B&G RD-75 dipole - baffle and or waveguide?

[hazard500]

So my ribbons are not yet complete but I can say that the ones made by Steve and Chris sound brilliant. Chris uses the same 50 watt class A mosfet amps that I do, in bi-amp mode, so I am able to make very good comparison. While the RD75 is very good (excellent) in all regards, these ribbons are better in every way and I have not found a weakness. They have incredible speed and air, and while I have not measured the response I am sure that it does have a more extended HF than the RD75. A great project for anyone. I can get details magnets etc if anyone wants.

[StigErik]

Yes, please share.

Measurements would be very nice to see. Is it possible for you to provide any?

The reason I ask, is that I've seen a few DIY ribbons, and they all measured badly. One pair was -15 dB at 20 kHz...

[Carl_Huff]

I hope that I am not being disruptive by posting here. I've just come across this thread. Early on there was a reference to 'airfoil shaped' baffles and their benefits. I have a pair that was cut on a CNC that have served me very well. Their shape was defined by the work that Rudi Blondia and John Whittaker did in the study sited previously. I used them for years and only recently pulled them out of service to experiment with other loudspeakers. In any case here they are. The last PIC does the best job of showing the airfoil shape.









I crossed them over to stock JBL Cinema cabs at 240Hz. They sounded great! My only complaint was that they got 'beamy' from 8kHz up. I was always gonna add some kind of tweeter array to the front of the baffle to fix that but never got around to it.

I highly recommend airfoil shaped baffles for these drivers!

[hazard500]

Carl, very interesting - do you know the reasoning to have ab aerofoil on one side, and no baffle on the other?

[hazard500]

Originally Posted by StigErik

Yes, please share.

Measurements would be very nice to see. Is it possible for you to provide any?

The reason I ask, is that I've seen a few DIY ribbons, and they all measured badly. One pair was -15 dB at 20 kHz...

Try this - Chris not sure but thinks he used software called WinMLS, and a panasonic microphone. Very flat up to 20kHz then a roll-off after that. Not sure if the roll-off is a function of the software, or a function of my power amp design (output stage is a mosfet in common source configuration, it will suffer higher input capacitance than a source follower which is more common, but I've never actually measured the response of the amp).

[Carl_Huff]

Originally Posted by hazard500

Carl, very interesting - do you know the reasoning to have ab aerofoil on one side, and no baffle on the other?

Yea, that came out of the Blondia & Whittaker study that was sited earlier. The airfoil shaped baffle that was published in AudioXpress magazine had the same feature. Altho I lived in the midwest at the time, I was in close communications with Rudi and John as they did their gymnasium experiments that are documented in their paper. I tried several baffle types (some are documented on that website). The airfoil baffle that I currently have is head and shoulders above everything else that I tried. The 'baffle missing on one side' definitively has an affect on soundstage, it improves it!

I just went looking for plots of the measurements that I took when I had the RD75s in service. I no longer seem to have them. I used the active crossovers in a Datasat AP20 at the time. The EQ that I used was Dirac Live that ran as part of the AP20. What I remember is that after extensive listening tests the RD75s sounded best when crossed aggressively at 240Hz. A knowledgeable person who uses this technology in a commercial product suggested that I try a lesser aggressive rolloff at 330Hz. I never got around to trying that.

I remember that the resonant peak in my RD75s were not at the same frequency. One was about a kilohertz higher than the other suggesting that the diaphragm was stretched tighter on that one.

That same knowledgeable person told me that their product has multiple diaphragm elements that radiate the high frequencies. That was how they got around the 'beamy' quality that I had described. He seemed confident that my tweeter array idea fixed to the front baffle and crossed between 6.5kHz to 8kHz would work just as well.

I remember after EQ the RD75s were quite flat out to 8kHz where they started rolling off. Without EQ they were well down by 12kHz and gone by 15kHz. I use a calibrated Earthworks M30 to take all of my measurements.

What I do remember about the RD75s was how wonderful they could sound when properly EQ'd. I am tempted to drag them out of that spare bedroom where they are stored and press them back into service!

[StigErik]

Hm..... my RD-75's are essentially flat from 200 to 15 kHz (apart from the response peak at 5.5 kHz), and approx 8 dB down at 20 kHz. I have not baffle at all, just some foam tubing at the edges to reduce diffraction.

[Carl_Huff]

Originally Posted by StigErik

Hm..... my RD-75's are essentially flat from 200 to 15 kHz (apart from the response peak at 5.5 kHz), and approx 8 dB down at 20 kHz. I have not baffle at all, just some foam tubing at the edges to reduce diffraction.

By 'down at 12kHz' I meant down 3dB and 6dB by 15kHz, not too much different than what you describe. Perhaps the 'one side baffle' and the 15 year age of my drivers plays into this as well. In any case I was very happy with them. My only annoyance was the 'beaminess' that I described.

[StigErik]

Mine are less than 1 dB down at 15 kHz, and I don't experience any beaming problems either. At 30 degrees off-axis they are flat to 10 kHz. Actually, a typical 1" dome is worse.

Could there the a difference between new and older RD-75's ?

[Carl_Huff]

Originally Posted by StigErik

... Could there the a difference between new and older RD-75's ?

That could be! Stranger things have happened. Maybe I should inquire as to it would cost to have mine rebuilt. BG in Carson City is about a 5 hour drive from Bakersfield.