Splitting frequencies between two BG RD-75's

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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.
 
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.
 
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?
 
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.
 
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.
 
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.
 
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".

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?

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.
 
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.
 
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. :D

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.
 
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.
 
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.
 
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.
 
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.

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.
 
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?
 
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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.
2012-11-18%252011.52.58.jpg
 
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.

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.
2012-11-18%252011.52.58.jpg

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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