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speakers & crossovers & my head hurts!!


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Hi!! dunno if any one can help??


Basically ... I had some speaker boxes with 150W 12" speakers that had an impedance of 8ohm & then APT tweeters that where 8ohm.

The total impedance was 8ohm (or so the box said)


The main speakers have been fried by a poweramp that went a bit nuts!


& now I have got some 12" speakers 400W (Peak) 200W (Music) they have a impedance of 4ohm


Basically my question is if I use the crossover (I dunno what the specs of the crossover are) with the 8ohm horns & 4ohm speakers what will the impedance of the box be?? I just need to know that it will not be below 4ohm as the lowest impedance on the poweramp for these boxes is 4ohm!


if any one can help I would be most greatful.



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The problem will be that the crossover is designed to work with 2x 8 ohm units. If you stick a 4 ohm driver in it, the output from this driver will be higher than the old 8 ohm driver and consequently you will hear more low/mid than highs.



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Hi Rooftop


thanks for this, however I am aware I will get more bottom end! this is not really a problem because these speakers are just going to be used as monitors for the drummer!!


basically I just need to know what the overall impedance will be?? is it just the same as connecting the units in parallell?? as I suspect or... as a sound guy at a vene we played said 'the crossover cancels out the horn impedance so all you need to consider is the impedance of the driver' if the second is accurate could someone please explain how exactly that happens?



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I'm afraid it's not that easy. Speaker impedance varies enormously with frequency - see http://www.churchsoundcheck.com/imp1.html for some examples. The crossover won't be working as designed with the 'wrong' kind of driver, so it's not really possible to predict what minimum impedance the amp will see without a circuit. On careful reflection, and after hours of calculations, my considered advice would be: suck it and see; but watch for the amp clipping at various points around the crossover frequency. There's a useful set of calculators here: http://www.carstereo.com/help/Articles.cfm?id=55
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I guess the question I need to ask is does any one know hoe the hell a basic passive crossover effects the impedance of a loudspeaker box!?


Cos the nominal impeedance with 2 8ohm units inside is 8ohm! & by reckoning this is an average impedance of the 2 units & by this logic if I have a 4ohm Unit & a 8 ohm unit with a crossover & knowing nominal impedance is basically just whatever the manufacturer wants to tell you I am guessing a stab at around 6ohms could be somewhere about right!!?? ;)


But I'm guessing some boffin is going to come along & tell me I'm way off the mark!


if the crossover was not involved I would be fine! but I just plain & simple don't understand crossovers!


A concurrent post has been automatically merged from this point on.


Just incase any one is iterested I have just rec'd an E-mail from a teccy fella!!


"Its a bit unpredictable mixing impedances that way however I can see no problem.

The Impedance of the cab would remain 4 ohms and the horn would be about 6 dB down on a 4 ohm one . That would be a good thing because compression drivers usually need some attenuation to sit well with the bass unit. JBL do a similar theing in some of their cabs.





- rmjloudspeakers"


is this guy telling the truth??

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OK. Bad analogy coming up. Hose pipe feeding two sprinklers, one for one end of the garden, one for the other. If you have both sprinklers on at the same time, the water can travel either route out to the ground. The flow of water is twice what you would get with just one sprinkler. Or you could say that two sprinklers connected in parallel have half the resistance of just one. That's like two drive units simply connected in parallel.


But if you have a valve that can gradually divert water from one sprinkler to the other, so that the total flow of water stays the same as you turn the valve from one sprinkler to the other, then the resistance to the flow of water is just the same as one sprinkler.


So an ideal crossover would keep the power going to each drive unit the same as the input frequency changed, and the impedance seen by the amp wouldn't change with frequency. That's how 2 ohm drive units through a crossover can still present a load of 8ohms - they aren't really 'in parallel', because the same signals aren't going to both. That's why a lower impedance drive unit for the HF will receive more power than a higher impedance one. In a perfect world, the load presented by the speaker would change from 4ohms at LF, where all the power was going to one drive unit, to 8ohms at HF, where all the power was going to the other drive unit.


In the real world, where very simple circuits are used to do the frequency division, the various elements interact, and results could be very different to this simple analysis.


My apologies if the analogy made things worse ;) !

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OK, how about this. Just for testing.


Take your two 4 ohm speakers & wire them in series so that you can connect an 8 ohm load to one of you crossovers. Measure the resistance of this with a multimeter. If you don't have one, there are some very cheap ones out there that will do this job & more.


Then take one of your 4 ohm jobbies away, then measure again. This should give you some idea as to what will happen to the impedance of you proposed new setup.


3 things that worry me:


The 4 ohm drivers are going to "want" a higher current from the crossover than the 8 ohm ones did - is the crossover built to be OK with this?


You old drivers were damaged by "a poweramp that went a bit nuts" - has this also done anything to the crossovers, and for that matter the horns?


If your speakers were cheap ones to start with, then changing some of the components will have unpredictable results on the sound. However if they're good well designed cabs, you'll probable make the sound worse by changing things about.

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