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Old 10th February 2013, 06:10 PM   #1
Rob41 is offline Rob41  United States
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Default Splitting frequencies between two BG RD-75's

Contemplating a possible project involving two pairs of BG RD-75 drivers. Here is what I have in mind. Using one pair for the upper range and the other pair for the mid frequencies. I haven't decided yet which route to take for the bottom end.

I'm asking about this because I'm wondering if there could be any advantage to doing this rather than letting just one pair handle the mids and highs even though they can. My hearing is good to about 17k-17.5k so I don't need to augment any higher than what they can produce.

I understand that splitting frequencies between mid and high can be troublesome because it's lands right where the transition can easily be heard, but would it be easily heard using these (exact same drivers) and placed right next to each other? Would the BG's potentially perform better having only half of the range to deal with?

I'd be using a miniDSP with this project to help dial things in.
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Old 10th February 2013, 09:35 PM   #2
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Kind of a dilemma, the BG RD-75 are weak (and have little excursion potential) in the bottom end, and the 6 dB additional output of running a parallel pair would be desirable if you like some headroom, but side by side placement will cause midrange and high frequency off axis peaks and nulls (comb filtering) due to the center to center distance of the two line sources.

If you really want to pursue a pair per side, I'd suggest a kind of 2.5 way arrangement, one run from around 150 Hz up, the other 150 Hz to around 400 Hz or so.
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Old 10th February 2013, 11:38 PM   #3
Rob41 is offline Rob41  United States
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So at the very least I'll want separation between pairs.

What is the advantage to running one from about 150 Hz up and the other 150 Hz to 400 Hz? I see one is running it's full spectrum while the other is running only up to 400 Hz. How does this work better than splitting the range somewhere in the middle?
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Old 11th February 2013, 04:31 AM   #4
gworrel is offline gworrel  United States
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Drivers should be less than one wavelength apart at the crossover frequency. At 400 Hz the wavelength is 33.9 inches so no problem there. When you get up to 2000 Hz the wavelength is only 6.78 inches so it might be tough to get the drivers close enough above that frequency.

Running two RD-75s per side would be very unconventional. Using two at lower frequencies could allow you to run them lower and with lower distortion compared to just one. At higher frequencies you would get comb filtering. There is a reason you do not see systems with two tweeters side by side.

The RD-75 has been often paired up with a line array of tweeters. Genesis does this in their extremely expensive systems. The reason is not because the RD-75 cannot produce output high enough. The problem is that it starts to beam a narrow pattern long before it reaches 17 kHz. If you are sitting in the sweet spot, then it can sound great, but off axis, not so great.

There is a guy on eBay that has been listing a speaker system with 3 BG RD-75s per side. Actually, he is using the higher performance commercial version that Wisdom audio used to use. I'm not sure how he is crossing them over. I think you can find it there if you search on 'bohlender". He is asking $7000 a pair.
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Old 11th February 2013, 02:30 PM   #5
Rob41 is offline Rob41  United States
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I see. It's a very different situation being side by side rather than vertical like a conventional driver.

The beaming explains why I see so many of them coupled with tweeters.

Thanks for the tip on the guy with three pairs. I sure would love to have those BG's to play around with. A lot of time could be spent tweaking with those.
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Old 11th February 2013, 03:56 PM   #6
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I find it a bit strange that people use tweeters with the RD-75, because it doesn't beam any worse than a typical 3/4" dome tweeter...
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Old 11th February 2013, 04:48 PM   #7
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StigErik, have you measured the off axis response? I haven't but I have noticed a difference in the high frequencies when I am off axis.

John Whittaker, who you're probably familiar with due to his baffle tests and measurements of the RD-75, had this to say in 2003 on the topica dipoles list:

"Today, like most, I crossover the drivers at a much higher frequency, and use them
in the approximate band of 500Hz - 5000Hz with L-R 4th order crossovers.

High frequency beaming of the RD-75 starts around 8KHz, and seems entirely
consistent with the physical width of the driver."

I find it interesting that the driver is used in so many different configurations and yet gets widespread acclaim. You prefer no baffle. Genesis uses a 40" wide baffle and crosses over to the woofers at 96 Hz. ThomasW prefers a 600 Hz crossover to Acoustat woofer panels. John Whittaker used them with his own 1 cm wide full height ribbon tweeter. The baffle that John thought worked best was a sloping asymmetrical baffle narrow at the top and wider at the bottom.

I am using them in your minimal baffle configuration and I think they sound great. I am considering trying some alternate ideas just to see if some of the other ideas have merit.
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Old 11th February 2013, 05:01 PM   #8
jcx is offline jcx  United States
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why not xo to a line array of mid/woofers?
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Old 11th February 2013, 06:18 PM   #9
Rob41 is offline Rob41  United States
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Quote:
Originally Posted by StigErik View Post
I find it a bit strange that people use tweeters with the RD-75, because it doesn't beam any worse than a typical 3/4" dome tweeter...
When I looked at their range it looked adequate in the upper reaches at least to my less than perfect range so I didn't want to go conventional for the highs. Your posts and experience with these also helped form my "wish list".

Quote:
Originally Posted by gworrel View Post
StigErik, have you measured the off axis response? I haven't but I have noticed a difference in the high frequencies when I am off axis.

John Whittaker, who you're probably familiar with due to his baffle tests and measurements of the RD-75, had this to say in 2003 on the topica dipoles list:

"Today, like most, I crossover the drivers at a much higher frequency, and use them
in the approximate band of 500Hz - 5000Hz with L-R 4th order crossovers.

High frequency beaming of the RD-75 starts around 8KHz, and seems entirely
consistent with the physical width of the driver."

I find it interesting that the driver is used in so many different configurations and yet gets widespread acclaim. You prefer no baffle. Genesis uses a 40" wide baffle and crosses over to the woofers at 96 Hz. ThomasW prefers a 600 Hz crossover to Acoustat woofer panels. John Whittaker used them with his own 1 cm wide full height ribbon tweeter. The baffle that John thought worked best was a sloping asymmetrical baffle narrow at the top and wider at the bottom.

I am using them in your minimal baffle configuration and I think they sound great. I am considering trying some alternate ideas just to see if some of the other ideas have merit.
If I read correctly, the beaming being consistent with this width of driver would also be applied to a line array of conventional drivers of similar width?

Quote:
Originally Posted by jcx View Post
why not xo to a line array of mid/woofers?
I didn't want to go conventional for anything accept on the bottom from around 20-25Hz to 350-400Hz. It seems like if I went with a line array of mids, I might as well use more conventional drivers the same way for the highs. Aren't the BG's known for their speed/accuracy/transparency? Maybe I'm just hoping for more than a conventional driver brings to the table.

I haven't decided yet weather to go with something like an array of woofers to augment the lower frequencies or to use sub woofer's to gently integrate with the rest of the system.....or both. Although I cringe at sloppy get your swag on bass, I do like more pronounced bass than a lot of folks.
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Old 11th February 2013, 08:07 PM   #10
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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.
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