Open Baffle - Phase 3...
December 10th, 2013, 3:03 pm
OK, some of you have been around long enough to have seen my open baffle progression over the last few years... For those of you who haven't, phase one was an open baffle as seen here: JE Labs Open Baffle with an 8" full-range driver, originally Altec 755Cs, more recently the Dayton Audio PS-220s. The phase one speakers ultimately found there way to CAF 2012 in a beautiful pair of baffles built by fellow DCAudioDIYer Jim Gerfin. The phase one open baffle is a great speaker when driven by a transconductance amplifier (an amplifier that puts out a current proportional to the voltage input rather than the more common voltage amplifier which puts out a voltage proportional to the voltage input). The biggest limitations with the phase one open baffle is the dynamics and the lack of decent low frequency response (although, they will give a surprising amount of bass for an 8" driver).
The phase two open baffle was constructed with the phase one limitations in mind, and used the Urei 801B 15" coaxial drivers. An ugly pair of the phase two speakers were in use at CAF 2013. While these open baffles did a much better job on the bass & dynamics, the drivers are quite difficult to source, and the crossovers used needed some upgrades to address some tonal balance issues, which is still a work in progress.
For those wanting a little light reading
a great writeup on open baffles can be found here: G. A. Briggs on open baffles
Now, on to the phase 3.... Inspired by chris1973's open baffle thread, I decided to get off my duff and do something with the original pair of phase one baffles. I had four suitable 15" drivers, but with the hole already in place for the original 8" driver, there was no way to do a 2x15" driver setup as the hole for the 8" driver wouldn't be covered. I also had a quartet of the Dayton Audio PA-460 drivers that I'd planned to use for the slot-loaded open baffle woofer system, but decided to put that on hold and use a pair for the phase 3 open baffles. Using a single, centered 18" driver, I was able to cover the original 8" hole, and move the Dayton Audio PS-220s full-range driver up about 9" to make a workable 2-way arrangement. I got them hooked up and working Sunday evening, so there's still a lot of tweaking (NO twerking!) to do. Currently bi-amping with the transconductance amp from the 2012 CAF for the bass, and the one from CAF2013 for the rest, xover is currently at 200Hz, but I expect that'll change as I play. Using a crappy old Behringer analog xover (ask Bruce) but the sound is quite promising. And, they can do some bass They do seem to have all the positive points of the phase one system, with the added low-end impact. The drivers seem to integrate pretty well (once I got them in-phase, the drivers are actually out of phase based on the red terminals, guess that's part of the price you pay for Chinese drivers 
). Plan to try the 300B monoblocks on the PS-220s tonight, they worked pretty well on the PS-220s in TQWPs at CAF 2012, so I'm optimistic. 
The phase two open baffle was constructed with the phase one limitations in mind, and used the Urei 801B 15" coaxial drivers. An ugly pair of the phase two speakers were in use at CAF 2013. While these open baffles did a much better job on the bass & dynamics, the drivers are quite difficult to source, and the crossovers used needed some upgrades to address some tonal balance issues, which is still a work in progress.
For those wanting a little light reading
a great writeup on open baffles can be found here: G. A. Briggs on open bafflesNow, on to the phase 3.... Inspired by chris1973's open baffle thread, I decided to get off my duff and do something with the original pair of phase one baffles. I had four suitable 15" drivers, but with the hole already in place for the original 8" driver, there was no way to do a 2x15" driver setup as the hole for the 8" driver wouldn't be covered. I also had a quartet of the Dayton Audio PA-460 drivers that I'd planned to use for the slot-loaded open baffle woofer system, but decided to put that on hold and use a pair for the phase 3 open baffles. Using a single, centered 18" driver, I was able to cover the original 8" hole, and move the Dayton Audio PS-220s full-range driver up about 9" to make a workable 2-way arrangement. I got them hooked up and working Sunday evening, so there's still a lot of tweaking (NO twerking!) to do. Currently bi-amping with the transconductance amp from the 2012 CAF for the bass, and the one from CAF2013 for the rest, xover is currently at 200Hz, but I expect that'll change as I play. Using a crappy old Behringer analog xover (ask Bruce) but the sound is quite promising. And, they can do some bass
They do seem to have all the positive points of the phase one system, with the added low-end impact. The drivers seem to integrate pretty well (once I got them in-phase, the drivers are actually out of phase based on the red terminals, guess that's part of the price you pay for Chinese drivers ). Plan to try the 300B monoblocks on the PS-220s tonight, they worked pretty well on the PS-220s in TQWPs at CAF 2012, so I'm optimistic.
Re: Open Baffle - Phase 3...
December 11th, 2013, 12:00 pm
Go get 'em, Roscoe! Let us know how they sound!
Re: Open Baffle - Phase 3...
December 11th, 2013, 12:15 pm
Sounds interesting!
Re: Open Baffle - Phase 3...
December 13th, 2013, 11:20 am
Well, I had an issue with the 300B monoblocks, bad 27s, didn't have time to look around for good ones, so pulled out an old QSC USA400 solid state pro-audio amp (TomP would be proud) to use for the bass amp. As expected, the bass is way over-damped with the QSC amp, putting 25ohms in series with the output helped some, but I need a higher source impedance to get the bass where it needs to be w/o EQ, but I've about run out of gain control on the xover to make up for the lost gain with the series resistor, so going to a higher resistance is not an option....
However, tomorrow I should have the cables to hook up a Behringer DCX2496 digital xover, which may give me some more room to play with the gain, but also will let me dial in some EQ....
More updates as things happen.
Roscoe
However, tomorrow I should have the cables to hook up a Behringer DCX2496 digital xover, which may give me some more room to play with the gain, but also will let me dial in some EQ....
More updates as things happen.
Roscoe
Re: Open Baffle - Phase 3...
December 13th, 2013, 12:21 pm
Roscoe:
Regardless of how much series impedance or amplifier power you have, you can't fight the laws of physics. With an open baffle that size you cannot get low bass. No matter how much drive you put in, you will increase both the front and back wave and cancelaltion will result. Gee, that might get you to think that is why they built enclosures in the first place.
You can certainly bump up the upper bass where the wavelength is shorter by increasing amplifier impedance or using an equalizer, but that results in exactly that, a bump, not flat response. If that is what you like, go for it. It is the same trick the mini-monitors use to create the illusion of bass. Look at the frequency response curves of them and you will always see a bump between 100 and 120 Hz to hide the fact that they have no bass. After listening to that for a while you can start to feel that real bass sounds like that. NOT!
Regardless of how much series impedance or amplifier power you have, you can't fight the laws of physics. With an open baffle that size you cannot get low bass. No matter how much drive you put in, you will increase both the front and back wave and cancelaltion will result. Gee, that might get you to think that is why they built enclosures in the first place.
You can certainly bump up the upper bass where the wavelength is shorter by increasing amplifier impedance or using an equalizer, but that results in exactly that, a bump, not flat response. If that is what you like, go for it. It is the same trick the mini-monitors use to create the illusion of bass. Look at the frequency response curves of them and you will always see a bump between 100 and 120 Hz to hide the fact that they have no bass. After listening to that for a while you can start to feel that real bass sounds like that. NOT!
Re: Open Baffle - Phase 3...
December 13th, 2013, 2:54 pm
tomp wrote:Roscoe:
Regardless of how much series impedance or amplifier power you have, you can't fight the laws of physics. With an open baffle that size you cannot get low bass. No matter how much drive you put in, you will increase both the front and back wave and cancelaltion will result. Gee, that might get you to think that is why they built enclosures in the first place.
That's a gross distortion of the physics involved. Except in the case where there's 100% cancellation (which, on-axis, only occurs at 0Hz) increasing swept volume will always result in an increase in output. Yes, the AMOUNT of cancellation will increase, but the PERCENTAGE of the signal that is cancelled will remain the same. Until one runs out of speaker displacement capability, one CAN compensate for the baffle roll-off (which is a fairly mild 8dB/octave for a baffle on a single surface, a.k.a. on the floor). See the Briggs article referenced in the first post.
You can certainly bump up the upper bass where the wavelength is shorter by increasing amplifier impedance or using an equalizer, but that results in exactly that, a bump, not flat response.
Increasing amplifier impedance will only give a bass 'bump' at those frequencies where the speaker impedance rises, which occurs a: near resonance & b: at frequencies where the voice coil inductance creates significant reactance. In my application, b is well above the frequency range the woofers are being asked to reproduce, which leaves only resonance. The woofers I'm using are specified to have a 28Hz resonance, not quite upper bass, at least not in my dictionary...
If that is what you like, go for it. It is the same trick the mini-monitors use to create the illusion of bass. Look at the frequency response curves of them and you will always see a bump between 100 and 120 Hz to hide the fact that they have no bass. After listening to that for a while you can start to feel that real bass sounds like that. NOT!
Of course not. Now, if we can design a 6.5" woofer that a: has a 6" linear x-max and b: can handle the power it'd take to move it that far in a small box, we'll be rich
Roscoe
Re: Open Baffle - Phase 3...
December 13th, 2013, 4:06 pm
Roscoe:
If what you say is true, why does the output on an open baffle drop at 12 dB/octave when the baffle dimensions approach the wavelengths being reproduced? Normally in a closed box that roll off happens below resonance. Remember. the wavelength at 200 Hz is about 68". Even on the floor at 8dB/octave the rolloff is still quite substantial. I guarantee you,that with the baffle sizes you are working with you will not find a driver that will have sufficient linear volume displacment to make up for that rolloff and give a reasonable output at low frequencies.
If you are satisfied with low amplitudes then you can get low frequencies with limited displacement. Or if you are working in a very small volume such as with in ear headphones the displacement requirements also get quite low. However, in a normal room you are fightiing a losing battle in my opinion. You also have to remember the effects of ear sensitivity, ie Fletcher/Munsion curves as the frequency goes down which further complicates the issue of perceived bass.
Believe me I am very much in favor of dipole bass as seen with my system. However, even with six high excursion 10" woofers per side and a total power of over 700 watts per side, I still only run it down to 90 Hz because of the inability to get much amplitude below that. The sealed box subs take over from there.
If what you say is true, why does the output on an open baffle drop at 12 dB/octave when the baffle dimensions approach the wavelengths being reproduced? Normally in a closed box that roll off happens below resonance. Remember. the wavelength at 200 Hz is about 68". Even on the floor at 8dB/octave the rolloff is still quite substantial. I guarantee you,that with the baffle sizes you are working with you will not find a driver that will have sufficient linear volume displacment to make up for that rolloff and give a reasonable output at low frequencies.
If you are satisfied with low amplitudes then you can get low frequencies with limited displacement. Or if you are working in a very small volume such as with in ear headphones the displacement requirements also get quite low. However, in a normal room you are fightiing a losing battle in my opinion. You also have to remember the effects of ear sensitivity, ie Fletcher/Munsion curves as the frequency goes down which further complicates the issue of perceived bass.
Believe me I am very much in favor of dipole bass as seen with my system. However, even with six high excursion 10" woofers per side and a total power of over 700 watts per side, I still only run it down to 90 Hz because of the inability to get much amplitude below that. The sealed box subs take over from there.
Re: Open Baffle - Phase 3...
December 13th, 2013, 4:32 pm
Roscoe:
I've just been thinking (danger Will Robinson). How about a meeting at your place sometime after the holidays where I bring my subs over (grunt). We can listen with your basic system and then add my subs while at the same time using a high pass to unload your system. In addition to the higher bass levels, the reduction in distortion that occurs when your woofer does not have to beat it's brains out trying to reproduce the low frequencies will be readily apparent in the rest of the range it is reproducing.
I have mentioned this is several articles for the magazine and in particular a test I did of the B&W 602 speaker. It was quite a good speaker but with material having substantial bass output, the upper bass and midrange got congested. Once I used a sub and a high pass on the 602, the upper bass and midrange dramatically cleared up and it could also play much louder.
Tom
I've just been thinking (danger Will Robinson). How about a meeting at your place sometime after the holidays where I bring my subs over (grunt). We can listen with your basic system and then add my subs while at the same time using a high pass to unload your system. In addition to the higher bass levels, the reduction in distortion that occurs when your woofer does not have to beat it's brains out trying to reproduce the low frequencies will be readily apparent in the rest of the range it is reproducing.
I have mentioned this is several articles for the magazine and in particular a test I did of the B&W 602 speaker. It was quite a good speaker but with material having substantial bass output, the upper bass and midrange got congested. Once I used a sub and a high pass on the 602, the upper bass and midrange dramatically cleared up and it could also play much louder.
Tom