know your logarithms, you know that
a negative number simply inverts your answer, so that 3dB corresponding
to double power is the same as -3dB corresponding to half power. There
are several other dB formulas; for instance, the voltage measurement is
dB=20log(V). For example, a doubling of voltage produces 20log2 =
6.0dB more output, which makes sense since power is proportional to the
square of voltage, so a doubling in voltage produces a quadrupling in
power."

I'm not sure how Anonymous got confused, but confused he is.

 

Bun beat me to it... 50 to 500 watt would be 10dB. 3dB as Bun said is double the power. (so 50 to 100watts)

 

WOW... thanks for all the replies. But I don't think I'm ready to buy amps on my own just yet. At least now I have a general idea of what to look for at an audio store.

What exactly is the difference between distortion and clipping? Or is clipping a form of distortion?
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http://www.anythingbox.net

 

Sound wave signals are supposed to look like nice, smooth, round waves.(Think back to Trigonometry and picture Sine waves.) When you push the power beyond what the head unit can produce, or the amp can power, the sound waves stop looking round. The tops are chopped off, effectively.
Speakers aren't built to handle that "clipped" shape.
The speaker is being told to function, mechanically, in a way that damages it.

That's my understanding.

 

Sorry, I didn't read your whole question!

Distortion, to me, is a general term. You could have a bad signal (distortion) due to any number of causes (poor FM reception, a bad MP3 recording, alternator whine, etc.)

Clipping is a power-related "distortion."