Crossover reverse calculation? |
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Andy Kos
Old Croc Joined: 15 May 2007 Location: Southampton Status: Offline Points: 3035 |
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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.
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just a guy with a warehouse and a few speakers... www.bluearan.co.uk
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snowflake
Old Croc Joined: 29 December 2004 Location: Bristol Status: Offline Points: 3118 |
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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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Tweeter_Box
Young Croc Joined: 23 June 2012 Location: London Status: Offline Points: 859 |
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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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odc04r
Old Croc Joined: 12 July 2006 Location: Sarfampton Status: Offline Points: 5483 |
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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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Tweeter_Box
Young Croc Joined: 23 June 2012 Location: London Status: Offline Points: 859 |
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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
Old Croc Joined: 15 May 2007 Location: Southampton Status: Offline Points: 3035 |
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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.
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just a guy with a warehouse and a few speakers... www.bluearan.co.uk
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studio45
Old Croc Joined: 16 October 2007 Location: United Kingdom Status: Offline Points: 3863 |
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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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Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA
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tv00
Old Croc Joined: 10 August 2009 Location: Denmark Status: Offline Points: 1886 |
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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 :-) |
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