amp load in 2 speaker cab

RXRUM · #1 10-26-2010, 10:28 AM

O.K., I know this will identify me as a novice, which I am, but I want to get better and smarter. If an amp(rated at 300w @ 4ohms) combo has 2x10" speakers (8ohms, 150w rated), am I getting the full 350w or would I have to add another 8ohm cab (lets say 1x15")? To figure the load, am not sure if you count the 2 speakers individually (8ohms/2=4ohms) or they count as one(8ohms), in which case I would not be getting the full 300w from the amp?!? I apologize in advance for my ignorance

perogato02 · #2 10-26-2010, 10:37 AM

It depends on how the speakers are wired.
Could be 150w@8 ohm, or 300@4ohm ...
But most of the time cabs have the rating for the entire cab (so including all speakers), so my guess would be 150@w8ohm.

We are talking about what combo?

bongomania · #3 10-26-2010, 10:37 AM

Depends on the specific amp/speakers, and where you found the specs. If the "8 ohm, 150 W" is printed on the back of each speaker magnet, then that is the data for each speaker, not the total. If the "8 ohm, 150 W" was printed in the manual, then look at how it's worded. Does it say that's for the cab, or for the speakers, or does it say something like "2x" that spec?

OldFender · #4 10-26-2010, 10:50 AM

If your amp is rated at 300w @ 4ohms then yes you can assume that the power output is about 150w @ 8ohms. On most amps the limiting factor for power output with an 8ohm load is the power supply voltage available, so you are getting all the power the amp has to offer. When you go from a 8 to 4ohm cabinet, the amount of current drawn by the speaker doubles, this is why the power rating doubles when you go from 8 to 4 ohms. As the speakers impedance gets lower most amps run out of current that there power supplies can provide safely. This is why most amplifiers do not recomend speaker loads of less than 4ohms.

It sounds like we are dealing with a combo amp with an external speaker output conection. If this is true then you will want to add a 8ohm cabinet, the speakers will be connected in parrallel so the impedance will apear to be 4ohms just as you reasoned. The power amp will then have to provide the extra current because of the lower impedance so the output power will double to 300 watts.

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