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Crossover reverse calculation?

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Andy Kos View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Andy Kos Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2014 at 1:33pm
The Peavey Cab I saw this in wasnt used at low volume, but the crossover did the job.
 
Reality is, unless you are prepared to put in a lot of time and effort, and spend money on the components, no passive crossover is going to be perfect, it's always a compromise.
 
 
just a guy with a warehouse and a few speakers... www.bluearan.co.uk
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2014 at 2:56pm
Originally posted by Saturnus Saturnus wrote:

Originally posted by Andy Kos Andy Kos wrote:

Originally posted by Saturnus Saturnus wrote:


It depends on the driver. For some compression drivers on a CD horn a 2nd order crossover in the 10Khz range actually gives a reasonably flat response and the best phase response without any attenuation and then applying CD horn compensation in the EQ. The reason is that the falling response of the CD horn is compensated by the filter and since the crossover frequency is so high it also compensates for the elevated response in the 4-6Khz region.

If it is paired with the right low frequency driver it is a perfect example of a superbly engineered keep-it-simple,-stupid crossover that actually takes the driver and filter responses into consideration instead of just adding more complex circuits like impedance compensation, CD horn compensation, tweeter attenuation and so on. All of which will affect the speaker negatively.

Ive seen this used in Peavey cabs, the crossover design seems to be wrong, but works, but as you say, just using a simple crossover to compensate for the HF response. In the instance of the Peavey cabs though, I felt they had overdone it a touch, leaving a bit of a hole in the vocal range.

Worked great for disco speaker though.


I probably wouldn't use the technique for a professional speaker. It works great in home speakers that rarely see more than a tenth of the power handling used but in a professional speaker the impedance of particularly the compression driver would probably vary too much as increased power changes the temperature of the VC and thereby the effective impedance. And since the filtering hinges on the impedance being stable you'd probably get that hole in the vocal range when it has to play louder. You can compensate for that to some extent by designing for the intended power range but then you'll have too much low high-range/upper mid-range at lower volumes instead. As noted. It works best if used on speakers that will practically never see even close to their full power handling potential utilized.

EDIT: As a note. A similar technique is used in the passive filter for a Boominator with the PHT407N tweeter but with a 1st order filter and limited attenuation instead. The PHT407N will at worst see about 1/8th of it's maximum power handling in peaks with the recommended amplifiers, so here it works quite well.

i used this method on one of my unity horn prototypes. as it is a CD horn if you get a driver with constant roll off from ~2kHz use one 1st order filter (~10kHz) to flatten the response and then another filter at ~1kHz to cross to mids. multiplying these two filters together gives a non-standard alignment with a crossover frequency ~3kHz. uses fewer components and saves wasting power in an L-pad. I agree that this is best used in low-power applications - for pro use an L-pad helps stabilise the impedance load on the crossover.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tweeter_Box Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2014 at 11:51pm
thanks guys, very interesting ready

ok so next question, if i needed to add attenuation to a passive crossover, what would be a good start?
as far as i know attenuation is added via the white ceramic blocks, whats the formula?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2014 at 8:32am
It's called an L-Pad, the ceramic blocks are high power resistors. It would be a very good idea to measure the impedance of the driver but you can approximate it if you don't.

Read this - http://sound.westhost.com/lr-passive.htm

I know no-one can be bothered with learning any maths round these parts and is always after a quick result but if you seriously want to make crossovers there's not a lot of alternative except a lot of component swapping.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tweeter_Box Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2014 at 12:48pm
Originally posted by odc04r odc04r wrote:

It's called an L-Pad, the ceramic blocks are high power resistors. It would be a very good idea to measure the impedance of the driver but you can approximate it if you don't.

Read this - http://sound.westhost.com/lr-passive.htm

I know no-one can be bothered with learning any maths round these parts and is always after a quick result but if you seriously want to make crossovers there's not a lot of alternative except a lot of component swapping.


thanks will check the link when im off work
i was goin to just add random value ceramic blocks till it sounded as desired, the bit i dont know of is how this would effect the ohms, Lpads are good but pretty pointless if inside the box imo
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Andy Kos View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Andy Kos Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2014 at 1:28pm
I can give you the resistor values if needed Tweeter, we probably have the parts you need too.

Just email me the details and I'll sort it.


just a guy with a warehouse and a few speakers... www.bluearan.co.uk
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 28 November 2014 at 5:02pm
L-Pads are easy to understand. Say you want to drop an 8 ohm tweeter by 6dB. That means you want to feed it half the power. So you put an 8 ohm resistor in parallel with it, and you can see how half the power will go through the resistor and half will go through the tweeter. 
But, now you have two 8 ohm resistances in parallel, giving a total resistance of 4 ohms. Your crossover will not like that if it is meant to be terminated with 8 ohms. So, you put a 4 ohm resistor in series, and that brings the total resistance up to 8 ohms again. Now the crossover is happy, the slope and frequency will be right, and the driver is receiving 6dB less power.
If you wanted to drop it by 12dB, that is 1/4 the power. So you would put a 2 ohm resistor in parallel. 2r paralleled with 8r = 1.6r, meaning you then need 6.4 ohms in series.
For other power reductions I would need to get my calculator out, or use the diyaudioandvideo.com online calculators (Very useful pro tip there ;)

For PA drivers having power handling up to 100 watts, especially in arrays that will be driven hard (ie reggae tweeter boxes) I think you should use the 50 watt gold coloured metal resistors in all crossover applications. The 20 watt white ceramic ones are not quite beefy enough IMO.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tv00 Quote  Post ReplyReply Direct Link To This Post Posted: 19 January 2017 at 12:46am
I found one that also did 3rd order, reverse and mess around online magic, free yet better than any casino, now I can't find it :-(

Just looking at some bms4590, missing one more 8/8 Passive xover,
The 8/8 here reads 5,5 / 3,3 / 0,1mH first must be 5,6uF
16 ohm version is: 4,4 / 2,2 / 0,15mH
So just wanted to check by going backwards with 16 ohm version and around :-)
www.facebook.com/babysoundsystem
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