Small pa - impedance question

[greenglaze]

I bought some good condition custom-built speakers at the weekend with Eminence drivers.

There are two 12" cabs with tweeters and two 15" cabs with tweeters.

I want the 15's for my keyboards and I want the 12's as the basis of a small PA - vocals etc.

However if I wanted to run all four cabs as a PA could anyone advise on the following:

The 12's are 4 ohms c400w rms

The 15's are 8 ohms c400w rms

If I wire them up in parallel is the impedance which the amp will run into over 2 ohms? Or under 2?

Is the equation 2/8 + 1/8 = 2.66666 ohms

Or 2/8 + 2/8 + 1/8 + 1/8 = 1.33333 ohms

Also one the 12's will be louder than the 15's as there is less impedance but the same rms. Is it a useless idea then putting the two together?

Any thoughts gratefully appreciated.

[RoadRanger]

Don't run them together.

[dennis a]

 

Is the equation 2/8 + 1/8 = 2.66666 ohms Or 2/8 + 2/8 + 1/8 + 1/8 = 1.33333 ohms

 

1.333

[Craig Vecchione]

The impedance, as noted, is too low for anything you're likely to be using to power them. The imbalance in output volume can be problematic, as well as the fact that the 4 ohm speakers are drawing more power (double) than the 8 ohm, so this may cause you probems if you try to get the 8ohm speakers louder. Plus it makes no sense to try to power 1600w of speaker with one amp, especially since they are two different types.

 

Your best bet would be two 2-channel amps, one that provides 400w per channel into 8 ohms, the other amp one that provides 400w per channel into 4 ohms.

 

Then the only problem will be whether the two different speakers cause phasing or comb-filtering effects when used for FOH together.

[imnotded]

Is there any benefit to use a 4ohm driver?

[agedhorse]

 

Is there any benefit to use a 4ohm driver?

 

 

In specialized applications yes, in general application more limited.

[philw44]

 

Is there any benefit to use a 4ohm driver?

 

 

A lot cheaper to get an amp that can push out the equivalent wattage at 4ohm vs 8ohm.

 

I'm surprised more one a side mid/tops aren't 4ohm rated really. Most modern amps seem to be able to handle 4ohm stereo punishment for tops

[RoadRanger]

I'm surprised more one a side mid/tops aren't 4ohm rated really. Most modern amps seem to be able to handle 4ohm stereo punishment for tops

Peavey has a 4 and 8 ohm version of their Impulse 1012 tops. If you account for the 4 ohm being slightly less efficient, having more power compression thereby, line loss, and that a typical amp only puts out 50% more at 4 ohms than at 8 they are about the same SPL for a given amp . I think I calculated the 4 ohm as 1/2 db louder but don't exactly remember.

[greenglaze]

Thanks for replies. I thought as much. Will keep the two sets of speakers separate.