"Ohmage" to amps and speakers
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 25th March 2004, 07:52 PM #1 z   diyAudio Member   Join Date: Mar 2004 Location: SW PA "Ohmage" to amps and speakers First post - impressed with the forums I've read, and other forums I've joined. This is one of the best uses of educational technology out there...real information from folks that know what they're talking about. I'm looking to secure a new bass amp head and speaker cabinet set up. The ohm/watts ratio are really throwing me, though. I have a decent mathematical head, but when I think I've figured it out, I purchase what I think I need, and end up blowing a speaker. Don't want to do that again. With that said, what's the relationship between amp and speaker ratings in terms of watts and ohms. Specifically, I'm looking at purchasing an amp head that puts out 300 watts at 4 ohms. I also have a line on a speaker cabinet that will handle 300 watts continuous, but the cabinet is rated at 8 ohms. Today's mathematical question - will I only be able to use half the power from my amp head, or will I get nothing out of speakers with the amp fully cranked. Thanks for your feedback!!!
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Join Date: Dec 2002
Location: Calgary, Alberta
Re: "Ohmage" to amps and speakers

Quote:
 Originally posted by z Today's mathematical question - will I only be able to use half the power from my amp head, or will I get nothing out of speakers with the amp fully cranked. Thanks for your feedback!!!
It is actually not a mathematical question, if you think that V=IR is the way to solve this. It is common to see a power rating into 4 ohms that is more than that into 8 ohms in amplifiers, but it is not a specific relationship and depends on the amp design.

If you really want to know what power your amp will drive into 8 ohms, ask that question of the manufacturer. But do not worry about a bad match between amp and cabinet, the speakers will work quite fine.
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 25th March 2004, 10:22 PM #3 sreten   R.I.P.   Join Date: Nov 2003 Location: Brighton UK Simply put it goes like this for bass amplifiers / cabinets. Speakers should be ~ twice the power of the head at nominal loading. So for for your 300W/4ohm head you need ~ 600w/4ohm speakers. Taking your 300w/8ohm cabinet, it will be fine used on its own with the head as the head will only put out ~ 150W. (probably nearer 200w in reality) For best performance you need two ~ 300w/8ohm cabinets. You can use one or two cabinets, depending on venue, using one cabinet will not cause it to blow. (Or just one 500W to 600W 4ohm cabinet) sreten.

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