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Old 26th January 2005, 06:32 AM   #1
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Default How to cross over

Hi again,

This is really a double checking thread to see if I'm thinking along the right lines here.

I'm building a small box active sub with electronic Q adjustment (something like a linkwitz transform although this part of the circuitry is commercial).

My surround preamp has the possibility of routing everything under 120 Hz to both the front pair (amp) and the sub, but offers a degree of control over the individual channels, thus I would like to use the 3 outputs here as a starting point.

The front pair is System Audio SA520. This reaches -3dB at 43 Hz, -6 at 40Hz and then falls off at 24dB/8ve. System Audio won't reveal any more than this.

I would preferably like to keep the components on the front signal path to a minimum and tailor the crossover on the sub channel as far as possible, so to start with I want to try with a single low pass filter on the sub channel. I've decided on a Linkwitz-Riley filter set at 40Hz to match the -6 dB point of the front pair. My thinking is that if I can't get this to work all I have to do is change a couple of resistors here, and build matching, high quality, high pass electronic filters for the front pair and crossing at, say, 60 Hz instead of 40.

The question is whether this is a reasonable approach or whether it's so stupid that it's bound to fail.

I'd like feedback on this before I get out the pcb etching kit :-)

Many thanks!
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Old 26th January 2005, 08:40 AM   #2
rabbitz is offline rabbitz  Australia
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I'm not sure what you are trying to achieve with this but for Home Theatre use the LFE channel determines the sub crossover. To make it easy it's just a matter of setting the fronts to small and the LFE to the sub but the sub must go up to 80-120Hz depending on the processor and the processor does all the work and you don't have to do a thing.

If it's for music then it's different as you only want the sub for the lower octaves that are missing from the mains and what you suggested seems sensible for that. Actually your mains seem to go low enough for most music.
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Old 26th January 2005, 08:59 AM   #3
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If only the processor crossed at <= 80 Hz thats just what I'd do. Unfortunately it's set in stone at 120 and I feel that that's way too high. Freq's over 80 are directional for a start and I only have 1 sub, and my front pair will be better at these frequencies. That's why I have to lower the effecive crossover freq.
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Old 26th January 2005, 01:16 PM   #4
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If the sub feed is crossed at 120 Hz what about the HP filter to the mains? If that's also an unalterable 120Hz you have no options except a better receiver.
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Old 26th January 2005, 05:51 PM   #5
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yep, but I do have the option of feeding <120Hz to the mains AND the sub. Hence the filter(s)
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Old 27th January 2005, 01:27 PM   #6
rabbitz is offline rabbitz  Australia
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When it's all said and done it's dependant on the LFE track anyway and has to be set high enough to cater fot that. I can't remember but I think it's around 90-100Hz. Even if the sub was set to 200Hz, it's only going to get what's in the LFE channel.
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Old 30th January 2005, 12:30 PM   #7
chrish is offline chrish  Australia
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Not sure if it helps you, but if you are doing an external crossover anyway, why not make it for your mains and your sub? This way you could tailor the crossover type/slope/frequency to exactly what you want. I am building stereo sub woofers, and as my processor only does mono, I am building an active crossover set at 80hz for each channel. My processor allows me to set the sub to "none" and the mains to "large", so all freqs and LFE go to the mains. If you still want mono sub, you could sum the outputs of the stereo low pass filters.

I have not finished this project yet, so cannot tell you how it sounds.

Just something extra to consider that may help you decide on a solution.

Good luck,

Chris
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Old 30th January 2005, 06:42 PM   #8
morbo is offline morbo  Canada
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You can do what I'm doing - set the mains to full range, then run the line level out for the L & R channels to an external crossover (in my case, the Paradigm X-30). I then send everything below 50hz to my sub, everything above to an amp that drives my mains. This way you can use whatever external crossover you want and have full control over slopes/frequencies/phase. From reading this thread it seems like several others are doing something similar as well.
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