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Why do watts go up when ohms go down?

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Old 04-12-2010, 11:48 AM   #1
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Why do watts go up when ohms go down?

I'm trying to match an amplifier to a bass bin. I've read everywhere about ohms vs watts and all that but nobody seems to ever answer this basic question.

Let's say I have a speaker that's rated 1400 watts @ 8 ohms (2800 peak) and then I have an amplifier that's rated (peak per channel) 1800 watts @ 8 ohms 2800 watts at 4 ohms. This means that if I plug in one speaker it will run at 8 ohm and push out up to 1800 watts. If I put a second speaker in the chain it will go down to 4 ohm and run at 2800 watts.

The speaker only gives one rating. Does it have a different rating (similar to the amp) if it's only getting 4 ohms? Or is giving it 2800 watts at 4 ohms actually very bad since it's only rated to handle 1400 watts?

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Old 04-12-2010, 06:27 PM   #2
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here's the same question but maybe phrased a little better:

If I have one speaker that's rated at 600w @ 8ohm program and 1200w peak then standard thinking is that I should mate it with an amp with maximum output somewhere between 600 and 1200 watts at 8ohms. Let's use as an example an amp that outputs 900 watts @ 8ohm.

Now if I plug two speakers into the same amp (on the same channel) then the impedance drops to 4 ohm but the output power goes UP to 1800 watts. Does the speaker have a different power handling rating at 4 ohm? Speakers only ever have one specification for this (1200w @ 8 ohm in this case). Is its peak power handling still 1200w regardless of ohms and therefore the amp will now damage the speaker if too much power is sent?
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Old 04-13-2010, 08:22 AM   #3
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Remember that watts is = V x I (volts x amps)...

Ohms = resistance

When resistance becomes low it will draw more amps thus increasing the wattage...

This is probably not the answer you are looking for...
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Old 04-13-2010, 11:14 PM   #4
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no, i think i figured it out.

if two speakers are plugged in to one channel then the ohms drop to 4 and the watts go up to 1800. but those 1800 watts are shared across both speakers. it's not going to each. so each speaker will get 900 watts at 4 ohms. therefor it will not blow up. (this applies to my second example).
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Old 04-15-2010, 12:11 AM   #5
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Originally Posted by mikeryan View Post
no, i think i figured it out.

if two speakers are plugged in to one channel then the ohms drop to 4 and the watts go up to 1800. but those 1800 watts are shared across both speakers. it's not going to each. so each speaker will get 900 watts at 4 ohms. therefor it will not blow up. (this applies to my second example).

That's right. Also, make sure that you do the math correctly. You have to have 2 cabinets with matching impedance, and when you connect them together, the impedance goes down. So if you want to connect 2 speakers to a 4 ohm output, they must be 8 ohm speakers. If you want to connect them to 8 ohms, they must be 16 ohm speakers. Otherwise, if it's just a single speaker, simply match the rating on the back of the speaker with the proper output and you should be ok.
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Old 04-15-2010, 01:04 AM   #6
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Originally Posted by maphi View Post
That's right. Also, make sure that you do the math correctly. You have to have 2 cabinets with matching impedance, and when you connect them together, the impedance goes down. So if you want to connect 2 speakers to a 4 ohm output, they must be 8 ohm speakers. If you want to connect them to 8 ohms, they must be 16 ohm speakers. Otherwise, if it's just a single speaker, simply match the rating on the back of the speaker with the proper output and you should be ok.
if an amp has ratings for 8, 4 and 2 ohms doesn't it go:

connect one 8 ohm speaker and get 8 ohms from the amp?
connect two matching 8 ohm speakers and get 4 ohms from the amp?
connect four matching 8 ohm speakers and get 2 ohms from the amp?
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Old 04-15-2010, 02:03 PM   #7
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Originally Posted by mikeryan View Post
if an amp has ratings for 8, 4 and 2 ohms doesn't it go:

connect one 8 ohm speaker and get 8 ohms from the amp?
connect two matching 8 ohm speakers and get 4 ohms from the amp?
connect four matching 8 ohm speakers and get 2 ohms from the amp?
Yep. But many amps have separate outputs for the different impedance, so just make sure you're plugged into the right one for the speaker configuration you are connecting, otherwise you could blow up the amp and/or the speakers.
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Old 06-15-2010, 10:48 AM   #8
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Quote:
Originally Posted by maphi View Post
That's right. Also, make sure that you do the math correctly. You have to have 2 cabinets with matching impedance, and when you connect them together, the impedance goes down. So if you want to connect 2 speakers to a 4 ohm output, they must be 8 ohm speakers. If you want to connect them to 8 ohms, they must be 16 ohm speakers. Otherwise, if it's just a single speaker, simply match the rating on the back of the speaker with the proper output and you should be ok.
Not exactly true - 2 8 ohm speakers could be connected as a 4 ohm load if wired in parallell, but 16 ohms if it's wired in series. If they were dual voice coil drivers (or if you had 4 speakers), they could be connected as a 2 ohm, 8 ohm, or 32 ohm load. Generally the more voice coils you have, the more you can play with that.
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Old 06-15-2010, 10:52 AM   #9
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Also...

Not all amps will double their power output as you halve the impedance. At BEST it'll double it, and many will give a more conservative 40-75% boost in power. Distortion, however, will double (at least).

Car Audio is where you tend to find amps that'll drive ridiculously low impedance loads. I remember Orion had one line of amps (HCCA) that were rated at like 25 watts per channel x4 when driving 4 ohms, but the amp was stable to like 1 ohm mono at which point it'd push a very conservatively rated 1600 watts.

Last edited by AgentSanchez; 06-15-2010 at 10:57 AM.
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Old 06-15-2010, 03:56 PM   #10
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Do some research on power matching

Impedance matching - Wikipedia, the free encyclopedia
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Old 06-17-2010, 01:48 PM   #11
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Originally Posted by Teflon View Post
I disagree - that link might teach you some of the theory behind it but that theory doesn't have much practical application when it comes time to design or build a system. These links are a bit more appropriate.

Speaker Impedance vs Power Output

Series Parallel Speaker Impedance
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Old 06-17-2010, 07:15 PM   #12
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Sorry. I didn't mean that wiki would give a step by step instruction manual. I meant it would be a good start to learn about power matching and why it matters.
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Old 06-17-2010, 07:34 PM   #13
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Originally Posted by AgentSanchez View Post
Car Audio is where you tend to find amps that'll drive ridiculously low impedance loads. I remember Orion had one line of amps (HCCA) that were rated at like 25 watts per channel x4 when driving 4 ohms, but the amp was stable to like 1 ohm mono at which point it'd push a very conservatively rated 1600 watts.
Those Orion amps were kick ass. You could rig the 2100 to run at 0.5 ohm mono, but it didn't last very long.
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