Author Topic: impedance / oddball loads  (Read 3584 times)

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Offline RxJxVx

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impedance / oddball loads
« on: July 23, 2007, 10:40:24 pm »
i am struggling to understand something, and reaching out for help.  if i am running an 8 ohm cabinet and a 16 ohm cabinet using the same amplifier, what kind of ohm load am i presenting the amp with?

i have been told that running an 8 ohm and a 4 ohm cab from the same head results in a 2.67 ohm load, or something very close to that, but i really dont want to run anything lower than 4 and i am considering rewiring a 4 ohm cabinet in series to make things a bit safer...just not sure how much power i'm going to lose by doing this

Offline JoeArthur

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Re: impedance / oddball loads
« Reply #1 on: July 24, 2007, 07:07:05 am »
i am struggling to understand something, and reaching out for help.  if i am running an 8 ohm cabinet and a 16 ohm cabinet using the same amplifier, what kind of ohm load am i presenting the amp with?

i have been told that running an 8 ohm and a 4 ohm cab from the same head results in a 2.67 ohm load, or something very close to that, but i really dont want to run anything lower than 4 and i am considering rewiring a 4 ohm cabinet in series to make things a bit safer...just not sure how much power i'm going to lose by doing this

First of all, stop worrying about power. Seriously. Even if you lost half the maximum power of your amp, you'd barely notice it. You'd be hard pressed to hear the difference in volume between 100 watts and 150 watts even though we're talking about a 50 watt difference. Power is related to volume exponentially, it is not a linear relationship.

Ok, since you put impedance in the title, Ed won't be answering this. Darn.  :-D

Calculating parallel impedances of two cabs or speakers is very easy as there is a shortcut. It'll be the product divided by the sum. Here is an example:

One 8 ohm and one 16 ohm.

Product = 8 * 16 = 128.
Sum = 8 + 16 = 24.
Product / Sum = 128 / 24 = 5.33

So our total impedance would be 5.33 ohms.

Not enough difference and certainly not enough volume loss compared with 4 ohms to notice.

Offline perato

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Re: impedance / oddball loads
« Reply #2 on: July 24, 2007, 07:31:08 am »
as said above, you can do this. the load is close enough to 4 that you should get away with it fine and i doubt you'd notice any discernible loss in volume. you will, however, get unequal power between the cabs. iirc, the 8 ohm cab will grab the majority of the power while the 16 ohm cab will be left with the rest. not the end of the world, but if i were you, i'd just rewire it so they were the same impedance just to keep things 'normal.'

Offline RxJxVx

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Re: impedance / oddball loads
« Reply #3 on: July 24, 2007, 08:45:24 am »
thanks for the responses, and ESPECIALLY for the formula...

Offline Isaac

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Re: impedance / oddball loads
« Reply #4 on: July 25, 2007, 09:31:36 am »
Right. The 8 ohm cab will get about 2/3 of the power, the 16 ohm cabinet 1/3. How loud they get will depend on their relative sensitivities.
Isaac

Offline JoeArthur

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Re: impedance / oddball loads
« Reply #5 on: July 25, 2007, 12:24:43 pm »
Right. The 8 ohm cab will get about 2/3 of the power, the 16 ohm cabinet 1/3. How loud they get will depend on their relative sensitivities.

I have no sensitivity for my relatives!!  :-D

Offline Soundmasterg

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Re: impedance / oddball loads
« Reply #6 on: July 25, 2007, 01:32:37 pm »
Thats relatively sensitive of you!  :-D

Greg

Offline JoeArthur

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Re: impedance / oddball loads
« Reply #7 on: July 25, 2007, 03:39:25 pm »

Relatively speaking, of course.  :-D