Question about ohms

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yeahboye

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Is it a problem to output combo amp (at 4 ohms) to 2 12 speaker cab at 8 ohms? Methinks it is, and I wanted to ask before there was a problem.

Any help would be appreciated.

Please don't laugh at me ;) ,

Tom.
 
I am trying to run a mesa boogie nomad 45 2X12 combo into a mesa boogie 2X12 rectifier cab. It say the outputs for external cabs is 4 ohms, but it says "(use with two 8 ohm speakers)".

i want to be ABSOLUTLEY SURE, ya dig :)

Thanks.
 
As a rule using 8 ohm speakers on a 4 ohm designed output stage of most amps will not damage the amp but will reduce the power that your speakers give. However using a lower ohm rating than the amp was designed for forces the output stage of the average amp to work much harder and deliver more power. This will be a problem for the power transistors in the output stage.

Most decent amps have shortcircuit protection and this may activate cutting output if the amp is driven hard into a much lower impedance than designed for. Could be embarressing on stage if this happens :D

just for reference:-

2 X 8 ohm speakers wired in parallel is equal to 1 4 ohm speaker

2 X 4 ohm speakers wired in series is equal to 1 8 ohm speaker

If you have a 4 ohm output stage on the amp and you have 4 X 4 ohm speakers you can wire 2 series pairs in parallel and still have a load of 4 ohms :D cool eh ?



(+amp) -----------4ohmspk-----4ohmspk---------- (-amp)
| |
--------4ohmspk-----4ohmspk-------

= 4ohms (R=(4+4)/2)

cmon guys that pic has got to be worth some rep points :D
 
Rule of thumb... take the rated minimum impedance (ohm rating) of the amp and you can feed it to a speaker cabinet rated at the minimum or higher ohm rating. So an amp rated at a minimu 4 ohms can safely work with a speaker cabinet rated at 4, 8, or 16 ohms. For more details, I would look on Google and look up parallel and serial wiring. Long story short, most cabs are wired in paralell, so the more speakers, the lower the ohm rating of the cabinet, the "harder" the amp must work. Amps are stupid beasts and just keep shoveling out power until they burn up. An ohm rating is a measure of resistance or "how hard" the speaker pushes back against the amp. (Other nerds, don't spin us off into damping... get my gist) So a speaker that pushes back hard (higher ohm rating) keeps the amp from putting out too much power (wattage) and burning itself out. So an amp may say it needs at least 4 ohms of "push back" to keep it from buring up. Make sense?
 
So basically I am in the clear, huh?

Thanks a million guys!

You guys made my day.

Regards to the nerd syndicate ;) ,
Tom.
 
yeahboye said:
Is it a problem to output combo amp (at 4 ohms) to 2 12 speaker cab at 8 ohms? Methinks it is, and I wanted to ask before there was a problem.
Click to expand...

FWIW, two cabs at 8 ohms wired in parallel would be 4 ohms.... :D
 
If the output rating of your amp is 4 ohms, you will be safe using an 8 ohm cab. The worst that will happen is you may loose a little volume, the up side is you should also get a little cleaner sound. The speaker jacks of many amps are rated at 4 ohm min due to the fact that many speaker cabs (most 4X12s for example) are wired at 4 ohms. Cabs with 8 or even 16 ohm ratings will not damage your amp. Running cabs in parallel untill the resistance drops below the amps minimum rating will damage the amp.
 
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