Looking for a sealed sub under $1500 to pair with KEF LS50

[Roen]

Unless you have a servo?

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[Verdinut]

Not many sealed enclosures have a servo. A ported box is a lot simpler to build.

 

[shadyJ]

Shady,
Talking of backspring pressure, this is why I never liked the idea of a sealed woofer enclosure. There is definitely some distortion caused by the non-linearity of the moving cone caused by compression and decompression of the moving mass. The extent will of course depend on the size of the box. It will be noticeable but that also depends on the excursion demanded from the transducer.

Once, I had the occasion to talk about that situation with a good friend of mine, the late Denis Ouellet, ex-president of Solen Electronics who was an EE. We were in agreement on this resulting behavior. Him and I have always built ported enclosures.

Unfortunately, when the listening room size, or the WAF does not allow the use of large enclosures, you are limited to the use of sealed cabinets. I wish everyone larger rooms and/or a more tolerant spouse in the future.

There are trade-offs in all designs. Sealed subs can do fine. To be honest, backspring pressure isn't really a problem unless the designer is completely clueless. Distortion from backspring pressure becomes a problem when the woofer is in far too small of an enclosure. It would be manifest as even-order harmonic distortions. I doubt the F15HP or E15HP have a problem with that at all. Still, it would be interesting to see how the slightly smaller enclosure does affect distortion behavior. Backspring pressure can be negated as a factor when the driver has a very powerful motor.

 

[shadyJ]

Unless you have a servo?

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In theory, one of the advantages of servo is that it would be able to compensate for backspring pressure. That would come at a cost though. A servo sub would be more likely to be over-driven if it isn't protected by filters. But even non-servo subs need limiters to prevent overdriving in sealed enclosures. The reality is that the sub designer is going to want to place the driver in a similar sized enclosure where that driver will operate comfortably, servo or no servo. And in a well-balanced design, the most significant distortion mechanism will come from coil vs gap overthrow or overtension on the suspension components, and servo drives wouldn't be able to handle that significantly better than ordinary subs.

 

[KEW]

In theory, one of the advantages of servo is that it would be able to compensate for backspring pressure. That would come at a cost though. A servo sub would be more likely to be over-driven if it isn't protected by filters. But even non-servo subs need limiters to prevent overdriving in sealed enclosures. The reality is that the sub designer is going to want to place the driver in a similar sized enclosure where that driver will operate comfortably, servo or no servo. And in a well-balanced design, the most significant distortion mechanism will come from coil vs gap overthrow or overtension on the suspension components, and servo drives wouldn't be able to handle that significantly better than ordinary subs.

Rythmik's ad copy makes some pretty strong claims relating to the benefits of their servo technology

You say "A servo sub would be more likely to be over-driven if it isn't protected by filters." If so, Rythmik is saying their Direct Servo design does not require them!

And Rythmik claims (bold is mine):

Our philosophy is to make audio products as simple as they can be. Less is more. Our subwoofers put as little into the signal path as possible. This approach applies to our amplifiers, Direct Servo technology and drivers. Most servo subwoofers use an accelerometer, which is another mechanical system with its own limitations. Also extensive protection circuits are required, and these further degrade the signal. As a result, accelerometer based servos are not as accurate as Direct Servo. The superiority of Direct Servo is derived from its simplicity.

Direct Servo uses a very thin sensing coil which is wound adjacent to the voice coil. The coil acts as a custom made microphone which creates a signal which is then used to correct any difference from the original signal. Any non linearities are instantly corrected.

I don't pretend to have a working knowledge of the Rythmik Direct Servo system!
However, are the protection circuits Rythmik mentions the same as the "filters for protection" you mention?

They also make some amazing claims (bold is mine):

A servo is essential in a subwoofer that aims to be true to the original. The benefits are clear:

No voice coil thermal-induced compression or distortions.

Spider and surround distortion reduced by 6 - 9 dB

Flat frequency response that is less dependent on T/S parameters.

Audiophile bass sound at an affordable price: articulate, tight, transparent, and well-defined bass

Applicable to all subwoofer configurations (including horns, dipole, infinite baffle and others)

Higher output (with better excursion utilization) for sealed, IB, and Dipole subs.

Some of the benefits of Direct Servo include:

• Reduction of the effects of thermal compression are eliminated under normal operation
• Higher efficiency allowing output which would normally require a more powerful amplifier
• Deep bass extension can be achieved using a low mass driver with superior transient response
• Mechanical and thermal memory effects are reduced, further improving transient response
• The effective Q value is 0.1 resulting in much tighter and accurate bass than conventional designs
• Dramatic reduction of the re-radiation of bass from inside the box

These benefits are all unique to a servo controlled subwoofer, and can't be achieved in a practical and affordable way in a conventional subwoofer.

I am used to manufacturers saying "reduced" Which avoids the issue of whether the difference is great enough to be significant. However, here they are saying "No voice coil induced thermal distortions"!
Maybe this is an obscure source of thermal distortion? If it is the most common source of thermal distortion, isn't this a huge deal?
Help me out, as I really don't know how much this matters!
The same goes for spider and surround distortion. What impresses me is they provide numbers instead of being nebulous the way most product descriptions are. Brian Ding seems like a true engineer like Dr. Hsu, and I have a hard time believing he would make this statement unless he measured it.

http://www.rythmikaudio.com/technology.html

If we "crowd fund" shipping costs, would you be willing to measure either an F12 or E15HP?

The only Rythmik subs that have been measured are the vented models. I find that strange because Rythmik was a "hold out" among ID sub manufacturers only producing sealed subs for many years. I always felt like market pressure drove him to produce vented subs. To this day, only 4 of his 14 models are vented.
http://www.rythmikaudio.com/products1.html

 

[shadyJ]

Wow, your reply is giving me a lot to chew on! I will have to address these comments individually.

Rythmik's ad copy makes some pretty strong claims relating to the benefits of their servo technology

You say "A servo sub would be more likely to be over-driven if it isn't protected by filters." If so, Rythmik is saying their Direct Servo design does not require them!

And Rythmik claims (bold is mine):


I don't pretend to have a working knowledge of the Rythmik Direct Servo system!
However, are the protection circuits Rythmik mentions the same as the "filters for protection" you mention?

I don't have intimate familiarity with Rythmik's servo mechanism either, but I think I get the theory, so take my comments as speculative. In theory, once the voice coil leaves the bounds of the sensing coil, that could trigger a circuit that reigns in excursion, so that could act as protection. How well it works is another question. I looked over the Rythmik site for some clues, and here is a page that does confirm what I thought about how the servo system could correct for backspring pressure. However, further on it says, "For each model of our subwoofers, we recommend a particular enclosure volume to avoid over-excursion." That looks to be like the servo system is not bulletproof driver protection. Also, comments in reviews by Ilkka, Ricci, and Paul Apollonio suggest that their Rythmik subs were susceptible to mechanical noise when over-driven. Take a look at this distortion chart from Ilkka of a 12" sealed Rythmik:

Obviously the Rythmik can be driven well past Xmax and into some heavy distortion. But can it bottom out and risk destroying the former? That I don't know.

They also make some amazing claims (bold is mine):

I am used to manufacturers saying "reduced" Which avoids the issue of whether the difference is great enough to be significant. However, here they are saying "No voice coil induced thermal distortions"!
Maybe this is an obscure source of thermal distortion? If it is the most common source of thermal distortion, isn't this a huge deal?
Help me out, as I really don't know how much this matters!

As with all things in speaker design, trade-offs are inevitable. Yes, the servo system could compensate for thermal compression, but the problem here is when a system starts compressing, that means it isn't adequately dissipating heat. In a normal system, what happens is the same amount of current is sent to the coil, but the heat causes the electro-magnetic resistance to build, and this reduces the magnetic force of the coil, so the motor system simply does not react as strongly to the same amount of current. The result is a loss of output vs normal operating temperatures. A servo system compensates for that output loss by shoving even more voltage in the voice coil to have the coil positioned where it is supposed to be regardless of thermal conditions.

So the servo system can virtually eliminate the EFFECTS of thermal compression, but in doing so it exacerbates the circumstances that would give rise to thermal effects. This, to me, is more problematic than the effects. Unless there is a temperature sensing system in the driver that can shut down the sub when it gets too close to red-lining (my guess is that there isn't), that puts the driver in greater danger of smoking its coil. The servo's solution makes the underlying problem worse.

The same goes for spider and surround distortion. What impresses me is they provide numbers instead of being nebulous the way most product descriptions are. Brian Ding seems like a true engineer like Dr. Hsu, and I have a hard time believing he would make this statement unless he measured it.

http://www.rythmikaudio.com/technology.html

http://www.rythmikaudio.com/technology.html

I don't think a feedback based system can really help a well-balanced suspension system. When such a system starts to place significant tension on high excursions, the driver is at Xmax, and that is it. I think a servo system might be able to help a suspension system that behaves asymmetrically. Look at some Kms(X) charts, they measure the symmetry of suspension tension both directions of a driver's throw. Here is a relatively good and balanced Kms(X) graph:

It is not perfect, and you can see some slight asymmetry, but that is pretty good. Tension will arrest driver motion at nearly the same time regardless of the direction of travel. The driver is not inhibited prematurely by one direction of travel having more tension than the other. Now let's look at a Kms(X) graph that is suboptimal:

You can see there is a lot more tension on one side of the driver's throw than the other. My guess is that the servo system can compensate for that a little bit, but even then there is only so much it can do. The real solution is simply to have a well-engineered driver. The symptoms of asymmetry in travel will be rapid promotion of even-order harmonic distortion as the amplitude is increased.

If we "crowd fund" shipping costs, would you be willing to measure either an F12 or E15HP?

The only Rythmik subs that have been measured are the vented models. I find that strange because Rythmik was a "hold out" among ID sub manufacturers only producing sealed subs for many years. I always felt like market pressure drove him to produce vented subs. To this day, only 4 of his 14 models are vented.
http://www.rythmikaudio.com/products1.html

While I am definitely interested in testing a Rythmik sub, I would prefer that Rythmik arrange for any testing. They haven't had a subwoofer tested for over 6 years, so I don't expect that to happen.