Friday, January 18, 2008

DIY Subwoofer - Rythmik Audio DS12 kit

Here is another of my speaker projects from 2007. What is it you ask, well its a subwoofer.My primary objective was to use a subwoofer to bolster the lowest frequencies of my DIY ESLs. But this was a unique situation as I did not want to have a situation where the bass sounded disembodied from the rest of the music like the Martin Logan ESLs!! So not just any subwoofer would suffice. I have listened to many subwoofers in my day. What I considered great in my youth (head-banging days), I now consider bloated and woolly, lacking definition and musicality. All the commercially available subwoofers just play catch up with the rest of the music. My ESLs called for a subwoofer that is both musical and accurate (some might even call it "fast") So my search for such a subwoofer thus began.

These subwoofers also had a second duty, which was to serve as the subwoofer to my Home Theatre setup. This double duty required a truly spectacular subwoofer, and at one point I considered DIYing 2 subwoofers. After much reading I came across a category of subwoofers using SERVO technology. Now subwoofers utilising servo technology is nothing new, several commercial maufacturers use it in their offerings. Servo subwoofers utilise feedback sensors to ensure that the driver is performing as it should. In addition to this servo tehcnology, I also opted for a sealed subwoofer as this sounded most accurate and musical to me. Enter a direct to consumer manufacturer called
Rythmik Audio. Rythmik Audio offered DIY Subwoofer kits based on TC Sound's drivers and specially modified plate amp.

The kit of my choice is the DS12 Standard kit for a sealed subwoofer design. The driver and plate amp were modified to utilise Direct Servo technology. In addition, the plate amp had adjustable circuits for bass extension and SPL. Wow this was the 2 subs in one that was needed to pull double duty in my setups.

Here is an extract from
Secrets of Home Theatre and High Fidelity article about the kit:

"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."

Having never heard these subwoofers and armed only with glowing reviews from owners, diyers and magazines, I proceeded to contact the designer, Brian Ding. A pleasant fellow, ever willing to answer your questions and help you in anyway possible. After several correspondenses with Brian Ding via email, I felt convinced that this was the kit for me. Musical, accurate, articulate, goes down flat to 10Hz, adjustable extension and SPL what more would I need. So I made the plunge and promptly ordered the DS12 standard kit. Shipping to Malaysia was not cheap. But after a couple of weeks, the kit arrived.... comprising of 1 12" driver unit and a 350 watt plate amp with humongous toroidal transformers and a bank of 4 caps!! Here are some pictures...

The DS12TC 12"TC Sounds based driver, and notice the large 2" magnet. This baby is heavy. Its not the most impressive driver from TC Sounds' stable, but it does its job well.
Top View. This version comes with a Black Anodized Aluminium Cone.Side View, showing the Sensor Coil. The sensor coil sends feedback plate amplifier's servo circuit about the cones movement and position. TC Sounds specially manufactures this sensor coil based on Rythmik Audio's design.Side View, showing Driver Coil.Bottom View.Here is the 350 watt A370 plate amp.So what's missing? The Cabinets of course. Cabinets were a simple affair, a cube with cross braced internals, thick walls.... and an open/glass top so that I can see what's inside. I made them using facilities in my Uncle's woodworking shop. I had them made out of MDF as that was all we had lying around. Here are the pictures of the cabinets. The internal cross braces.Front quarter view.Many people ask if I make the cabinets by myself. I do make it myself but I get a lot of help from my Uncle who is a carpenter by trade. He has a woodworking shop where I make use of the many equipment a home builder would otherwise have no access to. Here is a picture of part of the workshop. The installation of the plate amp and driver units were a simple affair, everything is sealed using silicone adhesives and screws. I use silicone as I had to ensure that it was air tight. The top glass cover is 1cm thick temperred glass. It is attached with nothing other than silicone. Here is a picture of the installed plate amp, notice the large toroid and bank of 4 caps!!Front view showing the 12" driver unit.Another view from the top showing the internals. Incidently, the internal walls of the cabinets have now been lined with 1cm thick foam to reduce internal reflections.Another view from the top showing the driver's backside. The rear view of the cone moving during high excursions is a sight to behold!!Feel free to leave your comments. Read my Listening Impressions or return to Home.

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