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 Subiectul mesajului: passive crossover?
MesajScris: Mar Noi 22, 2005 6:39 pm 
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i have noticed that many of the big name mid tops like the funktions use a passive crossover between the high mid and compression drivers. the major advantage of this is being able to use a 3 way active crossover/lms instead of the considerably more expensive 4 way or a pair of 3 in 6 outs, plus a similar saving from being able to use 4 pole speakons instead of 8s and significantly cheaper 4 core cables.

disadvantages are obviously lack of control over delays and levels, the crossover would really have to be right first time. linkwitz labs papers can really put one off using passives, but what are people's thoughts?

james.


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 Subiectul mesajului:
MesajScris: Mie Noi 23, 2005 2:10 am 
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Membru din: Joi Noi 17, 2005 3:22 pm
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James, good points brought up early in the design, which should rightly be addressed early.

Whilst there are the obvious advantages of control with full active crossovers, the additional equipment required is a major concern, especially if only two boxes were being used. In the case where you may run amp racks behind each stack, there would be the temptation to run both hi-mid and highs off each side of one power amp. This would result in the HF being connected to an amplifier probably at least double the power it needs to be. As long as everything is run clean and within limits, there is no problem, but an accident (feedback squeel, dropped mic, etc) could easily kill the horns unless carefully set limiters are employed.

I tend to favour using the passives on the top for the reasons you suggest, however the box needs to be designed to cater for this. If the hi-mid and HF drivers are mounted the same distance back from the front of the box (therefore time-aligned), a passive crossover is easy. If there is a small difference in distance, then various techniques like differing crossover slopes and frequency overlap/underlap can be used to compensate the time difference. Once the difference in distance exceeds 1/2 wavelength at the crossover frequency, then coherently using a passive crossover is probably impossible.

Time-aligning the drivers would suggest a very long horn for the HF, which could be very detrimental to its performance, but there are various ways around this, such as including the HF portion as part of the hi-mid flare (as I did in one of my dual 8"+1" boxes), or extending a short horn with a higher flare rate out to the full width of the mi-mid flare.

If the general concensus is to either use passive or have it as a serious option, then we need to allow for this BEFORE we get into the HF geometry. Although this would more than likely preclude the use of a commercially available HF flare, don't let this be a deterrant to using a passive crossover if most would prefer to do so.

As this is a community project that we would like any of us to be able to build, then the availability of FULL and accurate plans is a must. NOT just a couple of section views with dimensions like most plans available on the net, but full individual dimensioned parts diagrams and sheet cutting diagrams etc. With the 3D CAD setup I use, producing accurate parts drawings is easy. So even if the design is quite complex, and uses a lot of small, funny shaped bits with wierd bevels on them, IF you have accurate drawings of each bit, then cutting and assembling is a breeze.

The point of that last paragraph is:- if you would prefer a passive crossover, don't decide against it on the grounds that it may make the box too complex.

Cheers
Graeme


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 Subiectul mesajului:
MesajScris: Mie Noi 23, 2005 10:33 am 
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ho hum, regarding complexity. amongst other boxes, i have made some trapezoidal mid horns, they contained not one square bit of wood. we are talking about putting a bass and hf horn in such a cabinet too, it will be challenging carpentry.

for anyone looking for a simple box with as few mitres as possible, build a reflex cab. seriously.

i would really like to use a passive on the top section to keep operating costs and complexity down but i appreciate the difficulties, most particularly the length of the hf waveguide. on the bright side, as soon as the details are worked out a crossover is very repeatable and requires no specialist equipment or materials (assuming point to point wiring as opposed to using a pcb).

the hf and high mid waveguides are clearly needing to be addressed. i can't really do that much more with my mid bass horn until i have a better idea of how big these components need to be.

james.


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 Subiectul mesajului:
MesajScris: Mar Noi 29, 2005 6:51 pm 
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I think passive is the correct answer here. Graeme has already offered a couple of ideas and I'm sure we'll come up with more.

Jim


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 Subiectul mesajului:
MesajScris: Mie Noi 30, 2005 12:16 am 
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Membru din: Dum Noi 27, 2005 10:25 pm
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Would it be easier to desegn the box active as a starting point, this will allow experimentation with crossover frequencies and slopes.
Once the best results are found, publish the results for those who want to go passive. They can then get passive crossovers to those specs.


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 Subiectul mesajului:
MesajScris: Mie Noi 30, 2005 12:46 am 
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I think the active/passive decision is another to be left until we're a bit closer to a working system. There will be some people who will prefer passive or don't have the LMS/amps to run active. Those who prefer active setups will of course be free to run their boxes in that configuration.

We're still at the brainstorming stage really, lets not get hung up on details yet.


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 Subiectul mesajului:
MesajScris: Joi Dec 01, 2005 12:52 am 
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whilst i'm all for using an lms to work out what is likely to be the best settings for winding a passive crossover to, i believe that the physical constraint of the hf vs hi-mid waveguide length when it comes to aligning drivers (essential for accurate phase response with a passive crossover) means that it must be considered from the day one when designing the box.

there are plenty of designs which will work with time compensation on the drivers but few that can have a passive retro-fitted. passive crossover components are relatively inexpensive, some simple hf protection can be easily incorporated and the subsequent saving in cabling, amplification and processing can be channelled into getting well specced neo drivers.

i'd model for a pair of the awesome beyma lx60 12"s on the low mid if i thought i could bear lifting them onto a stack of martins.

james.


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