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Old 7th January 2007, 10:41 AM   #1
Did it Himself
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Question Subtractive Crossover for PA

I'm building a small PA system for when I DJ at house parties. I've made a pair of 12in bass bins and pair of 8in + tweeter mid/top cabs. I was planning on using a traditional 4th-order Linkwitz-Riley active crossover at 300Hz for the crossover between cabinets, but I'm wondering how a 4th-order LR for the high-pass and subtractive for the low-pass would fare. As I never thrash the system I reckon the stopband attenuation would not really be a problem.

Any comments on the sound subtractive vs traditional in this application?
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Old 7th January 2007, 11:58 AM   #2
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I don't know, so far I've just never got around to trying subtractive crossovers. If you do try them, I would be interested in your conclusions.

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Old 7th January 2007, 12:23 PM   #3
bjorno is offline bjorno  Sweden
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richie00boy,

I played with MathCAD and plotted a two-way LR4 subtractive filter, see submitted picture.

I recommend the use of Tolvan.com, Xdir for vertical polar dispersion evaluation of the filter.

b

1(1)
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Old 7th January 2007, 04:30 PM   #4
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Thanks bjorno. From your graph I pick out that the combined phase response is flat, this is to be expected from subtractive crossovers. The combined amplitude response I'm not sure is flat though, but maybe the bump at crossover will just give a bit of vocal projection, no bad thing really for a PA system. I've attached my simulated filter responses.

Click the image to open in full size.

Click the image to open in full size.
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Old 7th January 2007, 05:16 PM   #5
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Quickly doing a screen measure, the bump is a mere 0.4 dB, not anythng to worry about.
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Old 7th January 2007, 06:42 PM   #6
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The bump is 4.2561dB @ 380.189Hz according to the marker. I'm not sure this is the right trade off for perfect phase response.
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Old 7th January 2007, 08:36 PM   #7
Geoff H is offline Geoff H  Australia
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That bump is not that severe. If phase is taken into account it's flat. The subtractive process occurs in real time, not values merely added together, as in the graph.

What you need to watch is the roll off on the bass channel. Its only about 6db/oct. That could be a problem with edge resonance of the bass driver.

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Old 7th January 2007, 08:46 PM   #8
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Thanks Geoff that has helped. I did worry about the shallow roll-off on the bass as the graph showing the much steeper mid roll-off accentuates the problem.

The circuit is fairly simple so I might have a go and see how it turns out. After all a lot of PA systems run the bass driver fullrange or just with a passive crossover, at much higher frequency. The one I'm using has a foam roll surround so edge resonance might not be too badly damped.
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Old 7th January 2007, 09:13 PM   #9
Geoff H is offline Geoff H  Australia
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Your welcome Richie00boy.
If you ever want to shift the xo on the subtractive filter, you only have one set of components to change. You can even make it switchable.

I prefer simple passive xos if I must. But my favourite 15" with foam surround was a bit edgy about 750Hz on a 6db xo at 500.

Geoff.
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Old 9th January 2007, 08:17 PM   #10
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I decided to check time domain amplitude response of 300Hz sine input vs the two outputs. The HF output is out by 180 degrees, surely this should be in phase at the crossover frequency?

Click the image to open in full size.
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