Product Review

DIY # 14:  Rythmik DirectServo Powered Subwoofer Kit

April, 2004

Ralph Calabria



● Driver: 12"

● Amplifier: 350 Watts

MSRP: $399 for Basic Kit; Finished Enclosure is $199 Additional


Rythmik Audio


I'm always in the market for new speaker kits to share with Secrets readers. It gives the average Do-it-Yourselfer some nice options that will both save some money as well as satisfy a very rewarding hobby.

Kits offer additional advantages as well. Buying a kit leaves the speaker designing to the experts. When one considers the time and resources (not to mention equipment) speaker manufacturers put into each of their designs, that adds up to a considerable Research and Development (R&D) cost before the speaker goes to market. Through extensive measurements and redesign, the crossover is carefully �tweaked� so a speaker will play seamlessly across its frequency capabilities, particularly between the tweeter and woofer crossover point.

Since speaker manufacturers need to recover their R&D costs somehow, you as the consumer pay that cost, which is rolled into the price of the speaker. Although this R&D cost is still associated with a kit design, you save a considerable amount of money by buying the designer's components. For example, when Audax contracted Vance Dickason to design speakers around their drivers, Vance was undoubtedly paid for his designs. Audax, in turn, sold countless drivers to DIY'ers to build these designs. You, as the consumer, save money by assembling the kit yourself, while Audax makes money because they know their drivers will be used exclusively for their design.

Assembly is another key expense for a high-end speaker. Enclosure rigidness/bracing strategy as well as a quality finish can contribute significantly to the bottom line. However, the DIY'er has a plethora of resources at his/her disposal that can make a cabinet that rivals the best built consumer cabinets at a mere fraction of the cost.

The Rythmik Audio DirectServo sub kit is no exception, given its very reasonable price and ease of construction. And with Audio Concepts Inc. now out of the kit business, there's a small void needed to be filled in the DIY market. Very much like ACI, Rythmik Audio is a direct-to-consumer company, with no middleman, no distributor, and no salesperson. Rythmik Audio is a small mom-and-pop company (literally!) run by Brian Ding and his wife. They specialize in servo-controlled subwoofers, but also offer the non-servo variety. Currently, they cater to the DIY sector, but will eventually venture into selling assembled kits.

The DirectServo Design

There are many advantages to using a servo-controlled system for a subwoofer. As previously described in the review of the Titan sub, the servo design allows you to use a smaller cabinet to achieve the low-end response. The feedback controller also protects the driver from being overdriven, which would cause damage to the voice coil.

The feedback loop in the Rythmik sub is a combination of current sensing and velocity sensing feedback. Unlike most servo designs on the market, Rythmik takes on a minimalist's approach. There are no active devices in the feedback paths. The feedback path is purely comprised of resistor and capacitor networks. In contrast, in accelerometer based designs, op amps are used to amplify the motion signal, therefore, becoming a possible source of audible degradation.

The way Rythmik designed the extension filter was such that, without the extension filter, the frequency response (from the sub, not from the amp) will be flat to below 10 Hz. However, this type of design would not be practical. So Rythmik inserted an additional single 2nd order high pass filter with adjustable fs and Q values by changing the resistor and capacitor values of the filter. That sets up the ultimate extension of the frequency response, which allows the user to select between maximum extension and maximum playback SPL.

The Whole Kit and Caboodle

The following hardware is included with the DirectServo kit:

● One 12� long-throw polypropylene woofer
● One 350 watt rms amplifier module with variable active crossover
● Speaker gasket material
● Acoustic polyester stuffing (for damping)
● Hardware to install both the driver and amplifier

The following hardware is NOT included with the DirectServo kit:

● Wood to build the enclosure (enclosure with grille available at additional cost)
● Cabinet finishing materials
● Spike feet

The amplifier/crossover module comes completely assembled, so there's no need to solder anything. Even the driver comes equipped with connectors that essentially snap the wires on from the amplifier. So when I say, no soldering is required, I mean, put away you're guns and irons . . . you don't need them!

There really is no manual included with the kit, but quite honestly, there is no need for one since the assembly is THAT simple. The kit will function properly in a sealed enclosure between 2.1ft3 and 2.5ft3, and in a vented enclosure 4ft3.

Here are some details on the amp (also available from Rythmik's website):

● Thermal-coupled MOSFET differential input stage for the power amplifier
● No electrolytic caps on signal paths, nor feedback paths
● Adjustable bass extension control (with both frequency and damping)
● Audiophile grade MIL current sensing resistor
● Integrated 3/16" thick one-piece aluminum back plate and heat sink for better heat dissipation.
● Dimension: (W)10-3/8" x (H)12-7/16" x (D)4".
● Cutout Dimension: (W) 9 1/8" x 11 1/8"; 4" clearance in depth.

The amp has been optimized to work either in a sealed enclosure or in a vented design. By a simple flip of a switch on the amp (inside), the circuitry selects the optimum feedback control for the box design you've chosen. The amp is very flexible in terms of connection options and variable output. There are four sets of speaker level outputs as well as two pairs of L/R line level outputs. There is a volume control, variable crossover, as well as a 180� adjustable phase controller. The amp also has an auto on/off power switch, which automatically turns on the amp when a signal is detected.

Bass extension is adjusted using the frequency and damping toggle controllers. Frequency settings are 14 Hz, 20 Hz, and 28 Hz. The damping can be set to low, medium, or high. Setting the damping to low provides a 3 dB gain, and the medium gives you 1.5 dB gain. This adjustment is most useful when fitting the sub into your existing system and room. For example, if your room has a lot of gain and the sub sounds too boomy, you can set the damping to medium or high to reduce this artifact. If you want maximum bass, the low setting works best.

The Enclosure

If you're not good with power tools, for an additional cost ($199), Rythmik will provide a sealed cabinet. The cabinet is available in several different finishes: raw (unfinished), natural, red mahogany, and dark oak. The cabinet veneering and �" round molding was well done in the sample I received.

For the review, I opted for the raw cabinet so I could get a better idea what kind of carpentry went into the cabinet. Cabinet walls were a combination of MDF and particle board, totalling 1-� " in thickness. There was no bracing in the cabinet, which was a little surprising, particularly with a cabinet this size, and with the large cutouts on the front and back baffles for the driver and the amp.

Installing the Driver, Amplifier, and Damping Material

As I mentioned previously, putting this beast together was a cinch. The driver and the amp are coupled using a multi-wire connector that snaps into position, so there no need for soldering. The gasket material that is included with the kit is essentially worthless. The consistency of the gasket is similar to Silly Putty, only stickier. Don't even bother to use this gasket, as it was very difficult for me to get a good seal with it. I would recommend using the closed cell foam gasket material available from various companies that sell drivers and crossover parts. Fluff and poof the stuffing, then place in the cabinet before you install the driver. Should you decide secure the stuffing by using a spray adhesive, make sure you put the stuffing in first before you install the driver or amp. Wait for the glue solvents to completely dry before proceeding to the driver install.

The driver comes with a rubber gasket as well. Remove this before you install the driver, particularly if you are going to use the cabinet available through Rythmik. The screws that are included with the kit were more than adequate in securing the driver and amp module. The gasket that is part of the amp module is fairly hard, so you'll need to really bear down on the screws in order to get a good seal (you will need a good seal around the driver and amp to avoid air leakage out of the enclosure). Although I didn't use additional gasket material around the amp, it may make it easier to get a good seal around the amp without taking the risk of stripping the screw holes. Don't forget to drill very small pilot holes for each of the screws. This will reduce the risk of damaging the MDF.

The Measurements

I took several measurements, adjusting the variable frequency cutoff, the frequency toggle, and damping settings.

M = medium damping; L = low damping)

(M = medium damping; L = low damping)

As you can see, the sub has a pretty flat response between 20 Hz and 100 Hz. As one would guess, the low-end is extended further out using the low damping and 14 Hz frequency setting.

Movies or Music?

The age-old question once again arises, "Is this sub more suited for movies, or does it do music more justice?" Well, the times they are a changing, and in order to keep competitive with the other thousands of subwoofers on the market, it's essential for a sub to do more than an adequate job in both arenas. This is not only from a competitive standpoint, but also there is certainly a gray area when one considers all the music intertwined in every movie. Perhaps the distinction between movies and music comes from the fact that so much of the low frequencies in movies are synthesized, so a loose subwoofer will work for them. With music, particularly classical, the low frequencies come from instruments and are natural sounds, so a tight, clean bass is appropriate here. The Rythmik lets you choose this by the flip of a toggle switch.

There's no question that the DirectServo delivered all the low-end punch needed for a good home theater experience. The Lord of the Rings: The Fellowship of the Ring provided a good test. In the scene where the ring is almost thrown into the fires of Mount Doom in Mordor by King Isildor, the sub did a god job of reproducing the thunderous rumbles.

When the Fellowship are gathered in Rivendell, Boromir suggests returning the ring to Gondor and using it for �good�. Gandolf quickly rises up and speaks the language of Mordor, erupting into an extreme low-end frenzy. Again, the DirectServo showed no hint of the driver bottoming out or amplifier clipping. The sub had no problem handling the first �stomp� of the T-Rex coming out of �Jurassic Lunch�, which falls well below the 20 Hz frequency. The sub's sealed box design showed its composure on several other bass enriched movies including Terminator 3: Rise of the Machines, The Lord of the Rings: The Two Towers, and X-Men 2. I tried to stress the driver and amp in an attempt to overdrive either, but the DirectServo system showed resilience past 110 db.

The DirectServo also did a very nice job with music reproduction. Its flexibility in the way of damping and frequency setting created a smooth, balanced bottom end that seamlessly blended with my L/R speakers. Music sounded best using the high damping setting, which sets the sub to a Q = 0.6. The sub responded quite well to quick bursts of short, low notes when listening to the timpani in "Farefare for the Common Man" (Aaron Copland; Telarc). The subs good transient response did not sound boomy and was very natural.

Final Remarks

Rythmik has introduced a nice kit in its DirectServo for the DIY sector. I found the sub extremely flexible, in that the sub can be adjusted to work well for both music and movies by a mere flip of a switch, keeping it competitive with the best of subwoofers. The sub has enough output to deliver pounding bass in a large room that will provide a convincing home theater experience. The DirectServo's pricing is also very attractive. With a 350 watt amp (with all its flexibility) and a 12� long throw driver, this kit is quite a bargain.

Ryhtmik offers a cabinet to go with the kit. Certainly building your own cabinet offers additional savings. A cabinet suitable for this kit should be very inexpensive to build. It all depends on how confident you are using power tools. But then again, that's what DIY is all about!

 - Ralph Calabria -

Associated equipment for this evaluation included:

B&K AVP2000 preamp
B&K AV5000 power amp
Pioneer CLD-D703 combi player (modified for AC-3)
Panasonic CP-72 DVD player
Marantz DP-870 Dolby Digital Processor
DIY Audax A652 mains
DIY Audax Center Channel
DIY Titan Powered Subwoofer
M&K MPS-1625 tripole surrounds
Audioquest, BetterCables and Nordost interconnects

Notes from Colin Miller (Secrets Senior Editor) and Brian Ding (Rythmik Audio):

CM - We can get additional low-end response with EQ. What's the advantage with tailoring frequency response using a servo-system instead?

BD - While EQ is less expensive to implement, the system's frequency response is not as stable as true sensor-based servo-subs. For instance, we have seen some driver manufacturers boast that their drivers can put up with 3000 F or even higher temperature. However, what is not mentioned is when the voice coil is at 3000 F, the voice coil resistance is 50% or so higher than in room temperature. It can change the Q value as much as 50%, and the frequency response as well. Sensor-based servo subs completely eliminate this problem. That is, the system will have the same frequency response regardless of the temperature of voice coil.

CM- Some of us have had mixed experiences with different implementations of servo-controlled subwoofers based on accelerometers, some good, some not so good. I recall in particular how an accelerometer-based subwoofer that I owned sounded clean, �fast', and dynamic until it met its headroom limit (via the last elephant thump in the opening scene of The Lion King) in which case the woofer suddenly and conspicuously went BANG much harder than any conventional subwoofer.

BD- Our servo-sub works on a different principle. We use a sensor coil to detect cone velocity. Accelerometer-based servo-subs are difficult to work with. First problem: whenever one moves the sub when it is still powered on, one may see the cone move in and out, sort of like warbling (I like to call it induced transient response). This is because the accelerometer is referenced to ground/earth (as in physics, not electronics). The cone does not want to follow the movement of
the enclosure. Our servo-system does not have this problem because our sensor signal is referenced to the driver assembly, and therefore, the enclosure. The cone will follow the movement of the enclosure. Another common problem with accelerometer-based subs is stability. That is why we have heard comments like above to explain why accelerometer-based systems can create the BANG noise. In this regard, our DirectServo technology is absolutely stable because we use a combination of velocity sensing feedback and current feedback. When the voice coil is out of the gap, the current feedback plays the current limiting role. As a matter of fact, it is so stable we don't need to use relays in our amps. The power-on noise is still within acceptable level.

CM- One of the more well-known proponents and manufacturers of accelerometer-based servo-subwoofer systems addresses this, I believe, by incorporating limiting circuits to protect the driver, though it has been said that this may in some circumstances limit dynamic performance.

BD- This is a trade-off. Limiting circuits do sacrifice dynamic range. Sometimes the limiting circuits also serve the purpose of preventing the system from entering areas of unstable operation (such as amp clipping or out-of-gap). We have chosen not to use limiting circuits for sonic reasons. The excellent stability characteristic of the DirectServo sub allows us to do without a limiting circuit.

CM- So, then when the voice coil goes beyond the gap, the impedance drops, and the current-sensing portion of the circuit provides the protection and actually backs off amplifier output beyond linear excursion, and the system is more stable because it inherently tries to keep the driver within its linear range, as opposed to trying to linearize behavior regardless whether it's in its linear range or not. So that part of the feedback circuit protects the driver by its very nature? Then, functionally speaking, in addition to the protection from over-excursion provided by the driver suspension and mechanical stiffness of the enclosure air, you get the benefits of a smarter limiter that provides a soft kind of protection based on the most important aspect of protecting the driver-cone excursion, as opposed to typical limiters that operate on the basis of amplifier output level? That sounds smart.

BD- Yes.

CM- Can you explain �velocity sensing' vs. �current sensing'?

BD- We use a sensor coil to pick up the velocity signal. The velocity signal is similar to an accelerometer's, except that it's derived from the motion of the coil through the motor's magnetic gap, as opposed to an accelerometer's motion referenced to inertia. In our sub, the sensing coil is like the second coil of a dual voice coil driver, except that it's smaller to let the primary coil use most of the space and make the most efficient use of the magnetic flux of the motor. Current sensing looks at the current drawn by the driver itself.

CM- How does the current sensing adjust to the resonant impedance peak in the driver? For instance, if it were purely trying to make the current draw match the proportion of the input signal, it would increase the signal at the peak to force more current, and you'd get a big bump in frequency response. For that matter, how do you incorporate the different impedance curve caused by a vented design, or a larger/smaller cabinet?

BD- It is based on the concept of impedance loading. The effect of impedance loading can be created by using negative current feedback and negative motional feedback at the same time. The effect of impedance loading is to create a driver with a set of "virtual" T/S parameters that are very different from physical T/S parameters. The overall system response will act as if the driver possesses this new set of virtual T/S parameters. Since a sealed box and vented box work differently, it will require different sets of virtual T/S parameters, and hence, the feedback networks will also be different for the two.

CM- Wow. Thanks for the thorough responses to our questions.

BD- You're very welcome.

    Related to the article above, we recommend the following:

Primer - Speakers



� Copyright 2004 Secrets of Home Theater & High Fidelity

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