Build your own subwoofer kit

Introduction

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".

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