# Help me understand wattage

Discussion in 'Amps and Cabs [BG]' started by Jettond, May 11, 2011.

1. ### Jettond

Jul 16, 2009
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.

2. ### Selta

Feb 6, 2002
Pacific Northwet
Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
3. ### dbhokie

Nov 1, 2010
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.

4. ### Selta

Feb 6, 2002
Pacific Northwet
Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
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

5. ### Jettond

Jul 16, 2009
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?

6. ### Bassamatickeepin' the beat since the 60'sSupporting Member

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.

7. ### Jettond

Jul 16, 2009
Excellent...thanks guys.

8. ### Selta

Feb 6, 2002
Pacific Northwet
Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
9. ### ackWhy Can't We All Get Along?

Nov 19, 2006
Somewhere near Raleigh
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).

10. ### Jettond

Jul 16, 2009
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.

11. ### Selta

Feb 6, 2002
Pacific Northwet
Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
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).

12. ### dbhokie

Nov 1, 2010
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.

13. ### billfitzmauriceCommercial User

Sep 15, 2004
New Hampshire
Owner, Bill Fitzmaurice Loudspeaker Design
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.
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.

14. ### dbhokie

Nov 1, 2010
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.

15. ### Selta

Feb 6, 2002
Pacific Northwet
Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
Sub audio and full range live production are two totally different animals.

16. ### Selta

Feb 6, 2002
Pacific Northwet
Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
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.

17. ### Downunderwonder

Aug 26, 2009
New Zealand
If I was a moderator hokey boy would be getting a gag order.

18. ### dbhokie

Nov 1, 2010
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.

19. ### Selta

Feb 6, 2002
Pacific Northwet
Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
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.

20. ### aborgman

Jun 12, 2007
Ypsilanti, MI 48197
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.

I might put it at more like 99%.