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It's like a good meal - you have to start with quality ingredients. Yes, the end result is the most important thing.
The servo cannot correct everything.
No doubt! I think the Rythmik subs are quality ingredients. The end result IMO is awesome. In your opinion do you think that you would rather this (TC Sound) woofer over any servo driven unit?I look forward to more info on yours.
If I thought servo was the best way to design a subwoofer I would be doing it.
I don't think so, I believe a more appropriate question would be:To phrase it another way, would you buy an amplifier that relied on enormous amounts of negative feedback in order to make it listenable accurate?
Such as? You don't think the benefits outway the slight (in your opinion) negative?
If you had accurate speakers, the term "listenable" would be synonymous.Thanks for pointing out that this is not always the case!
OK that makes sense. What amp are you going to use then? Many of the well known subwoofer guys are going servo. Why do you think that is? These companies have many engineers / resources...Rythmik (which is a patented unit) obviously doesn't but Brian is a genious and has thousands of hours into this unit. I've had many conversations with him before purchasing my units from him years ago.
If you don't mind compression and limiting the ouput capability then it may work for you. Don Keele is visiting here and I spoke with him about it tonight. He mentioned the fact that our ears are less sensitive to low frequency distortion which is really the only reason for using a servo. The servo can limit cone movement but it can't correct for things like low motor strength or suspension limiting.
Actually a few reasons for them to do it that have nothing to do with sound quality. First, they can compress the output in the lower octaves which will save on warranty repairs from damaged coils due to extreme excursion. Second, they can reduce the driver cost due to cheaper motor structures / coils / suspensions.
Needs a serious box, like jtwrace's aluminum box!
I heard the paper GR/Rythmik OB subs, I was surprised that they had less resolution and weaker dynamics than my floorstanders with 11" Etons in reflex behind 2nd order passive xo. In contrast, I have heard a couple of Rick's sealed subs, with and without room correction - some of the clearest and most involving bass I've heard. I built a sub for a friend with TC 15" driver last summer. The driver is a monster. Needs a serious box, like jtwrace's aluminum box!
I don't know enough here to add anything. Maybe Brian (Rythmik) can add something...I certainly can't with my knowledge.
I don't have a problem with Brian explaining his design but I think it would be more appropriate to continue the discussion elsewhere. This thread has already steered way off course. You can imagine the problems it would create if one of my customers went on another manufacturer's circle and posted why everyone should buy a Selah Audio sub versus the "XYZ" circle manufacturer's subwoofer.
I am not trying to sell servo here. But this reply is more towards the questions that some members has asked. If I don't answer here. They would continue to carry those misconception and spread everywhere else. So just bear with me for a moment. First and formost, I respect Don Keele. His work on motor structure analysis is classic. I still have his paper in my paper collection. But two things are very imporant, first, just because someone is working in the field of acoustics does not mean he is authority on everything in that field. It is like doctors. You go to different doctors for different problems. That is why we have expert making wrong remarks like what Dr. Bose once said something to the effect that "in order to reproduce the sound of an instrument faithfully, the size of the speaker needs to be of similar dimension size. Or something like the citation Bob Carver used to get his patent approved and made the point that his compact sub with 2kW amp solves a problem that an audio textbook (I forgot which one) said cannot be done. For those ever apply for a patent know that solving a problem that "experts" deemed unsolvable is a strong ground for granting a patent. The patent was later disputed and revoked beause the author of textbook was wrong. I have been asked many times if my background is acoustics or electrical/electronics. Servo happens to be a field that requires knowledge from both fields. Second, it needs the right person with the right skils to make servo works. SEAS had been working several years on servo. One time I was introduced to their staff engineer in winter CES and he asked me if mine works because they have problem getting theirs t work. The trick is my patent uses a combination of sensing feedback and current feedback. It is the current feedback that solves many previous encountered problems. Now answers and clarification:1) "The servo can limit cone movement". A lot of customers during their first call told me servo could prevent bottoming. That is incorrect. I was surprised that they make comments like this. Amplifier does not have un-limited power. Once the amp is into clipping, it behaves the same as nonservo subs, that is, the feedback has no effect at all because there is no room for the amp to make further correction. So in servo subs from Paradigm and Velodyne, it is the limiter that prevents the amp from getting into clipping, and hence bottoming. But any nonservo amp can also add a limiter to prevent bottoming. Anyone can buy a 10 year old used Velodyne servo and bypass its limiter and see how the servo design can prevent bottoming. The answer is servo does not prevent the driver from bottoming. 2) "but it can't correct for things like low motor strength suspension limiting". Very true. That is why we use drivers with good motor size and uses deep basket to support the excursion that the driver would not bottom out during moderate to heavy amp clipping. Very interesting that you touched two subjects: motor strength and suspension. First motor strength. When I worked with NCA in north California (in early 90's), Glen told me a story that this customer designed a driver with huge motor structure, but later on found the high BL value (a trait from over-designed motor) has made it unsuitable for sealed sub application. Guess how the company solved the problem? Widen the gap. Instead of reducing the motor size, they chose to defeat the purpose of having a larger motor by widening the magnetic gap. In short, larger motor is often related to wide magnetic gap. In the era of Don Keele, that would be an absolutely no no. Now second subject: suspension. Spider and surround both have distortion and because it is the only part that actually change its shape during cone movement, they contributes a lot of dynamic distortion that measured like noise. Most peopl don't understand the difference between noise and distortion, not to mention dumb measurement equipments that require engineers to interpret the results. In Klippel test, it brings in another dimension time (which I will also touch later). 3)"A servo cannot correct non-linear distortions, like the type you'd see with poor Klippel BL measurements." But the servo do reduce the distortion. yet you do bring up a good point. Here is the missing link. We actually have two BL measuement curves: one for driver coil and one for sensing coil. They don't look the same at all. Our sensing coil is far more linear than driver coil because it is ultra overhang design. I will post one sensing coil curve when I return to office on Monday. But in short, it is the technique that does not affect the efficiency of driver coil. All techniques linearizing BL curve achieve the linearity by sacrificing the efficiency. If one looks at a normal BL curve from Klippel, it resembles a upside down U. The middle peak representing the max BL value location. What a linearization voice coil technique do is it will bring down the value of the peak BL value so that it can align with lower Bl values at say 10mm or 20mm excursion point. Our servo technique does not make that type of compromise. I would not elaborate further as I am just trying to make me point here in defense of the critics of servo design. 4) Sound improvement from servo vs nonservo. I personally compare sound from servo and nonservo over and over again. I only have one customer who bought a nonservo kit and servo kit at the same time and his comment was the nonservo is very good, but the servo is the sweet deal. He was so excited and asked me when I can extend extend the technology to midrange drivers. I wish I can have 48 hours a day to finish that project. My personal experience is the servo improves the spatial clue of the sound stage. On the other hand, the nonservo version of our drivers tend to have more noise that correlated only occasionally to the source. Listener cannot really connect that type of noise/distortion to the original source. so coherence is the key. with servo, instruments no longer warble or with ill-defined image. When I switch from 28hz to 14hz extension, the sound stage becomes deeper, wider, and taller. The sound is so coherent that it conveys the emotion of the performers in totality, giving me goosebumps. Now back to the coherence in BL plots from Klippel. For those who are familiar with Klipple system, the BL curve was a "curve fit" result from a test window. If one move the window in the viewer program, it gives a different BL curve. In short, when we are into finer and finer granularity of analysis, everything is more or less related t statistical behavior. Let us look at fast Fourier transformation (FFT), it is a result of averaging over a window. However, our hearing don't do averaging the way those measurement instruments do. That is why there seems to be a disconnect from measurement results to actual listening experience. We cannot control our hearing like how we manipulate testing results. not to mention limitation such as how we measure the depth of sound stage with instrument. We are still exploring the direction not formally investigated and yet we have people constantly quote findings from 40 years. The latter completely undermines any new findings. In short, servo is the technique to improve coherence in bass. Lastly, sorry for the long post. It is simply to hard to me to read tthe critics without answring them, in particular when I am in a hotel in Las Vegas that my wife is waiting for me to get out of Hotel room.Plase just thnk of as a point of record.
Richidoo,Were those the units you picked up for Carl and Carl later sold to George? If so I have read that George had to rewire them to get them to play properly. I believe this was done during a GTG at George's if I remember correctly. It is possible you have not heard them at their best, unless of course those aren't the set you are referring to.Best,Ed
