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Frequently Asked Questions

Rythmik Audio Subwoofers

Is Direct Servo stable?
Do servo subs need a Linkwitz Transform?
How is the frequency response measured?
What is the damping factor between driver and amp?
Do I really need a flat response down to 14 Hz for music?
How does Direct Servo achieve a flat response in so many different configurations?
What is the amount of equalization present in Direct Servo?
Why do non servo subwoofers seem to have a higher crossover frequency than servo subwoofers?
Does the Direct Servo sensor make the sound slower?
What don't we sell passive speakers or non-servo amplifiers?
How does push-pull technology compare to servo technology?

Custom Install products & Do It Yourself (DIY) issues

What are the important issues in building a sealed enclosure?
Why can't I use a sealed enclosure larger than the recommended size?
Can I use a 1 cu ft sealed box?
What is the trade-off between sealed and vented box?
Do the Damping factors, Bass Extension controls work with the vented solution?
What happens if I build a vented solution and plug the ports, like the SVS and HSU subs? Are there any changes required to the circuit boards to "convert" from vented to sealed?
What are the important issues in constructing a vented or PR enclosure?
How much polyfill do I need in the enclosure?
Are the drivers used for Direct Servo subs different from regular drivers?
Can I use a regular dual voice coil subwoofer in a Direct Servo sub?
What is an aperiodic enclosure?

I have a very large room, should I choose vented box over sealed box?
When should I use the XLR version over PEQ version
Is is possible to achieve a higher crossover point by removing the low pass crossover?
There are quite a few large, high excursion drivers with heavy moving mass and/or high inductance. The supplier recommended a low crossover point. Is that a sound idea?
How does Direct Servo affect the physical property of drivers so that I can feel it in action? And how does it act differently than accelerometer-based servo subwoofers?
What is the purpose of rumble filter?
Does the phase control on the plate amp provide a fixed phase shift to all frequencies?
How long does it take for a subwoofers to fully break-in?
Can memory effect be measured?

Advice on choosing a subwoofer

Can a subwoofer reproduce bass as accurately as high end speakers?
Do I need a Direct Servo subwoofer?

What is the difference between a subwoofer designed for music or home theater?
Which of your subwoofers should I buy?
Which has more SPL – subwoofer A or subwoofer B?

Sound Quality

What makes the bass sound fast, is there really a thing called fast bass?
What is the difference between noise and distortion?
Why do I need to add a sub when I already have an audiophile grade bookshelf speaker system?
I am concerned that applying filtering to my speakers will degrade the sound quality?

Other issues

I hear hum from the subwoofer, how do I get rid of that?
How do I check and replace fuse?
How do I connect your subwoofers to my integrated amplifier for 2-channel setup?
Can I connect your subwoofers to my computer?
Isn't each driver is designed for an optimal box configuration?
Can I connect the speaker output from my digital power amplifier to the speaker input of the sub?
How hot will my subwoofer amplifier become?

 

 

Rythmik Audio subwoofers

Is Direct Servo stable?

When it comes to DIY servo subs, the first concern that comes to mind is the stability. Servo subs at one point had bad press because too many poorly executed projects were out there. These were too often done on a trial-and-error basis, without considering the control system or theories behind them. We can assure everyone that Direct servo system is absolutely stable.

A few of the features of the Direct servo system can further demonstrate that. First, the system is stable even when the amp is in clipping. Most accelerometer-based systems are not stable when the amp clips. So the protection circuit in those systems is a necessity, rather than a value added feature. We choose not to put in any anti-clipping circuitry, because such circuitry reduces linear dynamic range. Second, there is no relay in our amps. Some servo system cannot work without relays of instability during power-on. The power on noise of our system is just like that of a regular non-servo system.

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Do servo subs need a Linkwitz Transform?

Servo subs in general do not need Linkwitz Transform circuits. Our Direct servo subs don't. The Direct servo subs are closed loop systems where a servo signal that monitors the movement of driver's cone is fed back to the amplifier to achieve lower distortion, as well as a more coherent sound over a wide range of working conditions. LT-based subs are open loop systems. Flat response is achieved by predicting the transfer function between the amplifier output and the speaker output and pre-equalizing the amplifier output with the inverse of that transfer function. However, as the driver parameters change due to signal level, break-in, and fluctuating voice coil temperature, the actual transfer function between amplifier output and speaker output would also change. When that occurs, the output from the subwoofer will no longer be flat.

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How is the frequency response measured?

We measure all subwoofer frequency response outdoors, using the near-field technique. For vented subs, measurements from the woofer and vented are taken separately. These two measurements are then summed after the levels of both measurements are adjusted so that they have same output at 1.4 times the enclosure/port resonance frequency.

measuring a driver

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What is the damping factor between the driver and amplifier in Direct Servo subs?

Close to infinity. Keep in mind damping factor is not the same as the damping of bass extension. More accurately stated, damping factor is the ratio of speaker impedance vs amplifier output impedance. For most amplifiers, damping factor ranges from 20 (tube amplifiers) to 200 or even higher. In the past, higher values are preferred because they imply the frequency response of the amplifier is less sensitive to the speaker's impedance. In a conventional (or non-servo) amplifier-speaker interface however, higher damping factor does not translate to real-world benefits because it does not include the impedance of speaker wires, or even the DC resistance of the voice coil for that matter. One should keep in mind, the voice coil DC resistance is not constant, rather it is temperature-dependent. A high power subwoofer driver means the voice coil can tolerate higher voice coil temperature. What is not often mentioned is that when the voice coil is at a higher temperature, the DC resistance also increases as a result. Copper wire has a temperature coefficient of 0.4% per degree C. A 125C (or 225F) temperature increase causes a 50% increase of the voice coil DC resistance, which in turn causes the Q value of the speaker response to change by a similar amount. In a Direct servo sub, the subwoofer response is completely independent of the voice coil temperature. Or to be more precise, the resistance on the path from amplifier output to speaker, including resistance of speaker wire, connectors if any, and voice coil resistance, does not change the frequency response at all. As a result, its damping factor is infinite, even by the most stringent definition.

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Do I really need a flat response down to 14 Hz for music?

The simple answer is yes. The more accurate, more complicated answer is that it depends on the music you are listening to. On dance mixes and music with extremely high bass contents, one does not need 14 Hz extension because of what is called the "masking effect". The masking effect occurs when a high amplitude signal is present simultaneously with other smaller amplitude signals. The perceived loudness of the smaller amplitude signal is drastically reduced. In addition, human ear's sensitivity decreases in the lower frequencies. Therefore for this type of music, one may even want to set the extension setting higher with lower damping factor setting (in our A350/A370 amps). That will make the bass sound clean (by removing ambient information) and punchy. It essentially allocates more amplifier power to the signal that we hear and care about most. On the other hand, when no such signals are present (for example, in jazz, solo and other types of music) and masking effect is not strong, one can clearly hear the difference that a 14 Hz extension makes (vs say 28 Hz extension). It is not a difference that will have you jumping out of your seat say "there's the bass". Rather it is an improvement in two areas: ambience and dynamics.

Ambience: good recordings normally capture a lot of ambience information which is predominantly low frequency signals. It is generally agreed that the lower the sub plays, the deeper, taller, and wider the sound stage becomes (or one may say open and spacious).

Dynamics: during musical transitions, a lot of the non-harmonic content is produced. Most of this content is in the very low frequency band, which will be filtered out if the bass extension is not low enough.

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How does Direct Servo achieve a flat response in so many different configurations? It sounds too good to be true.

Direct servo can be thought of as an adaptive equalization system. To be more specific, it performs a transformation on the physical T/S parameters into a set of virtual T/S parameters. By doing so, the amplifier adjusts its output so that the frequency response behaves as if the driver possesses these virtual T/S parameters. As a result, the Direct servo can be analyzed just like normal drivers and the technology can be applied to any known subwoofer configuration. All one needs to do is change the feedback network.

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What is the amount of equalization present in Direct Servo?

The amount of equalization present in the sealed box configuration can be as high as 10 dB at 20 Hz if the extension is set to 14 Hz. That is also why we use a high power amp and driver so that it has enough headroom for equalization.

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Why do non servo subwoofers seem to have a higher crossover frequency than servo subwoofers?

Our Direct Servo subwoofers utilize a frequency region that voice coil inductance has the least effect in impairing the cone control. That is the reason we limit our crossover to frequencies lower than non servo subwoofer. It is possible to achieve a crossover as high as non servo subwoofers, but performance is compromised as the effectiveness of the servo is reduced.

In practical use, many subwoofers lack adequate accuracy to be crossed as high as ours. Often one needs to cross low in order to "minimize the damage", since most subwoofers have inferior sound quality to the bass of the main speakers.

Does the Direct Servo sensor make the sound slower?

Ask any one of our many satisfied customers and they will tell you emphatically "definitely not!" If by "fast bass" you mean more true to the original, then quite the opposite is true.

Our servo is based on sensing coil, and it does not have any additional mechanical assembly (unlike the accelerometer-based design, which is a mass attached to a pressure sensitive material to sense the inertia of mass acting on the material). There is no mechanical delay in our voice coil assembly. Also the location of the sensing coil is "co-centered" with the driver coil, immersed in the same magnetic gap. The moment the driver coils starts to move, the sensing coil will detect that and start reacting. In other words, the distance between sensing coil and driver coil is "zero". This is very different to accelerometer-based approach, which place the sensor on the cone under the dust cap. There is a finite distance between the voice coil and the accelerometer, and as a result there is a response time. It is the same with another approach used in Germany that put the sensing coil in its own magnet gap. We don't have these problems.

If the word "slow" is used with reference to the extra length on the voice coil, that is even less understandable. The voice coil is used both for driving and sensing. If the voice coil is slow for sensing, it will also be slow for the for driver coil? Does that mean all voice coil based speakers will be slow? The wire length used in voice coil is probably shorter than the transformer in a tube amp. Does that mean the tube amps will sound even slower? Better yet, get rid of the inductors in crossover networks.

What don't we sell passive speakers or non-servo amplifiers?

The musicality of our subs come from the synergy of the amplifier and the driver operating under servo control. If you have only our amp, or just our driver, the synergistic effect is lost. That is why Rythmik is very unique and why we don't bother getting into the component business.

Our subs sound very musical because they have very low memory effects. All passive drivers driven with non-servo amplifiers exhibit three major drawbacks: (1) the temperature of the voice coil changes the frequency response, (2) the hysteresis memory effect of the spider reduces the clarity of the sound, and (3) because the spider and surround are mechanical parts, the impact to frequency response changes both in short term and long term. Our servo operation improves all of the above and it achieves a level of sound quality beyond what the same driver and amplifier combination can achieve without servo control. Consequently, we prefer to sell customers the driver and servo amplifier combination which allows those components to reach their full potential.

How does push-pull technology compare to servo technology?

We still think our servo subwoofers offer more advantages:

1) The idea of push-pull is good. But the driver with reverse cone has smaller cone surface (because the voice coil is exposed), so the distortion cancellation is not as good in practice. In practice, we found just 2x or 3x reduction on even order harmonic distortions. In order to get best distortion reduction, the driver with reverse cone needs to have 20% more excursion. In addition, push-pull only reduces even order harmonic distortions, which are more forgiving to start with. Odd order harmonic distortions are more detrimental and push-pull cannot address those. Our servo addresses all order of harmonic distortion. So it is more effective. There are other types of distortion that servo addresses while push-pull does not. For instance, the distortion caused by voice coil temperature and other mechanical nonlinearity that causes variation on amplitude response variation. When the cone puts less than what it is supposed to provide, the servo feedback will enable the amplifier to provide more power instantaneously. The opposite is also true (to stop standing wave or when the cone moves too much). In this case, the servo can even make the amplifier generate negative voltage, not just 0 voltage, to provide better braking power.The net result is you hear a much more dynamic and clearer sound.

2) How does an 8" driver achieve 20hz extension? It is through equalization. If one works with manufacture ring, then he will know there is a so-called unit-to-unit variation (some call it tolerance). The EQ is set up based on a "typical" driver model. So you may get a unit that deviates from this "typical" driver model and the EQ will not be ideal. In addition to this unit to unit variation, we also have batch to batch variation, and year over year variation (aging) and in the presence of all these variations, it becomes clear it is difficult use "one size fits all" approach. Servo is more advantageous because servo takes cone velocity into the feedback and it address all these 3 types of variations on the fly. This unit to unit variation problem feeds back to the first problem where a perfect cancellation can be even problematic in the presence of unit to unit variation.

Custom Installation series & DIY issues

What are the important issues in building a sealed enclosure?

First, the enclosure should be airtight. This is mainly because a sealed box uses pressure to limit the excursion. Without this proper pressure, the excursion from the driver can lead to bottoming (exceeding driver maximum excursion). Make sure to apply additional wood glue between every wall joints from enclosure interior to seal off any possible air leak. Second, apply internal bracing if possible (however it is not absolutely necessary). Sealed boxes are in general smaller than vented boxes so the free panel areas are quite small. To further improve on that, the internal bracing needs to be substantial to be effective. Using minimal bracing only gives us a slight improvement. Third, use only a moderate amount of polyfill. In Direct servo subs, the purpose of the polyfill is not to control the damping of bass roll-off, because the damping of bass roll-off is controlled by the servo loop. Rather, it is used to control the tonal balance of the mid bass range.

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Why can't I use a sealed enclosure larger than the recommended size?

Our recommended enclosure size is based on an actual test enclosure with our amplifiers. To achieve the same excursion in larger enclosures, one should really use smaller amps. The consequence of not doing so is the driver can be pushed beyond the mechanical excursion limit. On the other hand, it is very easy to reduce the enclosure's net volume if necessary, by placing 2x4 blocks inside an enclosure, so an oversized box is always preferable to an undersized box.

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Can I use a 1 cu ft sealed box?

Yes you could. But be aware of the fact that the output below 30 Hz can by reduced by 4-5 dB, as compared to a 2 cu ft box. We think the 1.5cu ft box is the best compromise for those who look for small subwoofers.

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What is the trade-off between sealed and vented box?

A vented box provides higher output between the tuning frequency (which is 20 Hz in most cases) to 40 Hz, and almost no improvement above 50 Hz. Most of the vented box gain in output comes from the enclosure size. That is, a vented box needs a larger enclosure to gain more SPL output. Distortion is lower in the region where the vent has significant output. The transient response of a vented box is inferior, and below tuning the driver unloads completely. This means there is a risk of bottoming and for high output a rumble filter is required. Many subwoofers do not provide this, and as a result, they will bottom when deep bass is required at high output unless the subwoofer is used well below its maximum output. Another downside of the vented box is that output from the rear of the driver will always escape through the vent. This output is outside of the intended bandwidth of the vent.

The trade-offs of a vented box are generally considered most appropriate for home theatre use. A sealed box will generally suit accurate music reproduction better.

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Do the Damping factors, Bass Extension controls work with the vented solution?

The extension controls still work for the setting above the tuning frequency. For instance, if one tunes the box to 20 Hz, the usable settings are 20hz and 28hz. Setting extension to 14hz in a box tuned to 20hz does not extend the frequency because the low end extension of vented box is mainly determined by the tuning frequency. The extension filter cannot undo that.

What happens if I build a vented solution and plug the ports, like the SVS and HSU subs? Are there any changes required to the circuit boards to "convert" from vented to sealed?

Plugging the port in a vented box does not make it a sealed sub. Rather it makes it an aperiodic box. In sealed box, the enclosure volume act as a very linear spring device to mitigate the non-linearity of spider/surround of the drivers. In an aperiodic box, that volume is not fixed and can change due to internal pressure. This unpredictable volume change can lead to a memory effect. That is, the shape and position of the port stuffing material (such as foam) can change due to the pressure inside the enclosure and cannot recover to its original position instantaneously. In fact, this dependency of the previous pressure level and the fact that it cannot recover to its original position causes a memory effect. More about memory effects and how we reduce them >

As a result, we don't recommend this option. We suggest that instead, it is better to choose a design which best fits the application. For those who believe a vented design will suit their needs best, we recommend our DS1500ci subwoofer. This product will best suit a vented box, and can later be converted back to a sealed box. If you are leaning towards a sealed box but are concerned about the output for home theatre, we recommend multiple subwoofers and all of our products are suitable for this application.

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What are the important issues in constructing a vented or PR enclosure?

A vented enclosure is normally larger than a sealed enclosure, therefore heavy bracing is needed. Without bracing, a vented box will sound boomy. Second, use double flared ports (that is, flared opening at both ends). They do make a difference in reducing any 'chuffing' noise that may result from port turbulence. Third, apply 1" thick polyfill to the wall. Adding too much polyfill will increase the enclosure loss and therefore make the ports less effective. A passive radiator enclosure should ideally have two radiators on opposing sides of the box so that their forces act in symmetrically opposed directions. As a result, the net forces cancel. Care should be taken to ensure that the passive radiator will not be required to use too much of its maximum excursion, as the mechanical linearity of the suspension system will be less, resulting in higher distortion. Passive radiators are often used up to their excursion limits, and this is often not considered a problem since the distortion performance of the driver itself is poor.

In a high end Direct Servo subwoofer, the distortion is very low, so great care should be taken not to compromise the accuracy by adding a poorly executed passive radiator. Audiophiles who wish to add a passive radiator should keep in mind the effects of mechanical memory. This is discussed in our technology section. In brief, Direct Servo dramatically reduces the memory effect in the mechanical suspension system, and this is a major part of the highly accurate performance with musical reproduction. When you add a passive radiator, you are adding another mechanical system with its own memory effects which Direct Servo can’t correct, because it is outside of the closed loop. Audiophiles who wish to obtain low bass extension for home theatre for high output are advised to consider a second subwoofer as an alternative to buying two passive radiators.

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How much polyfill do I need in the enclosure?

Polyfill is used for absorbing standing waves in an enclosure. In non-servo subs, it is also used to occasionally adjust the Q value of bass roll-off. In this case, a heavy amount of polyfill will be needed to achieve the desirable Q value. The downside of this practice is that low level resolution is somewhat compromised. This is mainly caused by the nonlinear absorption rate of polyfill. Anyone who has built their own full-range speakers knows that too much polyfill is not always a good thing. Overstuffing the enclosure can make the sound less dynamic.

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Are the drivers used for Direct Servo subs different from regular drivers?

All the drivers used in Direct Servo subs are just like any other drivers, except for the sensing coils. The gap space occupied by the sensing coil is only 6 mils. This makes it very easy to take an existing driver design, put in a sensing coil, widen the gap by 2x6=12 mils and get a driver that can be used for Direct servo subs. This change only alters the T/S parameters slightly. As a result, all Direct servo drivers can also be used in a non-servo sub. We publish their T/S parameters just in case our customers want to do so (even though practically, there is no reason to do so).

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Can I use a regular dual voice coil subwoofer in a Direct Servo sub?

It is possible but certainly not preferable. The results will not be worth the effort. If you use one of the coils in a dual voice coil subwoofer, you will severely compromise the performance. Both voice coils are required for the driver to perform well, and the a regular voice coil does not perform well as a sensing coil.

The sensing coil in our driver is wound with the smallest gauge of wire. It is not only lighter, but also space efficient in the magnet gap. With a regular dual voice coil driver, half the gap space is wasted because only the half is used for producing electromagnetic force. The extra weight from the voice coil which is now being used as a sensing coil can increase the moving mass and hence increase the magnetic distortion of the motor.

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What is an aperiodic enclosure?

An aperiodic enclosure is an enclosure with a high loss opening to release the pressure in the enclosure. The lossy opening is achieved using highly flow-resistive materials. Aperiodic boxes have a 2nd order roll-off characteristic. Most computer simulation programs do not include them because it is too difficult to model them correctly. However, it is a convenient way to convert a vented box into something close to a sealed box. Our recommendation is to tightly pack the vented opening so very little air can escape from it. It also gives the user a convenient way to compare the sound from a vented box and a sealed box.

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Advice on choosing a subwoofer

Can a subwoofer reproduce bass as accurately as high end speakers?

Our products are ideally suited to those who feel subs just don’t cut it when it comes to music. Perhaps past experience has led you to believe this is the case. Or perhaps you have bought subwoofer and been disappointed with what it did to your music.

It is with good reason that many hifi salesmen will tell you as if it is a fact that subwoofers are for home theatre only. The reason is that they don’t have any subwoofers that are accurate enough to reproduce music. They probably haven’t heard any that can reproduce music with accuracy either.

One thing is clear – anyone who believes subwoofers are not suited to music has not heard a Rythmik Audio Direct Servo subwoofer.

Our subwoofers achieve a level of accuracy that is not possible with conventional subwoofer technology. This may sound like a bold claim, but we encourage you to examine our site. You will find that this is not sales hype, but it is in fact the only logical conclusion. Our technology section gives a background to how we achieve such a high level of accuracy.

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Do I need a Direct Servo subwoofer?

Our subwoofers are designed primarily for audiophiles who want the most accurate musical reproduction possible. Due to their SPL capabilities, our subwoofers are also suitable for home theatre applications, which can benefit just as much from the low distortion, highly dynamic bass that they reproduce.
Not everyone needs subwoofers as good as Rythmik Audio makes. In fact, not everyone needs a home theater or music room either, or even a sound system for that matter. But if you want to have the ultimate subwoofer that can perform as well for music as it can for home theatre, you do need a Rythmik Audio subwoofer. If you buy another subwoofer expecting to achieve the same level of accuracy and performance, expect to be disappointed. You will most likely pay more and get a lot less. Would you like to know how we can make such a claim? Review our technology section and it will start to make sense.

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What is the difference between a subwoofer designed for music or home theater?

Conventional thinking puts it this way – accuracy for music, maximum SPL at 20 Hz for home theatre. The problem with this is that movies contain music, and often contain bass from familiar sounds, so we know what we are hearing isn't accurate.

We think differently. Both music and high end home theater demand accuracy. The primary difference between a subwoofer optimized for each relates not to accuracy but the nature of what is being reproduced. With home theatre, relatively high SPL capability at 20 Hz is important. Due to the masking effect, deeper bass will often not be noticeable, unless at very high in amplitude as a special effect. Of course, there is music with high amplitude bass that will mask very low bass. Some movies will also benefit from bass below 20 Hz.

Our recommendation for a subwoofer for music are our sealed Direct Servo subwoofers. If your interest is also home theater as well, one or a number of our sealed subwoofers will still be an ideal choice. If you are a home theater enthusiast on a modest budget and don’t listen much to music, you might consider one of our vented subwoofers. We provide advice on choosing a suitable subwoofer in our product selection guide.

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Which of your subwoofers should I buy?

That will depend on your room, your SPL requirements and your tastes in music and/or home theater. Without knowing this, there isn’t a quick and simple answer. In many cases, one of our 12” or 15” sealed subwoofers would be the best choice. Our product selection guide will help you with this choice. If you are unsure on what to choose, contact us and we can help.

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Which has more SPL – subwoofer A or subwoofer B?

The important consideration here is that there are two numbers we can arrive at as an answer to this question. One is maximum SPL which is the most that you can measure without damage to the subwoofer. This is relatively simple to determine for a given frequency. The other number is the maximum SPL which can be reproduced with low distortion, definition and accuracy. In practice this is the more significant limitation of a subwoofer. Most subwoofers lack accurate bass at any level. Typically even subwoofers normally considered high end are not able to reproduce what we would consider accurate bass at 10 dB less than their absolute maximum SPL.

While our subwoofers are not designed to impress with high SPL, the SPL they can produce without high distortion is higher than competitive subwoofers.

Maximum SPL at low frequencies (without considering distortion) is determined by the amount of air that the subwoofer can displace. In a sealed subwoofer, this is the only factor, but in vented subwoofers, the output from the vent must also be considered. In theory, a vent or passive radiator can contribute 6 dB in the bottom octave, but this gain can be reduced by port compression in vents and by the increase in mechanical stiffness of the passive radiator at high excursion.

The amount of air displaced can be limited by a number of factors that are difficult to estimate accurately. In practice, a maximum estimated SPL capability is of little use since it is not related to the distortion involved.

The following points should be considered:
*Higher excursion drivers will not necessarily have higher SPL
*Drivers designed for a very high excursion have significant compromises which result in an increase in distortion
*Higher amplifier power does not always mean more output
*While a vented subwoofer can have similar output at 20 Hz to two sealed subwoofers, the sealed subwoofers will have higher output above 40 Hz

If high output is important to you, in most cases the best way to achieve it is with a number of well-designed subwoofers. Rather than buying one very expensive subwoofer with ultra high excursion and a massive power amplifier, we advocate multiple subwoofers with a sensible and intelligent design. This removes the compromises used in the ultra high excursion ultra powerful subwoofer which we consider to be more useful for making noise than for reproducing music. Our subwoofers are superior in the areas that count the most – accuracy and value for money. In many cases they are superior in SPL and cheaper as well. This is only possible because we use superior technology.

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I have a very large room, should I choose vented box over sealed box?

There are many factors to consider other than the size of the room. Typical program content and personal tastes are important, as well as considering the construction of the room. If your room is very solid (sealed air volume with concrete floor, masonry walls with no windows), a sealed box may be more appropriate even for home theatre in a large room. If your room is very light weight in construction (drywall timber framed walls and ceiling with timber floor, large glazed areas and open to other rooms), then a vented box may be more appropriate. This is especially true where home theatre is a significant priority and high output is desired.

Another important factor is standing waves (room modes). The roll off is different for sealed and vented boxes and will impact how room modes are excited.

When should I use the XLR version over PEQ version

The first factor to consider is the distance of the subwoofer from the signal source. If greater than 10 ft or more, the XLR version will avoid signal contamination from the long interconnect.

The second factor to consider whether you are using multiple subs or not. The XLR version has a master/slave mode where one can use a single sub to control multiple subs by a daisy chain connection. If you want to alter the settings and have multiple subs, it is much quicker to only change the master.

If neither of these factors are relevant, our PEQ version has a single band of parametric EQ which many will consider more useful.

Is is possible to achieve a higher crossover point by removing the low pass crossover?

With conventional subwoofers, removing the crossover (low pass filter) would mean the upper response would extend as high as the driver allows. With our servo, roll-off occurs above the motional impedance and inductance crossover point. Even if we remove the crossover, it will not extend to 300 Hz. We don't believe this is a significant limitation, as it is usually better to cross within the range that our subwoofers are designed for.

There are quite a few large, high excursion drivers with heavy moving mass and/or high inductance. The supplier recommended a low crossover point. Is that a sound idea?

We don't believe in using drivers that require a low crossover point to "minimize the damage." The above recommendation is and admission that there is an adverse effect of high inductance and moving mass. All other things being equal, another driver with a lower inductance or low moving mass can be considered superior. Once a driver is designed with high inductance, there is no simple solution. In the process of developing our products, we have attempted to use such products. We found that the results did not meet our goal of accurate bass that is true to the original. Even with our Direct Servo technology, we were not happy with the result.

If the output of a single driver is not adequate, we recommend multiple drivers. Firstly this means we don't need to make compromises that will take away from the accuracy. As a bonus, multiple subwoofers can be placed in a room in a way that is better optimized for room modes. This may not sound as exciting as recommending a uber driver, but we cater to customers who consider accuracy their highest priority.

How does Direct Servo affect the physical property of drivers so that I can feel it in action? And how does it act differently than accelerometer-based servo subwoofers?

Our Direct Servo technology improves cone control in the form of increased damping. The best way to feel servo in action is to connect the Direct Servo subwoofers in free air and then gently push the cone. One can feel the cone as if it is immersed in a highly viscous oil in both fast push (high frequency) and slow push (low frequency). That is how the driver cone in our Direct Servo subwoofers dissipates unwanted energy from inside the enclosure.

On the other hand, accelerometer-based will make it feel like pushing a heavy brick. In other words, its net effect is like adding a heavy mass to the cone. That means more of the internal reflected energy bounces back off the driver cone until other elements in the system such as enclosure walls dissipate that energy. Adding damping is good but adding cone mass should be avoided.

What is the purpose of rumble filter?

Only PEQ and XLR models have a rumble filter, so the following discussion only applies to those two models.

The rumble filter is a 3rd order high pass filter with corner frequency at 20 Hz to provide a steep roll-off below 20 Hz. It is useful when playing LP records, or when there is unwanted subsonic signals in both movies and music that makes the cone move in and out wildly without making audible audible sound. When it is on, it changes the roll-off of sealed subwoofers from 2nd order to 5th order, and vented subwoofers from 4th to 7th order.

Those who are more familiar with filter design will be interested to know that our extension filter is a 2nd order filter to set up the roll-off frequency for both sealed and vented subwoofers. With the difference of order between rumble filter and extension filter, one can actually get more flexible roll-off slope. For instance, in the following we list 3 possible roll-off slopes.

Configuration 1: a sealed box with rumble filter off and extension filter on at 20 Hz has a 2nd order roll-off at 20 Hz.

Configuration 2: for the same sub, if one wants to have a 3rder roll-off to attenuate some subsonic signals, he/she can set the extension filter to 14 Hz med damping and turn on the rumble filter.

Configuration 3: if one set the rumble filter to on and set the extension filter to 20 Hz, then one gets a 5th order roll-off characteristic from this combination.

So it can configure the sealed sub in 2nd order, 3rd order, and 5th order depending on the desired slope of roll-off. For the case of a vented box, it can be configured as 4th order, 5th order, and 7th order with configuration 1, 2, and 3, respectively.

Does the phase control on the plate amp provide a fixed phase shift to all frequencies?

No, it does not. It should be called "delay control." However, since its purpose is to adjust the phase alignment between the subwoofer and front speakers, most people use the term phase control for this function.

How long does it take for a subwoofers to fully break-in?

Normally 1-2 weeks.

Please note: Continuous sine wave signals should not be used to speed up break in. This will void warranty if damage results. A one third duty cycle should be used, where 30 seconds of signal is followed by a 60 second pause.

Can memory effect be measured?

It is very difficult to measure memory effect or even quantify it. To be able to capture it, the signal has to be constantly changing. It cannot be impulse, nor sine wave, nor square wave. Recently there has been a report that a new circuit element called memsistor has been introduced. Hopefully this is help us to understand better about memory effect. So far we can show how to reduce the thermal memory by having a servo not dependent on the voice coil temperature.

Sound Quality

What makes the bass sound fast, is there really a thing called fast bass?

This is one of the most commonly debated subjects in any discussion group. At the end of the discussion, we sometimes read comments like "the best bass is no bass", which we completely disagree with. If indeed the best bass is no bass, wouldn't performers use particular brands of bass instruments that produce less bass energy, or even do away with bass instruments altogether? The problem is that most subwoofers can’t accurately reproduce the live experience of a performance. This so called "fast" bass is really a bass sound with very good resolution, so that the bass sound precisely follows the rest of the signals. The sound should start immediately when the signal appears and stop without any remnant energy when the signal stops. Both of these requirements are equally important. In practice, however, it is the second requirement that most subwoofers don't meet. To make the bass sound stop quickly, the Q value of the bass roll-off should be low, so that the woofer cone can return to its resting position as quickly as possible. In addition, the subwoofer should have very low memory effects. Coincidentally, these are both areas where the Direct Servo subs excel. The result is a very coherent bass sound even on the most dynamic music passages. Our customers often describe the bass sound from Direct servo subs as "transparent", having "excellent definition", and "dynamics" yet, most importantly, with "convincing weight and slam".

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What is the difference between noise and distortion?

Many people categorically put any spurious signal component that is not in the original signal as distortion. Noise, when beginning to correlate to the signal, becomes distortion; Distortion, after being totally randomized in relation to the original signal, becomes noise.

In the real world, it is never black and white. Distortion can have various degrees of correlation to the original signal. And that is the key to explaining why some distortions are more pleasant to the ear because they have strong correlation with the signal (for example, distortion generated by tube amplifiers). This also explains why some distortions are less measurable, and often leaves us scratching our heads why the sound is not as good as our distortion measurements lead us to expect. In our opinion, the most subtle distortions are often those that are not quite measurable (such as memory effects). Very often the effects of such distortion is to obscure the life-like dynamics of the music and/or limit the resolution of the system. Now going back the difference between noise and distortion: the best example of this is digital dithering. Digital quantization error (or distortion) is one of the nastiest sounds that can be introduced into the recording/playback chain. The objective of digital dithering is to randomize this quantization error so that it becomes noise. The result is very dramatic. One of the Chesky Record test CDs has 2 demo tracks that demonstrate the effect of dithering. The track without dithering is just unbearable. The track with dithering, in comparison, is much more listenable despite a high amount of background hiss noise (as a result of dithering).

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Why do I need to add a sub when I already have an audiophile grade bookshelf speaker system?

Our recommendation for those who have bookshelf front speakers is to use a high pass filter on them. This achieves two goals. First, it will relieve the front speakers from the loading of low frequency signal and therefore improve the dynamics and midrange clarity. Second, most bookshelf speakers have bass roll-off with a rather high Q value. This has been a common practice since the introduction of BBC LS3/5A. The above graph shows close field response of a typical bookshelf speaker, in this case, the NHT SB3. The reason speaker manufacturers implement this "cheat" is to create the illusion of more extended bass. Long term listening of such speakers often reveals the bass to be unnatural. By applying high-pass filtering to the front speakers, one can undo this "cheat" and achieve more natural bass response.

typical bookshelf response

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I am concerned that applying filtering to my speakers will degrade the sound quality?

It is true that all op-amps have coloration to some degree. If you can hear the coloration of op-amps, we recommend a first order RC passive network with corner frequency of 100-150 Hz. The reason for this high corner frequency is to compensate for the slower roll-off of a first order filter. It is important that this RC network be placed close to the power amp input terminals (or even inside the power amp if that is possible), to avoid signal degradation. Essentially the output of this filter has a high drive impedance below 200 Hz. Pictured above is the RC filter that we made with one RCA jack and one RCA plug. Keep in mind this option is somewhat expensive, as one needs to buy a pair of each. It may run from $40-$100 depending on the brand used (in our case, Vampire). The jack is not soldered to the plug, as the plug has a cable holder screw which is used to hold the two together. Only the resistor and capacitor are soldered. The recommended resistor value is 5k ohms for solid state pre-amps and higher for tube pre-amps.

bookshelf modification

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Other issues

I hear hum from the subwoofer, how do I get rid of it?

To identify the cause of hum, one needs to unplug signal cables from the subwoofer amplifier and only connect the power cord.

First put the power slide switch to OFF position (red light). If the hum noise occurs at this position and appears only come from the transformer, the hum is most likely from a smaller amount of DC offset injected by small appliances such as toast oven, hair dryer, and others that draw large current and yet use less expensive circuit to regulate the power. Some computer printers (for instance, Brothers MFC7360) have been shown to inject small amount of DC offset back to wall power.. On the other hand, there are printers (for instance, Samsung MFC that we tested) that do not inject DC offset. It is an issue in the switch mode power supplier design. Alternative, one can also buy a power conditioner that specifically removes the DC offset to get rid of the hum noise.

Next put the power slide switch to ON position (green light). If the hum noise is absent and only come back after you plug in the signal cables, the cause can be one of the three: 1) ground loop, 2) broken grond connection on the signal cable, or 3) poor cable shielding on the signal cable. If the hum noise is present even when no signal cable is plug-in, the customer should contact us immediately for a further investigation.

Lastly, one common source of group loop is set-top box coaxial cable connection. If the hum is gone after disconnecting set-top box coaxial cable, one can use an coaxial cable isolation transformer to eliminate the ground loop. Some surge protectors also have coaxial input/output which can also break the ground loop.

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How do I check and replace fuse?

Fuse box is located beneath the power cord socket as shown in the photo. Use a set of small tweezers to open the box. The box constains two fuses: one in use in an open housing and one spare in a close housing. If the one in use is blown, one can use the spare to replace the blown one. Please contact us if the second fuse is also blown after replacement.

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How do I connect your subwoofers to my integrated amplifier for 2-channel setup?

Most integrated amplifiers have pre out and main in RCA sockets. One can purchase a set of high quality RCA splitters connecting to the pre out. One pair of outputs from the splitter go to the main in sockets and another pair goes to the plate amp on the subwoofer.

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Can I connect your subwoofers to my computer?

Yes. You will need to connect the output from your computer sound card into the line level input of the plate amplifier. For this you will need a splitter so that you can connect the same output to the other speakers. You will also need a cable with an RCA connection on the plate amplifier end, or a suitable adaptor. These can be found at your local electronics store.

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Isn't each driver is designed for an optimal box configuration?

This is a common misconception. If you have access to simulation programs, you can easily see why so-called "optimal box configurations" are in fact engineering compromises.

First, let us look at the vented box. Set the box size to 100 liters and tune it to 20 Hz. You can plug in any reasonable values for T/S parameters and play with them. You will then see that maximum output at 20 Hz depends only on two parameters: Re (voice coil resistance) and BL (actually, it depends on 3 parameters: Re, BL, and enclosure size, which is fixed). All other T/S parameters will change the response curve, but not the maximum output at 20 Hz. So the job of driver designers is to find a set of T/S parameters that will level off the response above 20 Hz so that we get a flat response. But in the process, the efficiency at the mid-bass band (40 Hz-100 Hz) is sacrificed. The mid bass from professional drivers is generally very good. This is mainly because of their high efficiency. The goal of our Direct Servo driver is to borrow that idea so that the mid-bass efficiency is as high as possible. In terms of frequency response, we rely on the servo technology to give us a flat response. So that we have best of both worlds: good mid bass sound and great bass extension. A similar argument applies for sealed box designs. Overall speaking, there are 3 parameters that we care most: Re, BL, and enclosure size.

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Can I connect the speaker output from my digital power amplifier to the speaker input of the sub??

Our speaker inputs aredesigned to accept regular single ended power amplifier outputs where the black terminals are the ground. Most digital power amplifiers have complementary outputs. That is, the black terminal is not true ground. This can cause problem problem even when the user accidentally short the black terminals of left and right channels together. Using outputs from complementary output amplifiers not only can damage our amplifier, it can use damage the source power amplifier itself. Please confirm with manufacturer of your digital power amplifier before using speaker level inputs.

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How hot will my subwoofer amplifier become?

The class A/B and Class H amplifiers have low idle power consumption. That means you can keep the amplifier ON all the time and the heatsink is barely warm. However, the class D amplifiers have higher idle power and therefore should be placed at AUTO position all the time. If one suspect the amplfiier runs too hot, the first thing to check is the idle power: amplifier ON for class A/B and H, and power switch at AUTO for class D.

The heatsink temperature depends on usage and signal frequency. Subsonic frequency in sealed subwoofers consume a lot of power. If you find the cone move a lot and you don't hear anything, it is best to turn on the rumble filter. This only applies to sealed subwoofers. If one has a black color cone, it is very easy to place a dot at the edge of surround and observe the cone movement in action.

If one uses multiple subwoofers, it is recommended to calibrate both subwoofers with near field measurement technique to ensure both have the same output. Otherwise, the one works harder can also get warmer.

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