Help me understand wattage

Jettond · #1 05-11-2011, 08:56 AM

Forgive my ignorance but would someone explain to me how an amp can be rated at 300/500 watts depending on ohms? I dont understand that concept.

Selta · #2 05-11-2011, 08:59 AM

Let me google that for you

dbhokie · #3 05-11-2011, 09:00 AM

In Layman's terms Ohms is how much your equipment resists the current passing through it. So something with higher resistance is going to mean that less of the power gets through to it. Then it gets into wiring parallel and series, etc. Really you just need to check on your head, and see what it is designed to push, then get a speaker cab to match.

Selta · #4 05-11-2011, 09:02 AM

Quote:

Originally Posted by dbhokie

In Layman's terms Ohms is how much your equipment resists the current passing through it. So something with higher resistance is going to mean that less of the power gets through to it. Then it gets into wiring parallel and series, etc. Really you just need to check on your head, and see what it is designed to push, then get a speaker cab to match.

The OP, I believe, wants to actually know the electrical theory behind it. If he reads up on Ohm's Law then he'll get a good dose of what he needs in that regard. My snarky reply above will get him a ton of useful links on google for Ohm's Law

Jettond · #5 05-11-2011, 09:02 AM

Well lets say I buy an amp that is rated 300/500 watts depending on ohms...how do i get the 500? using more speakers?

Bassamatic · #6 05-11-2011, 09:04 AM

An Amp can push 300 watts of power through a cab with an impedance of 8 Ohms. Impedance is the resistance to the current flow.

If the impedance is lower, to 4 Ohms, more current can flow and the amp can deliver more power - in this case 500 Watts. You can get 4 Ohms by using two 8 Ohm speakers - the current has 2 places to go, so it can double, or you can use a single speaker cab rated at 4 Ohms.

Just FYI - Double the Watts is not double the volume to your ears - you can barely hear the difference.

Jettond · #7 05-11-2011, 09:08 AM

Excellent...thanks guys.

Selta · #8 05-11-2011, 09:09 AM

Another good reference for the general amps/ohms etc. is in the F.A.Q.
AMPS FAQ!! Info on OHMS, Allsize RIGS-OverUnderPowerCabs DIY TechTalk-Links

But, basically, yeah, don't worry about soaking every watt out of your head... more speakers will do you more for volume than more wattage will.

ack · #9 05-11-2011, 09:11 AM

Being new to all this myself, this is the way I understand it:

If your amp is rated at 300W @ 4 Ohms / 500W at 2 Ohm's, then:
1. With one 4 Ohm cab (say, a 115), you'll be able to push out 300W to it.
2. With two 4 Ohm cabs (say a 115 & a 210), you'll be running at 2 Ohms (able to push out 500W).

Jettond · #10 05-11-2011, 09:14 AM

The reason I'm asking is that I'm about to purchase a Markbass CMD 102P and its specs read "500W @ 4 ohm / 300W @ 8 ohm" and I wondered how to reduce the resistance to get the 500 watts.

Selta · #11 05-11-2011, 09:17 AM

Again, don't worry about it. First, see how the 300W/2x10 combo does for you volume wise. If it's not enough, then get an 8-ohm extension cab (IMO, a 2x10 would be best, but that's me).

dbhokie · #12 05-11-2011, 09:28 AM

Speakers all have different resistances according to how they were built.

You can get to 4 ohms with two speakers, they would just need to be 2 8 ohm speakers wired in parallel, or 2 2 ohm speakers wired in series. Alternatively you could do it with 1 410 cab with 4 16 ohm speakers wired in parallel.

They were right though, if you really want to know that much and it isn't just a generalized question google it. I would just caution you to not run a speaker combination that takes the resistance lower than the amp specs out.

billfitzmaurice · #13 05-11-2011, 11:15 AM

Quote:

Originally Posted by Jettond

Well lets say I buy an amp that is rated 300/500 watts depending on ohms...how do i get the 500? using more speakers?

Yes, but divest yourself immediately of the notion that watts and output levels (measured in decibels) have a 1:1 relationship. They don't. It's a 10:1 relationship. If you want twice the perceived loudness that 300 watts gives you don't need 600 watts, you need 3,000 watts. I'd venture that 95% of the time inadequate loudness is caused by not having enough speakers, not by inadequate power.

Quote:

Speakers all have different resistances according to how they were built.

Speakers do not have resistance, they have impedance. And there's a very good resource only a mouse click away that explains how impedance works.
AMPS FAQ!! Info on OHMS, Allsize RIGS-OverUnderPowerCabs DIY TechTalk-Links

dbhokie · #14 05-11-2011, 11:19 AM

In my experience with a lot of sub audio, lack of decibels is caused by lack of air movement (which of course can be speakers), but also can be easily achieved by grossly underpowering speakers, causing them to have hardly any excursion.

Resistance, there is still resistance within the voice coils in speakers, they are rated in Ohms they don't possess resistance in the strict rationalist fashion. They have a rated resistance measured in Ohms, I don't understand why everyone here is always so quick to split hairs.

Selta · #15 05-11-2011, 11:23 AM

Quote:

Originally Posted by dbhokie

In my experience with a lot of sub audio, lack of decibels is caused by lack of air movement (which of course can be speakers), but also can be easily achieved by grossly underpowering speakers, causing them to have hardly any excursion.

Sub audio and full range live production are two totally different animals.

Selta · #16 05-11-2011, 11:24 AM

Quote:

Originally Posted by dbhokie

Resistance, there is still resistance within the voice coils in speakers, they are rated in Ohms they don't possess resistance in the strict rationalist fashion. They have a rated resistance measured in Ohms, I don't understand why everyone here is always so quick to split hairs.

Because resistance does not equal impedance, and impedance does not equal resistance. There IS a difference, and when it comes to audio, it's significant.

Downunderwonder · #17 05-11-2011, 11:28 AM

If I was a moderator hokey boy would be getting a gag order.

dbhokie · #18 05-11-2011, 11:29 AM

Well then just gag me for the terrible things I have said. I never said ohms equals resistance. Though it is a measurement of resistance in a circuit. Next time I will be sure to scrutinize every word.

I know that full range audio is quite different, however when it comes to the low ends of bass and the elongated waves, excursion and air movement is quite important.

Selta · #19 05-11-2011, 11:32 AM

Sub audio is also vastly different in that the speaker design is hugely different. Those subs are terribly inefficient in comparison to the typical bass cab driver.

aborgman · #20 05-11-2011, 11:35 AM

Quote:

Originally Posted by billfitzmaurice

Yes, but divest yourself immediately of the notion that watts and output levels (measured in decibels) have a 1:1 relationship. They don't. It's a 10:1 relationship. If you want twice the perceived loudness that 300 watts gives you don't need 600 watts, you need 3,000 watts.

Depends on frequency and some other things.

I'd say the research says that a doubling of perceived loudness generally corresponds to somewhere between 3-10dB.

Quote:

Originally Posted by billfitzmaurice

I'd venture that 95% of the time inadequate loudness is caused by not having enough speakers, not by inadequate power.

I might put it at more like 99%.