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Foolish question incoming


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I'm really unfamiliar with gauging ohms of resistance on a cab to what sort of amp head I should use. So now that I'm looking at an Ashdown ABM 500 EVO II head, which can run at 4 or 8 like many heads, and have my hypothetical choice of two sister cabs that are only different in their resistances (4 or 8 ohms), I really have no idea which would be the better choice. I would only be running the one cab, I think, not two cabs. I know that has something to do with it, because you reduce the resistance that way, or some such. What's my best choice here, and why? Is it an issue of sound, or speaker health?


Sorry! I've never internalized this information well enough. Maybe this experience will make it more "real" to me and it will stick.

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I would only be running the one cab, I think, not two cabs.


That I think is pretty important.


You can run a single 4 ohm cab and get more power. I don't really think you'll find much tone difference between a 4 ohm and 8 ohm cabinet, they just eat the power differently.


Multiple cabs add up like this:


2 @ 4 ohm cabs = 2 ohm load

2 @ 8 ohm cabs = 4 ohm load

4 ohm cab + 8 ohm cab = 2.66 ohm load


If your amp can only run down to 4 ohms (safely) you'll need to either stick with 1 cabinet or get the 8 ohm cab now.


Safe bet = 8 ohm cab

Better use of power = 4 ohm cab

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The total resistance of a set of resistors in parallel is found by adding up the reciprocals of the resistance values, and then taking the reciprocal of the total:


1 / R = 1 / R1 + 1 / R2 + 1 / R3 +...


ie for NUTS excellent example

1/R = 1/8 + 1/4 = 3/8

Therefore, R = 8/3 = 2.66 ohms




"We will make you bob your head whether you want to or not". - David Sisk
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Very cool. Thanks for the helpful formula. Now, is it possible to run two speakers "in series" into an amp, if it has two inputs in the back? Or is that still parallel, the same as if you ran the speakers in a chain?


Additionally... looking closer at that aforementioned Ashdown 575 W head, I can't find anything about the power it runs at per ohms. Is it possible it runs at the same power regardless (4 or 8 only; it is not rated safe for 2), or is it just as simple as cut 575 in half to get 262.5 W, if you were running the thing into a single 8 ohm cab?


Thanks again for all of this sage advice. This aspiring young bass player is grateful.

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If you are using standard cables, you are running the cabs in parallel, no matter where you plug them in. You can run the cabs in series, but it will require specially wired cables. One of the guys on this forum did that a few years back (Edendude).


As to the power, it's not as easy as "half". The specs need to be provided by the manufacturer.




Acoustic Color


Be practical as well as generous in your ideals. Keep your eyes on the stars and keep your feet on the ground. - Theodore Roosevelt

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Simply, if you know your only going to use one cab, the 4ohm option will be better because you will maximize the power and headroom potential of your amp.


Most amps will put out more power with lower loads. Look at the specs on most amps, they'll usually (not always) show power output at 8 and 4 (and sometimes 2)ohm. ie. 1400 watts @ 4 ohms / 800 watts @ 8 ohms. Note, you never want to use a load that is lower than the manufacturer says the amp can go.


The only drawback to having one 4 ohm cab is that you will not be able to hook up a second cab if you need it. That will lower the load seen by your amp, most amps cannot go below 4 ohm though there are some that can. So if you find yourself in a situation where you would need to move more air and need a second cab, you will have to get two 8 ohm cabs.


Hope this helps.

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