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Old     (hogger)      Join Date: Sep 2008       05-26-2011, 8:16 PM Reply   
I have 4 Polk MM651UM speakers that are 3 ohms each.
I am wanting to power them with my Rockford P500-2 amp that''s rated at 125W x 2 @ 4 ohms. 250W x 2 @ 2 ohms, and 500W x 1 @ 4 ohms.
Two speakers are in a double tower speaker can, with one set of out output wire terminals.
Right now the speakers are wired in parallel, which drops them to 1.5 ohms, which I think is too low for the amp, and might cause some problems.

If I wire them in series they will be at 6 ohms. Any idea what kind of power the speakers might see at 6 ohms? And is that safe for the amp?

I was also thinking about wiring them in series inside the cans, then hooking them up in parallel at the amp. Only problem is that it will be one channel at 3 ohms, not sure if the amp can do this.

Why on earth doesn't Polk make speakers with normal impedances.....

Last edited by hogger; 05-26-2011 at 8:21 PM.
Old     (hogger)      Join Date: Sep 2008       05-26-2011, 8:55 PM Reply   
I also have the option of adding a pair of Polk 2 ohm speakers.
I've been playing with some calculations, and think this might be the way to go.
I could run 2 of the 3 ohms and one of the 2 ohms in series giving my 8 ohms per side. Then paralleling them for 4 ohms to one channel. But I would really like to try and keep the left and right separate.
Old     (david_e_m)      Join Date: Jul 2008       05-27-2011, 5:03 AM Reply   
The MM UM product is 4-ohms, not 3-ohms. You may be reading the DC resistance which is very different from the AC impedance. Two in parallel per channel at 2-ohms should be fine for most any contemporary amplifier.
Only the MM (non-UM) series are 2.7 ohm.
You do not want to run speakers of different impedances on the same channel because more current will flow through the one with the lower impedance and that speaker will play louder and also create a power handling inequity.
Btw, The Polk MM651UM is an excellent in-boat speaker but a questionable choice for anyone looking for decent projection from the tower. Its okay as a surf speaker or when at rest.

Earmark Marine
Old     (hogger)      Join Date: Sep 2008       05-27-2011, 10:35 AM Reply   
Thanks again David.
I'm running either 16 or 18 guage wire(its not labeled) about 15' in the 250W x 2 at 2 ohms mode. Is the speaker wire too small?
Old     (david_e_m)      Join Date: Jul 2008       05-27-2011, 12:39 PM Reply   
18 gauge is definitely too light. 16-gauge 4-conductor is minimum. 14/4 would be ideal for your present set-up. As long as you are upgrading the wire then go to 12/4 as this will allow you to upgrade to any HLCDs at a later date without revisiting the tower harness again.

Earmark Marine


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