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Old 15th February 2010 , 03:27 PM
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Default Impedence question on subs with passive crossover

I have Carlsbro Alpha 400W RMS 8ohm subs with an inbuilt passive crossover that feeds, via a speakon cable, to Alpha tops 250W RMS also 8ohm.

I understand if I that usually chaining speakers together (in parallel) I will get 4Ohm in this instance but is the total impedence in this setup with the crossover now 8ohm or 4ohm. I assume still 8ohm somehow because of the crossover?

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Old 15th February 2010 , 05:22 PM
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Off the top of my head, it would entirely depend upon whether or not the crossover adversely affects the overall impedance... my gut instinct would be that it probably has a high additional impedence rating, doesn't hugely affect the overall impedance and so Ohm's law still applies and the load of 2x 8 ohm speakers in parallel through a crossover will still be 4 ohms. The only way to know for sure is to check the specs of the crossover.
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Old 16th February 2010 , 09:24 AM
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Hi Dave

Thanks for the quick response. The subs are Carlsbro Alpha 15s but the Carlsbro site is down at the moment so can't get any spec sheets - can else anyone help?

I found elsewhere something that says they have "a filtered O/P for tops" which I guess relates to the crossover but this is all a bit above me. The back of the speaker says "Output MIDS/HIGHS only (1,-1)" I don't know if that helps?

Oh and something else I've just noticed the Subs are 4 Ohm! The tops are 8 Ohm but they were purchased as a pair from the dealer... a few years back and seem to work but had always assumed they were the 8 Ohm version.

The reason for the question is that I run them via a Peavey PV2600 which pushs 900W per channel at 4Ohm and 550W at 8Ohm so was wondering how much is actually being used. To be honest I don't really understand what happens to a speaker cab that is rated at say 250RMS @ 8ohm when part of a 4Ohm overall load. Is it likely to cause issues.

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Old 16th February 2010 , 11:23 AM
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8ohms and 4ohms in parallel would give an overall load of 2.6ohms.

There is a formaula to work it out:

1/R = 1/R1 + 1/R2 (1 divided by total load = 1 div by speaker1 load + 1 div by speaker2 load)

or to make it easier, use this online resistance calculator: http://www.diracdelta.co.uk/science/...ce/source.html
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Old 16th February 2010 , 07:05 PM
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Thanks Dave. So what happens to the speakers then when rated at 4 and 8 Ohms but the load is actually 2.6. Are they capable of handling more that the RMS figures given or am I likely to blow them if the amp is turned up?
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Old 17th February 2010 , 11:23 AM
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I don't know the kit involved but..
Passive crossovers must be designed for the impedance of the particular drive unit(s).

A crossover for a 4R system (assuming the same turnover f's) will have the inductance values halved and the capacitances doubled ref an 8Ohm network.

I cannot see any great harm being caused but things might not sound right!

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