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Old 11th February 2013, 08:20 PM   #11
Rob41 is offline Rob41  United States
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Quote:
Originally Posted by StigErik View Post
I have measured the RD-75's of course. They stay flat until 10kHz at 30 degrees off-axis. At larger angles, they start to drop off from 5kHz. That is very much like dome tweeters, so if you want to find a dedicated tweeter with less beaming it has to be very very small.
Thanks for your input StigEric. I've enjoyed reading about your BG's.

What is your opinion regarding splitting the mid/high frequencies between two RD-75's per side given they are far enough apart to prevent combing?

Can you see any advantage to this approach or do you think it would just be wasting money?

I'm beginning to think I'd be better off just using one per side and focusing my attention on the mounting of the BG's in something like the beautiful surrounds I've seen used with the BG's before. That and tinkering around a lot with the crossover point/bass implementation.
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Old 11th February 2013, 08:27 PM   #12
jcx is offline jcx  United States
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add a "acoustic lens"?

probably worse effects than the problem though
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Old 12th February 2013, 02:15 AM   #13
gworrel is offline gworrel  United States
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To prevent combing it is a question of getting them close enough together, not far enough apart.

I would take the second pair and use them for surround sound. Or get 5 and have a full home theater set. You still have to use an array of low frequency drivers for each RD-75 or something like what StigErik has.
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Old 12th February 2013, 04:52 AM   #14
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If you really want to use more than one BG-75, you could always run them in series and bypass one with a 50 uF capacitor. On-axis reinforcement between the two units will keep the response flat even though the impedance is doubled at low frequencies, and excursion below 500 Hz is halved.

I've tested this trick with cone drivers, and it works a treat.
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Old 12th February 2013, 12:09 PM   #15
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If the point of using multiple RD-75's is to get more dynamics at low frequencies, I think it's a better (and cheaper) idea to move the XO point somewhat upwards, or use a wide baffle. The latter will increase the efficiency more than 10 dB.

I XO at 200 Hz, and even without any baffle I don't have any problems, but then I don't play very loud, and I listen at very short distance.
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Old 12th February 2013, 03:49 PM   #16
Rob41 is offline Rob41  United States
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Quote:
Originally Posted by gworrel View Post
To prevent combing it is a question of getting them close enough together, not far enough apart.

You still have to use an array of low frequency drivers for each RD-75 or something like what StigErik has.
I thought they needed to be closer but wasn't sure. I'm still pretty new at this. Wouldn't the RD-75's be able to get close enough to prevent combing even if the surrounding flanges overlapped?

Yeah, some type of array of LF drivers like StigEric has would to the trick.

Quote:
Originally Posted by StigErik View Post
If the point of using multiple RD-75's is to get more dynamics at low frequencies, I think it's a better (and cheaper) idea to move the XO point somewhat upwards, or use a wide baffle. The latter will increase the efficiency more than 10 dB.

I XO at 200 Hz, and even without any baffle I don't have any problems, but then I don't play very loud, and I listen at very short distance.
I think you're right. It looks like the cost vs benefits isn't going to pan out using two pairs. Using a wide baffle and raising the XO point may be prudent in my case.
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Old 12th February 2013, 04:52 PM   #17
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I thought they needed to be closer but wasn't sure. I'm still pretty new at this. Wouldn't the RD-75's be able to get close enough to prevent combing even if the surrounding flanges overlapped?
It looks like the cost vs benefits isn't going to pan out using two pairs. Using a wide baffle and raising the XO point may be prudent in my case.
Overlapping the panels would cause HF problems, and even as close as possible the wavelength of the center to center distance of 4 11/16" would cause problems in the midrange.

Since there are many woofer choices that can cover the range of 25-250 (or even higher) nicely, raising the XO point would be prudent, let the RD-75 do what they do best. At 200-300 Hz, you won't get much directivity change in the woofers unless going floor to ceiling, a single woofer per side could easily suffice in a bi-amped set up.
As far as adding tweeters, that could be done later if you feel the upper dispersion is too narrow, but it seems there are not too many with that complaint.

Have you found any polar charts for the RD-75?

Last edited by weltersys; 12th February 2013 at 04:55 PM.
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Old 12th February 2013, 07:40 PM   #18
gworrel is offline gworrel  United States
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Here is a gallery of RD-75 systems put together by ThomasW who runs the Cult of the Infinite Baffle, and is a moderator at htguide.com Mission Possible DIY.

Projects update and iformation page


Here is a picture of my "work in process" RD-75, hanging bare with only pipe insulation on the edges, next to a 24" wide baffle with 15" TC Sounds dipole woofers. I cross over at 280Hz using a DBX Driverack PA.
Click the image to open in full size.
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Old 13th February 2013, 04:58 AM   #19
Rob41 is offline Rob41  United States
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Quote:
Originally Posted by weltersys View Post
Overlapping the panels would cause HF problems, and even as close as possible the wavelength of the center to center distance of 4 11/16" would cause problems in the midrange.

Since there are many woofer choices that can cover the range of 25-250 (or even higher) nicely, raising the XO point would be prudent, let the RD-75 do what they do best. At 200-300 Hz, you won't get much directivity change in the woofers unless going floor to ceiling, a single woofer per side could easily suffice in a bi-amped set up.
As far as adding tweeters, that could be done later if you feel the upper dispersion is too narrow, but it seems there are not too many with that complaint.

Have you found any polar charts for the RD-75?
I have not found any polar charts. I don't think I'll have any issues not adding additional high frequency drivers so I'll let the RD-75's do their thing.

I think using a single set of RD-75's, playing around with the XO point and experimenting with different sizes and shapes of baffles is going to yield me my best bang for the buck.

I see now how problematic using two pairs would be and likely unnecessary.

Quote:
Originally Posted by gworrel View Post
Here is a gallery of RD-75 systems put together by ThomasW who runs the Cult of the Infinite Baffle, and is a moderator at htguide.com Mission Possible DIY.

Projects update and iformation page


Here is a picture of my "work in process" RD-75, hanging bare with only pipe insulation on the edges, next to a 24" wide baffle with 15" TC Sounds dipole woofers. I cross over at 280Hz using a DBX Driverack PA.
Click the image to open in full size.
Thanks for the link. There are some beautiful implementations of the RD-75. What does the pipe insulation do? Nice looking bass drivers; what are they?
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Old 13th February 2013, 07:42 AM   #20
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The pipe insulation is there to round off the edges and reduce edge diffraction. It works! It also extends the effective baffle width.
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