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Old 01-06-2014, 05:19 PM   #1
ChrisBlock
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2 cabs, different ohms

Hey guys. I just bought myself some gear and i got two sweet gallien kruger cabs. The one problem is that the 4x10 is in 2 ohms and the 1x15 is in 4 ohms. At what ohm should i put my head in order to make everything work together and not cause any damage ?

Question 2 : My cabs added up together have a power handling of 1200 watts (800 for the 4x10 and 400 for the 1x15). So this means i could use a 1000 watt bass head without damaging anything, right ?
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Old 01-06-2014, 07:21 PM   #2
saberrocks
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As for question one, Ohms are still a mystery to me.
As for question two, I'm using a 2000 watt head on a 500 watt cab, so it should work fine.
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Old 01-06-2014, 08:52 PM   #3
Ziphoblat
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Eh, combining 2 ohms (an odd impedance for a 4x10, are you sure that's correct?) with 4 ohms will create a decimal 1.something load (I'm not sure exactly what it would be). Mixing like that isn't a good idea at the best of times, especially not when you're wanting to run lower than any conventional power amp is designed to. Your best bet would be to simply buy a second power amp to drive the 2 ohm cabinet and run your amp into the 4 ohm cab.
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Old 01-06-2014, 09:03 PM   #4
dark Mass
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IIRC almost all GK's 4x10s are 4Ω.
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Old 01-07-2014, 02:06 PM   #5
Phil Starr
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Sorry this all sounds a bit negative but here is the information you need.

First of all you need to check the impedance of your cabs, someone has probably been messing with the 4x10. It is quite possible that it is two ohms if someone has wired all the speakers in series and they are 8 ohm drivers. they should be rewired to the original series/parallel and the cab will then be 8 ohms. It is equally possible that the person who sold them to you has given you duff information.

Most amps aren't happy pushing full power into 2 ohms, the current draw is too high. That's even true of most of the amps that say they are happy into 2 ohms. If they don't say they are suitable for 2 ohms then the cab will either cause them to shut down when you turn the volume up or the amp will break. If you plug the second cab in then you will present the amp with a 1.3ohm load which no amp will work with for very long. You could wire the speakers in series to get a 6ohm load but this will be quieter than using either cab alone and will almost certainly sound worse, so why would you? In any case you'd need a tech to make up a special lead to have the speakers in series, using any normal connection will end up with them in parallel (with the 1.3ohms and blown amp).

Your last question is about power handling. This depends upon how the cabs are wired up and you need to be able to understand Ohms law and probably Kirchoff's law to work it out.

You have to work out where the power is going because the speakers won't all be getting the same power in this set up because the resistance of each speaker is different. Then the power handling is determined by which speaker is going to blow first, the one with the lowest watts usually. If both cabs were the same ohms the total power handling is twice the power handling of the weakest cab, but they aren't the same ohms

Generally speaking it is really hard to use two cabs with different ohms together effectively and your best bet is either to use the cabs separately or look to get two 8 ohm cabs.

Hope this is helpful advice even if it isn't what you'd have wanted to hear
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Old 01-07-2014, 03:37 PM   #6
Dave_Mc
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Assuming this all works the same way as guitar speakers (I assume so, it's basic physics/electronics but you never know )

"My cabs added up together have a power handling of 1200 watts (800 for the 4x10 and 400 for the 1x15). "

That's not how it works, normally, you can't just add up the wattage handling of each cabinet. Assuming impedance is the same, the load is split evenly between the two cabinets, you'd have 500 watts going to each cabinet in that case. So the 1x15 would technically be overpowered- though whether that'd actually cause any damage, I dunno. It depends on how much of a risk you're willing to take.

In your specific instance, the impedance of the 4x10 is 2 ohms (assuming that's correct) and the 1x15 is 4 ohms- so double the power would go to the 4x10. which would be 666.67W to the 4x10 and 333.33W to the 1x15. Which would be ok in terms of power handling.

You'd have a load of

1/R= 1/R1 + 1/R2 (assuming the cabs were linked in parallel, which they probably would be)

1/R = 1/2 + 1/4

1/R = 3/4

R = 1.3333 ohms... which is well below most amps' minimum impedance requirements (normally it's 4).

However (and this is the important bit) most solid state amps don't want to see less than their minimum impedance or they, er, fail, so you basically don't much want to be using that cabinet at 2 ohms, either on its own or combined with the other cabinet, because either way you're under the minimum impedance

Some ways round it... you could attempt to rewire the speakers, as Ziphoblat and Phil Starr suggested. That would work if someone has been messing with the interior wiring.

Weber (and possibly some other companies) manufacture an impedance switcher. Never used one, no idea if it works, but might be worth considering.

You can get extension speaker cables which will link cabinets together in series (I know award session in the UK offers them, and they can't be the only people who do). Slightly less safe if something fails in either of the cabs (since you'd have no circuit i.e. no load though that might be less important with SS bass amps than tube guitar amps), but that'd give you a total overall load, when using both cabs together, of 6 ohms which'd be ok. I'm not sure how the wattage handling would be affected by that, normally when I'm working out wattage handling it's between cabs in parallel.

Quote:
Originally Posted by Phil Starr
Sorry this all sounds a bit negative but here is the information you need.

First of all you need to check the impedance of your cabs, someone has probably been messing with the 4x10. It is quite possible that it is two ohms if someone has wired all the speakers in series and they are 8 ohm drivers. they should be rewired to the original series/parallel and the cab will then be 8 ohms. It is equally possible that the person who sold them to you has given you duff information.

Most amps aren't happy pushing full power into 2 ohms, the current draw is too high. That's even true of most of the amps that say they are happy into 2 ohms. If they don't say they are suitable for 2 ohms then the cab will either cause them to shut down when you turn the volume up or the amp will break. If you plug the second cab in then you will present the amp with a 1.3ohm load which no amp will work with for very long. You could wire the speakers in series to get a 6ohm load but this will be quieter than using either cab alone and will almost certainly sound worse, so why would you? In any case you'd need a tech to make up a special lead to have the speakers in series, using any normal connection will end up with them in parallel (with the 1.3ohms and blown amp).

Your last question is about power handling. This depends upon how the cabs are wired up and you need to be able to understand Ohms law and probably Kirchoff's law to work it out.

You have to work out where the power is going because the speakers won't all be getting the same power in this set up because the resistance of each speaker is different. Then the power handling is determined by which speaker is going to blow first, the one with the lowest watts usually. If both cabs were the same ohms the total power handling is twice the power handling of the weakest cab, but they aren't the same ohms

Generally speaking it is really hard to use two cabs with different ohms together effectively and your best bet is either to use the cabs separately or look to get two 8 ohm cabs.

Hope this is helpful advice even if it isn't what you'd have wanted to hear


to the bolded bit- don't you mean parallel there?

EDIT: I just realised 90% of what I wrote was already covered in your post
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Last edited by Dave_Mc : 01-07-2014 at 03:39 PM.
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Old 01-08-2014, 12:50 PM   #7
Phil Starr
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yep it should read parallel not series in the first paragraph. Oops!
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Old 01-08-2014, 03:45 PM   #8
Dave_Mc
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No worries, I always do silly typos like that too
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