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07-07-2011, 08:12 PM
| | | | Mixing a 4 ohm and 8 ohm cab...good or bad idea?
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I use an ampeg Portaflex 500 head, 500 watts at 4 ohms. I want to combine my 8 ohm 2x10 cab that handles 450 watts and my 4 ohm 4x10 cab that handles 1000 watts. This would lower the 4x10 cab's resistance to 2 ohms, correct? is this a bad idea? thanks! |
07-07-2011, 08:14 PM
| | Registered User | | Join Date: Feb 2010 Location: Melbourne, Oz. | | | It would be about 2.6 ohms and yes it is a bad idea.
Last edited by Bill Posters : 07-07-2011 at 08:17 PM.
Reason: maths fail
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07-07-2011, 08:16 PM
| | Registered User
Owner, Bill Fitzmaurice Loudspeaker Design | | Join Date: Sep 2004 Location: New Hampshire | | | The load on the amp will be 2.7 ohms. It has no effect on the 4x10s impedance. Using an 8 ohm 2x10 and 4 ohm 4x10 is the correct procedure, as it gives equal power to all six drivers. Assuming the amp can handle less than a 4 ohm load, of course. |
07-07-2011, 08:18 PM
| | | | your amp can not handle less then 4 ohms, do not do it. it is a bad idea. |
07-07-2011, 08:20 PM
| | | | mixed responses...still not sure what to do. |
07-07-2011, 08:21 PM
| | Banned
Endorsing Artist: MLaghus Custom Basses | | Join Date: Apr 2004 Location: Boca Raton - FL | | | Mixing a 4ohm and an 8ohm cab is not a bad idea per se, but in this case it can NOT be done, since the amp can't handle the load...
2ohm amp = good idea
4ohm amp = bad idea |
07-07-2011, 08:22 PM
| | | Quote:
Originally Posted by allexcosta  Mixing a 4ohm and an 8ohm cab is not a bad idea per se, but in this case it can NOT be done, since the amp can't handle the load... | hm understood, what makes it not able to handle the load just so i know? |
07-07-2011, 08:22 PM
| | Registered User | | Join Date: Feb 2010 Location: Melbourne, Oz. | | | They're all saying much the same thing (even if I got the maths wrong initially). It will decrease the load on your amp to less than 4 ohms. You're amp cannot handle a load of less than 4 ohms. Therefor it's a bad idea and you shouldn't do it.
edit: wow, quick replies ITT... |
07-07-2011, 08:23 PM
| | | Quote:
Originally Posted by Bill Posters They're all saying much the same thing (even if I got the maths wrong initially). It will decrease the load on your amp to less than 4 ohms. You're amp cannot handle a load of less than 4 ohms. Therefor it's a bad idea and you shouldn't do it. | gotchya, thank you sir! |
07-07-2011, 08:24 PM
| | Banned
Endorsing Artist: MLaghus Custom Basses | | Join Date: Apr 2004 Location: Boca Raton - FL | | Quote:
Originally Posted by brodgers91 hm understood, what makes it not able to handle the load just so i know? | The guys who make it say so... Ampeg: Portaflex Series - PF-500 |
07-07-2011, 08:27 PM
| | | | So far, the responses are correct, but the way you asked the question had a mistake in it. I think of impedence as a property of the amp/speaker system; you don't reduce the impedence of one of the cabinets by adding another, you change the system impedence. And the problem is with the amp. Most amps can handle a minimum of 4 ohms; less than 4 ohms and the amp will basically kill itself.
But some heads are designed to handle as little as 2 ohms. Apparently your is not (based on the responses).
The formula for calculating impedence in parallel is 1/(1/cabinetA + 1/CabinetB). In this case it's 1/(1/8 + 1/4) = 1/(3/8) = 2.67 ohms (as stated previously; I thought the formula would be useful here).
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Last edited by FretlessMainly : 07-07-2011 at 08:30 PM.
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07-07-2011, 08:39 PM
| | | Quote:
Originally Posted by brodgers91 hm understood, what makes it not able to handle the load just so i know? | Sorry for the double post, but to me, it's kind of the opposite of the amp not being able to handle the load. The amp needs something to prevent itself from overloading itself - it needs a certain level of impedence from the speakers or it will burn itself out.
I suppose it is "not being able to handle the load" if you think of the load as coming from the amp. I think of it as the speakers not providing enough load at 2.67 ohms.
Either way, unfortunately, the recipe is almost always to get two 8 ohm cabs.
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07-07-2011, 08:41 PM
| | | Quote:
Originally Posted by FretlessMainly Sorry for the double post, but to me, it's kind of the opposite of the amp not being able to handle the load. The amp needs something to prevent itself from overloading itself - it needs a certain level of impedence from the speakers or it will burn itself out.
I suppose it is "not being able to handle the load" if you think of the load as coming from the amp. I think of it as the speakers not providing enough load at 2.67 ohms.
Either way, unfortunately, the recipe is almost always to get two 8 ohm cabs. | thanks for clearing that up! |
08-06-2011, 07:45 PM
| | Registered User | | Join Date: Dec 2009 Location: Tucson, Arizona | | | Okay, So now here is my question... I have an Ampeg pb800 which is rated 300 watts @ 8 ohms, 550 watts @ 4 ohms, and 800 watts at 2 ohms. So now, I have a 4ohm 2x10 cab and want to add a 4x10 cab. If I add a 4 ohm 4x10 cab I know that it would then be 2 ohms, and my head says it can handle that, but is it still a good idea?
...Also, If I was to add an 8 ohm 4x10 cab would that make one louder than the other? |
08-06-2011, 07:55 PM
|  | Registered User | | | | Quote:
Originally Posted by ClayD Okay, So now here is my question... I have an Ampeg pb800 which is rated 300 watts @ 8 ohms, 550 watts @ 4 ohms, and 800 watts at 2 ohms. So now, I have a 4ohm 2x10 cab and want to add a 4x10 cab. If I add a 4 ohm 4x10 cab I know that it would then be 2 ohms, and my head says it can handle that, but is it still a good idea?
...Also, If I was to add an 8 ohm 4x10 cab would that make one louder than the other? | Adding an 8 ohm 410 would make them all the same loudness, This would be at 2.67 ohms. |
08-06-2011, 09:50 PM
|  | Player Characters fear me... Moderator | | Join Date: Nov 2002 Location: Middletown CT, USA | | | Look at the link in my sig -click it - that's the AMPS FAQ. go look for the OHMS FAQ. Click that. |
08-06-2011, 10:10 PM
|  | Supporting Member | | Join Date: Nov 2008 Location: Lake Havasu City, Az USA | | Quote:
Originally Posted by FretlessMainly Sorry for the double post, but to me, it's kind of the opposite of the amp not being able to handle the load. The amp needs something to prevent itself from overloading itself - it needs a certain level of impedence from the speakers or it will burn itself out.
I suppose it is "not being able to handle the load" if you think of the load as coming from the amp. I think of it as the speakers not providing enough load at 2.67 ohms.
Either way, unfortunately, the recipe is almost always to get two 8 ohm cabs. | Go read the FAQs. While nice of this poster trying to help, he has this all backwords.
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08-06-2011, 10:34 PM
| | | | short answer: an 8Ω 2x10 with a 4Ω 4x10 will combine to make a nicely balanced 6x10 cab with all 6 speakers getting the same power.
you just need an amp that will go lower than 4Ω to safely run it.
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08-06-2011, 10:59 PM
| | Registered User | | Join Date: Feb 2010 Location: Nude Zealand | | Quote:
Originally Posted by walterw short answer: an 8Ω 2x10 with a 4Ω 4x10 will combine to make a nicely balanced 6x10 cab with all 6 speakers getting the same power.
you just need an amp that will go lower than 4Ω to safely run it. | Correct. Quote:
Originally Posted by aqsw Adding an 8 ohm 410 would make them all the same loudness, This would be at 2.67 ohms. | Total impedance correct; relative impedance of cabinets required in order to achieve identical power to each speaker in setup, incorrect. See above.
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