Sean,
In both my OB designs the crossover for the Eminence Alpha 15A is second order at 200 Hz. At 2000 Hz, the input signal is down 40 dB according to my quick calculations. Are you worried about a 10 dB peak for a signal already down 40 dB with respect to the reference SPL? I am not too worried about it and have heard no evidence of a peak at 2000 Hz from my Lowther OB system which uses two Alpha 15A's per side.
Hope that helps,
Martin
In both my OB designs the crossover for the Eminence Alpha 15A is second order at 200 Hz. At 2000 Hz, the input signal is down 40 dB according to my quick calculations. Are you worried about a 10 dB peak for a signal already down 40 dB with respect to the reference SPL? I am not too worried about it and have heard no evidence of a peak at 2000 Hz from my Lowther OB system which uses two Alpha 15A's per side.
Hope that helps,
Martin
Nelson Pass said:Yep. Designed with OB and FR in mind. I have a nice one
now, but it's going through a revision. If you come to RMAF
you can listen to it.
Thanks!
I'll be building an OB soon. Yesterday I played a bit with some cardboard and passive x-overs. It was a bittersweet experience. In the OB, the sound is very relaxed, but I was disappointed by the passive xo performance and efficiency mismatch between Visaton B200 and Eminence Beta 15. My main amplifier is ZEN V9 without feedback (so, a transconductance amp), and I have hoped that, even if run via passive Xo-s, it could provide some bass boost (As Dick Olsher noticed in his review of the Visaton No-box). However, it didn't work that well. I may repeat the experiment when I build the proper baffles. And I am not really willing to put an L-pad to Visaton B200.
So, my next plan would be this:Biamp. Run Visatons via ZV9, with only a PLL Xo, first order. There could be a small cap at the input, or, since ZV9 output is capacitor coupled, just reduce its output cap at a small value (50-100 uF) to obtain a first order rolloff.
Then, use active Xo/EQ and another SS amp to drive the Betas.Experiment with crossover value and slope until I find the right match. ( I have a gainclone amp just collecting dust, but I doubt that it would be powerful enough after EQ).
Hope I am not far off-track. Any suggestion is welcome.
Regards,
Vix
Quoting Vix: "So, my next plan would be this:Biamp. Run Visatons via ZV9, with only a PLL Xo, first order. There could be a small cap at the input, or, since ZV9 output is capacitor coupled, just reduce its output cap at a small value (50-100 uF) to obtain a first order rolloff. "
It's better to perform the roll-off in front of the amp. This reduces bass energy causing the amp to clip. The damping factor of the amp can still control bass resonance of the upper driver.
Geoff.
It's better to perform the roll-off in front of the amp. This reduces bass energy causing the amp to clip. The damping factor of the amp can still control bass resonance of the upper driver.
Geoff.
Instead of doing a trial and error set of experments, why not do the design work up front and build it once. The chances of you getting an optimum result by cut and try and playing with crossover points and slopes are not so great. Doing the design work will save time, money, your efforts, materials, and produce an advanced design right out of the gate. How do you know that the components you have selected will ever work well in an OB speaker system under any conditions?
The whole point of my article was to optimize by design, rule out the not so great ideas, and understand the significant contributors to the final SPL response. In my opinion, it will take as much effort to design an OB speaker system as it does to design any of the boxed speaker systems.
MJK said:
Thanks Martin! That's where I will need some help from the experienced builders.
The idea was to build a Visaton No-Box: http://www.visaton.com/en/bauvorschlaege/breitband/noboxbb/index.html
However, there are some changes, but I don't see too much difference:
1. Bass Driver: Visaton uses its BGS40. I will use Eminence Beta 15, because it was recommended to me for OB use, and it is quite cheaper than Visaton. (and doesn't look worse)
2. Visaton went with passive X-overs, I want to go active. It should be at least equally good, or better. But, I 'll need some help here, as I am not that good with software...
Best regards,
Vix
Hi Gio,
The polarity does not need to be reversed. If this were a textbook case, pure resistance load and coincident point sources, it would need to be reversed. The picture below shows the SPL and phase of the two sources at the listening position.
http://www.quarter-wave.com/Responses.jpg
The polarity does not need to be reversed. If this were a textbook case, pure resistance load and coincident point sources, it would need to be reversed. The picture below shows the SPL and phase of the two sources at the listening position.
http://www.quarter-wave.com/Responses.jpg
I think that is a terrible idea. A phase plug will have no impact on the frequencies being produced by the woofer in this design (40 to 200 Hz). I have never heard a ringing problem from the four Alphas I am using in my open baffle system. In my opinion the risk of screwing up a good driver are far greater than any slight potential for improvement. Have you tried the Alpha 15A driver in this type of system and heard a ringing at 2 kHz?
Wow, great article. I love it when someone can explain something complex simply. Its a sign of mastery.
I had a question: at the end , "Figure 13: Final Calculated System SPL Response". This graph doesn't take into account the actual drivers frequency response - if it did, the resulting SPL response would no longer be within +/-2dB anymore would it? Some sort of sum of the peaks/dips of fig. 13 and the drivers - but the point of this article was to design the best baffle/xo given the woofers limits on sensitivity, fs and Qts, correct?
Is the inverse of this article true? That if one were to use an active crossover/equalization based on measurements, selection of drivers, baffle shape/size and crossover frequency would not be as critical?
I had a question: at the end , "Figure 13: Final Calculated System SPL Response". This graph doesn't take into account the actual drivers frequency response - if it did, the resulting SPL response would no longer be within +/-2dB anymore would it? Some sort of sum of the peaks/dips of fig. 13 and the drivers - but the point of this article was to design the best baffle/xo given the woofers limits on sensitivity, fs and Qts, correct?
Is the inverse of this article true? That if one were to use an active crossover/equalization based on measurements, selection of drivers, baffle shape/size and crossover frequency would not be as critical?
MJK said:
Hmmm, Impulse response will show in its trail edge. You won't hear it clearly ringing but you will hear it by its absence if fixed. A hobby knife taking out a dust cap is nothing to be afraid of on a cheap woofer. It can be easily glued back. No I haven't done it on an Alpha. But you thinking its an outright terrible concern, maybe it isn't.
I quote Tom Danley: "A bad sign (regardless of cone material) for a direct radiator is a large peak(s) associated with its hf roll off. It is assumed that a steep or low crossover fixes this problem but it is not gone. So, picture a driver with a big mound in its response associated with breakup.
Lets say that mound is 10 dB high above the “flat” response zone, this represents an frequency dependent acoustic gain element AFTER the voice coil.
So, here is where it is not fixed even with a brickwall crossover.
The driver’s motor produces harmonic distortion, that acoustic gain amplifies it by 10X (10dB) for harmonics when the fundamental is N fraction of the peak.
In other words, if the mound were at 1200Hz, the third harmonic of a fundamental at 400Hz is amplified by the acoustic gain even if one had a 120,000dB / octave digimatic crossover at 500Hz. Here it is better to fix the source.'' Its taken from
here . In the attachment is the published Alpha 15A chart.
Attachments
Thank you very much for the positive feedback.
Your understanding is exactly correct. This is the kind of study I typically do when considering a new project for a specific driver. If I decided to pursue the project, the next step would be to aquire the drivers and measure their responses. Then I would make a second pass through the calculations using the actual measured results to see if any tweaking of the design was required. If the driver produced a good listening impression when auditioned on the bench, I would expect the design to be a success.
I think it would be a slightly different set of design challenges. If I were looking to use an active crossover, I would still look for roughly the same driver parameters/relationships and use the crossover for its conveninece and adjustability. I would not use a signficant amount of EQ or boost and would rely on the acoustics of the baffle and drivers. Using a little boost for the bass is fine but if you apply a lot of boosted power to the wrong woofer it will run out of gas. The high Qts Alpha 15A is very efficient and its motions when music is played, even at a loud volume, is very small. I think that this is a big advantage.
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