Speaker Resistance

Skip Ellis · Post by **Skip Ellis** » 24 Oct 2009 11:25 am

Approximately what dc resistance should I read when measuring an 8 ohm speaker? I've got a 15" out of an old Polytone bass amp that I thought about putting in a cabinet and using for an external speaker for my Roland Cube 60 which calls for 8 ohm. I thought the 15 might make the amp a little more suitable for steel.
Thanks!!

Ray McCarthy · Post by **Ray McCarthy** » 24 Oct 2009 11:35 am

I don't believe you can determine the impedance of a speaker by measuring the DC resistance--the impedance would be determined by several factors, including the magnet's properties. If it is a 4-ohm speaker, It will allow the amp to produce a little more power, but it should work OK.

Skip Ellis · Post by **Skip Ellis** » 24 Oct 2009 11:43 am

Thanks, guys!

Greg Cutshaw · Post by **Greg Cutshaw** » 24 Oct 2009 1:32 pm

You can see from this chart that the rated impedance of a speaker is 20% higher than the speaker's measured DC resistance.

Greg

Gary Jones · Post by **Gary Jones** » 24 Oct 2009 1:51 pm

The general rule of thumb is to measure the DC resistance of a speaker (with nothing else attached) and multiply that by 1.3, so an 8 Ohm speaker should measure around 6.15 Ohm dc.
Of course this is only a ballpark figure, but since musical instrument speakers usually only come in 4, 8, or 16 Ohm varieties, that measurement should be enough to tell them apart.

Here is a good article on speaker impedances. http://www.epanorama.net/documents/audi ... dance.html

Gary Jones
Yes, I'm still alive, I just smell funny.