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Old 20th January 2015, 10:15 AM   #1
djn is offline djn  United States
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Default Need help designing a crossover

Hi All, I got these a couple weeks ago and they have two obvious problems. The drivers are the ScanSpeak 21W/8554 woofer and the Dayton DC28 silk dome tweeter. The crossover is a Dayton prebuilt 2-way crossovers designed using a second order (12 dB/octave) Linkwitz-Riley alignment

1. the ScanSpeak woofers absolutley needs to be in a vented cab so I did the WinISD thing and will vent them this week.

2. the midrange is increadable boomy and if you look at the FRgraph, you can see an 8 to 10 db peak at 3.5Khz. The issue is that the crossover is set at 3.5khz so they are crossing right at the top of the peak.

Question; can I, should I, just put in a notch filter and use that crossover? If so, where in the line does the filter go? Or should I go with, say, a 2khz crossover third order for the woof and second order for the tweet?

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Last edited by djn; 20th January 2015 at 10:21 AM.
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Old 20th January 2015, 11:36 AM   #2
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I would add a second order low pass filter between the woofer and cross-over and just roll it off from 1kHz. Making a notch filter is an overkill. Moving the system cross-over freq down may affect the power-handling of the scanspeak.

BTW beautiful cabinets.
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Old 20th January 2015, 11:40 AM   #3
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Before stuffing up the boxes with a bass reflex port, make very sure you need it. Once the hole is made you cannot close it.
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Old 20th January 2015, 11:40 AM   #4
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IMHO, the drivers you have can work together, but you have to completely redesign the crossover (reasons below).
I assume you don't have measurement capabilities, so you have to rely on simulations only. There is a sticky in this forum you should read for your info, I find that the method described at first is way too complex and doesn't take into consideration the fact that there are (for free) crossover simulation SW. I think that the best method is the one linked at post #20.
Introduction to designing crossovers without measurement

I would try a crossover at 2KHz mainly for directivity reasons, or better below (1.5-2.0KHz). You should aim at 4th order acoustical LR slopes. I obtained good results for phase tracking with a 2nd order electrical on the woofer and a 3rd order electrical on the tweeter, with a shallower slope on the woofer. The problem with your cabinet is that the tweeter is mounted on a different plane from the woofer, you should take care of that in the simulation, and maybe reverting to a 2nd order for the tweeter.
The bump at 3KHz you are worried is not really a problem as a carefully crafted 2nd order electrical crossover can take care if, but you need to simulate the effect of the baffle step and diffraction of your cabinet summed to the natural FR of the driver. What you'll find is that the sensitivity will drop from 90dB to something in the 84-87dB range (depending on your personal taste and room placement). So the tweeter needs to be tamed (as it is 89dB), a single resistor before the crossover usually is fine.

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Old 20th January 2015, 11:53 AM   #5
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Quote:
Before stuffing up the boxes with a bass reflex port, make very sure you need it. Once the hole is made you cannot close it.
That is a sensible suggestion for a general case, but the specs tell that this woofer would work in a closed cabinet only if you want to add a subwoofer under 70-80Hz, if you want some bass you have to vent the cabinet.

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Old 20th January 2015, 05:35 PM   #6
djn is offline djn  United States
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I modeled the woof in this cab in WinISD and it indicated that I needed a 2" port 3" long. So I took the tweeter out and made a plate with a port in it that fit right into the tweeter hole. Solid bottom end now, but there is still the huge bloaty are in the midrange.
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Old 20th January 2015, 07:09 PM   #7
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I modeled this crossover for an audax HM210C0 and a seas millenium T25. I think it should work with your drivers.
1500Hz crossover frequency.
You should adjust the level of the tweeter by adding a resistor (3.3 or 4.7ohms) before C2, 10uF on its crossover.
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Old 21st January 2015, 06:02 PM   #8
djn is offline djn  United States
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Thanks Jerome. I'll give that a try. It looks like a good starting point. I'm glad I have a big junk xover box.
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Old 24th January 2015, 04:32 PM   #9
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Old school designer here. Try a 6db around 800hz on woof and 18db on tweeter around 2k or 2.5k. I'm aways looking for the simplest solution and less is more. I've used similar design with excellent results.
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Old 24th January 2015, 04:34 PM   #10
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Oh and lpad tweeter as needed.
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