Impedance in series/parallel

[B.Flat]

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Hi,
I have a simple question. If I wire two 8ohm speakers in parallel, giving 4 ohms, and then I add a third in series what will the total impedance be and is there any draw back in this configuration?
I have three 8ohm 250 watt speakers which I want to use on an amp giving 630 watts at 4 ohms. I want to draw maximuèm power but realise I cannot make these speakers give a 4 ohm load. Or can I add a dummy impedance in some way?
All replies gratefully received.
This is my first post so I hope I have observed all the correct protocols!

[flyfisher]

The basic answer is 12 ohms.

The main disadvantage is that you won't get anywhere near the max power output of your amp (assuming it's rated at 4 ohms).


A better configuration might be two in series (16 ohms) and the other in parallel = 5.33 ohms. (http://www.1728.com/resistrs.htm)

But you won't get an even distribution of power through the speakers. Half the power will be in the single parallel speaker (so 315W) and the other half will be in the two series speakers (so 157.5W each).

You can add a dummy load if you want but that won't really help as you'll just turn some of the amp's power into heat in the dummy load.

Power-wise, I wouldn't worry too much about just using two parallel speakers (giving 500W handling) with a 630W amp, unless you're in the habit of running your amp at 100% full volume - in which case things will probably sound crap before anything breaks.

Edited December 2, 2010 by flyfisher

[B.Flat]

[quote name='flyfisher' post='1044813' date='Dec 2 2010, 11:59 AM']The basic answer is 12 ohms.

The main disadvantage is that you won't get anywhere near the max power output of your amp (assuming it's rated at 4 ohms).


A better configuration might be two in series (16 ohms) and the other in parallel = 5.33 ohms. (http://www.1728.com/resistrs.htm)

But you won't get an even distribution of power through the speakers. Half the power will be in the single parallel speaker (so 315W) and the other half will be in the two series speakers (so 157.5W each).

You can add a dummy load if you want but that won't really help as you'll just turn some of the amp's power into heat in the dummy load.

Power-wise, I wouldn't worry too much about just using two parallel speakers (giving 500W handling) with a 630W amp, unless you're in the habit of running your amp at 100% full volume - in which case things will probably sound crap before anything breaks.[/quote]