Rythmik Audio Direct Servo DS12s Subwoofer Build

This document details the construction of a Rythmik Audio DS12 Direct Servo Subwoofer kit in the recommended 2 cu ft sealed enclosure.

Background

After owning three less than impressive subwoofers which came in a 5.1 setup I decided that the time had come to buy my first 'proper' sub from a dedicated sub manufacturer. This one would hopefully not have me wanting to upgrade it after a few weeks, or leave it switched off altogether for listening to music and would provide useful low frequency extension to my Mission Elegante E83 front speakers, which measure in-room down to 31.5Hz.

Thanks to the wealth of knowledge on avforums.com I had a very short shortlist of subs considered to provide excellent performance at a reasonable cost. These included the BK Monolith-DF, SVS SB12+ and SVS 20-39PC+. The upcoming SVS PB13 Ultra and PC Ultra subs were ruled out due to their physical dimensions and the fact that if I'm honest I probably don't need something that is capable of the sound pressure levels these are. Unfortunately the physical dimensions of the BK Monolith soon saw this crossed off the list as well.

Either of the remaining SB12+ or 20-39PC+ would have been good choices. The SB12+ is very compact in size, which is a necessary evil in my domestic situation, and has the added bonus of a single band parametric equaliser (PEQ) which would allow me to tame my significant 40Hz room node. I did have one or two doubts about the low frequency extension of the SB12+ though as the frequency response really does seem to tail off pretty rapidly below 40Hz, partly due to the compact size and partly due to SVS wishing to protect the driver from low bass. With some room gain, I'm sure the SB12+ would have been a perfectly acceptable choice for me though. By virtue of the 16” diameter footprint of the 20-39PC+ it would also have been possible for me to have accommodated one of the SVS cylinders in my chosen location. The 20-39PC+ has the greatest sound pressure levels of any of the three subs on the short-list and the ability to tune down to 16Hz to extend the otherwise pretty rapid tail off of the frequency response below 20Hz again. The lack of the PEQ was not a deal breaker as I would probably end up with a Behringer Feedback Destroyer (BFD) anyway.

I had pretty much decided to buy the SVS 20-39PC+ and slept on it a few nights whilst keeping an eye on the avforum classifieds, just in case anything special came up second-hand. During this time I chanced upon an ex-demo Sunfire True EQ Signature sub at half price, which seemed perfect from a physical size and SPL perspective, but I couldn't find many positive comments about the sound quality of these subs at low frequencies. It was also pretty expensive, even at half price.

Then by chance I happened upon a discussion of the Rythmik Audio Direct Servo subs on avforums and did some reading up on their performance during the last round of Ilkka's sub tests. I was very impressed with what I read and also re-assured that there were a number of experienced bass-heads on avforums who were also very interested in Rythmik's subs. I briefly flirted with the idea of building a DS1500 sealed kit, but even at a relatively tiny 3 cu ft it was a little larger than a Monolith, which I had previously discounted because of its size. However the DS12s 2 cu ft sealed enclosure seemed ideal. The DS12s actually has the same frequency response as the DS1500, albeit around 3dB down in terms of sound pressure level across the range, with a -3dB point of 12.5Hz. I thought this was pretty remarkable for such a small sized sub and would certainly provide the low frequency extension I was looking for. There was also a great deal of positive discussion about just how great the DS12s is with music, which is probably more of a priority for me than home theatre. This was borne out by looking at the test results showing low levels of distortion in mid and upper bass and extremely low group delay figures. Doing some quick sums revealed that I could most likely build a good looking sub for under £400 as well, which was 2/3 of the cost of the already inexpensive 20-39PC+ and have the enjoyment and satisfaction of building it myself.

Ordering the Kit

The first thing to do is to order the kit from Rythmik Audio. I chose the DS12 standard kit.

I ordered the kit before I started to build the enclosure as I expected it to take a couple of weeks to arrive from the US and this would give me plenty of time. In the end the kit took exactly four weeks to arrive, which comprised 2 days to leave the US, 19 days to arrive in UK customs, 4 days the clear UK customs and 2 days to arrive with me

Here is the package that arrived. It was pretty heavy at 19Kg but all seemed well:

Inside was the box containing the plate amp, fixing screws and power lead:

And the driver, which was particularly well protected:

Here is a picture of the DS12TC 12” driver out of the box, showing the separate wiring to the driver coil and from the servo coil, both of which were already terminated with a push-type connector. This part accounted for $200 of the order:

Here is a picture of the 370W RMS (4 ohms) servo amp electronics. The large toroidal transformer and capacitors bode well. This part accounted for $259 of the order:

And a close-up of what looks like the feedback circuitry from the servo coil, which appears to be common to the R12s. R15s and R15v enclosures:

I have to say that I was really pleased with the DS12s kit. It was very well packaged, the driver was very substantial and as a chartered electronic engineer myself I couldn't fault the plate amp at all.

Building the Enclosure

Cutting Out

Plans are available for download from the Rythmik Audio website for a number of enclosures, including the 2 cu ft sealed enclosure.

A little work was required to determine the exact dimensions to cut the enclosure material as the plans are specified in inches and nominally 3/4” MDF is actually 18mm thick. I produced the following cutting plan for a sheet of 2440mm x 1220mm (8ft x 4ft) MDF:

The cutting plan was worked out to produce the minimum amount of cutting of the sheet of MDF and to provide some spare panels should any unforeseen disasters happen during the assembly process. If cutting by hand it could also provide some scope for picking a panel which fits best taking account of cutting tolerances.

The sheet of MDF should first be cut across into 1220mm lengths of 394mm, 510mm, 510mm, 492mm and 358mm and then each of these lengths cut to provide the final panels. This will ensure that all panels are the same size, for example the 510mm side panels onto which the top and bottom are fixed.

• Two 394x394mm (15-½” x 15-½”) panels are for the top and bottom of the enclosure.

• Two 394x510mm and two 358x510mm panels provide the four sides of the enclosure.

• One 358x492mm panel is required for Brace 2.

• Two 170x492mm panels are required for Brace 1.

• One 358x358mm panel provides the innermost bottom plate into which the driver is fixed.

I sourced the MDF from my local B&Q, specifically because they provide a timber cutting service free-of-charge. This allowed me to buy my MDF pre-cut in accordance with the cutting plan and be sure that all of the edges were square.

Here are the pre-cut panels on the workbench ready for final shaping and assembly:

Which after a couple of hours work were turned into the following pre-cut panels.

One 394x510mm side panel with 9-1/8” x 11-1/8” cut out for the plate amp:

Two panels which form Brace 1:

Brace 2:

One 15-½”x 15-½” panel with12-9/16” (319mm) diameter hole for outer bottom panel:

One 358x358mm panel with 11-3/16” (285mm) diameter hole for inner bottom panel onto which the driver will be mounted:

Assembly

The enclosure was glued and screwed with a combination of No.6 x1-½” screws and No More Nails adhesive. All screws were pre-drilled with a 3mm pilot hole, which is the diameter of the screw shank, and chipboard screws were used because they have a very sharp thread which would cut into the MDF and minimise the chance of splitting. It is worth noting that a 3mm pilot hole is pretty big for a No.6 screw and there isn't a huge amount of material for the thread to cut into. This means that the screws must be tightened carefully, ideally by hand rather than with a drill, to avoid stripping the thread. There is however just enough 'bite' for the screw to pull the panels together, squeeze out the excess adhesive and hold the panels whilst the adhesive dries. The plans state that PVA glue will have sufficient strength for the enclosure, but it would be much more difficult to assemble the enclosure and hold it tightly clamped while the adhesive dries without screws.

First of all I assembled two sides of the enclosure around the inner bottom panel, one 394mm wide and one 358mm wide side panel. This provides a rigid three sided box into which the Braces can be assembled. I then attached one half of Brace 1, onto which Brace 2 was attached:

The third side panel and other half of Brace 1 was then attached:

Followed by the fourth and final side panel with the cut out for the plate amp:

Here is a different view down towards the base of the enclosure looking through the cut out for the plate amp:

The top was then fixed in place. This picture shows a close up of one of the corners to emphasise the benefit of getting the panels accurately cut with straight, square edges so that they but together perfectly thus ensuring that the enclosure is air-tight:

Finally the lower bottom panel was attached:

I did consider using 1” MDF rather than the 3/4” MDF specified in the plans, but decided to stick exactly to the plan, as to be honest I know no better. At first I was a little concerned with how lightweight some of the parts felt, particularly the half of Brace 1 with the cut out for the amp plate. However once assembled together, the completed enclosure feels absolutely solid with no give in any of the panels, perhaps unsurprisingly given the extensive bracing, and weighs in at 13Kg.

Veneering

I used two sheets of 2500mm x 255mm (actually 260mm) x 0.6mm Iron-on American White Oak veneer. This is a real wood veneer which is coated on the back surface with an even layer of hot melt glue. The process for applying the veneer is very simple. Firstly cut a piece of veneer for the part you wish to cover, allowing an extra 3-4mm overlap as the veneer will shrink across the grain when it is applied. Then heat the veneer by passing an iron over it on the hottest setting. I placed a piece of plain white paper between the iron and veneer to minimise he chances of burning the veneer or transferring anything from the veneer onto the iron which might make its way onto one of my shirts later. As soon as the glue melts you need to rub down the surface with a small wooden block, applying sufficient force to push down any ripples or bumps, paying particular attention to ensuring that the edges are well bonded. At this point with part of the veneer bonded and part unbonded the veneer will probably be quite distorted as the ironing process dries out the veneer and causes it to shrink. However once it is all bonded in place the shrinkage will be more uniform and you will get a flat uniform finish with a little help from the block of wood. Here is a picture of the completed veneered enclosure:

With the veneer being only 255mm wide it was necessary to use two lengths of veneer on each side of the enclosure. The instructions which came with the veneer recommended using veneer tape to hold the two edges together whilst ironing on the veneer from the joint outwards. I tried using some masking tape, but it just left a sticky mess and a 3-4mm gap between the two sheets. In the end I abandoned the masking tape and simply butted the veneer sheets together before ironing and then cut a thin strip of veneer to fill the gap. By carefully matching the grain of the veneer, the end result isn't actually very noticeable. You will see a thin strip of veneer down the centre of the panel in the picture below:

One of the things I have observed with veneers is that they tend to get damaged on exposed edges. With this in mind I had decided to finish the edges of the enclosure with some 6mm quarter rounded beading which will be fitted flush with the surface of the veneer and thus prevent it from becoming chipped off. Here is a close-up of one of the edges of the enclosure before the beading was attached:

One unforeseen problem with the veneer was that due to it being a real wood veneer there was a little cracking in some places where the shrinkage had pulled the grain apart. This was only minor and could possibly have been minimised by applying just enough heat to the veneer to melt the glue but lifting the iron off before the veneer had dried out too much. I also found that the cracking was minimised by starting to iron the veneer in the centre and working my way out to the edges rather than from one side to the other. It will be necessary to use a little wood filler in the cracks before applying a final finish to the veneer. Here is a close-up of some of the cracking:

Edges

For the edges of the enclosure I had obtained some 6mm quarter round hardwood beading, which first of all needed a 6mm square rebate cutting into the edges of the enclosure:

I was then able to glue the 6mm quarter round beading in place. Here is a close-up of one of the edges:

And a close-up of one of the corners:

With all six sides of the enclosure finished with the 6mm quarter round beading the enclosure is starting to look a lot more finished:

And from the opposite side:

Feet

You may have noticed in the previous two pictures that the enclosure has now gained a set of feet. These were 100mm long tubular steel furniture feet which I cut down to provide 2-½” tall feet as specified by the plan when using a recessed driver. These also come with screw in levelling pads, which aren't shown in the picture as I am going to put spikes in. Here is a picture of the feet from the underside. Not the most aesthetically pleasing feet perhaps, but they are very sturdy and aren't going to be on view:

Staining & Varnishing

The enclosure is required to match our sitting room furniture, which is solid oak finished with pure tung oil to give it a more natural looking matt finish. Rather than being made from solid timber, the sub enclosure is veneered in oak and I wanted to use a surface finish that would provide some additional stability to the veneer, such as with a varnish, but without the high gloss finish of varnish. In the end, with a little trial and error, I was able to reproduce the colour of the sitting room furniture using a combination of two coats of medium oak Danish oil (which is tung oil with woodstain and varnish added) and three coats of teak coloured satin finish varnish. The Danish oil brought out the grain of the timber by staining it a medium oak colour and the teak coloured varnish gave the chestnut tone required to match my furniture.

Here is a picture of the enclosure with two coats of the medium oak Danish oil. I actually really liked the natural oak finish of this and it would be a great match to a set of real oak veneered speakers:

And another of the other two sides:

With three coats of teak coloured varnish the enclosure was now a good match for my furniture. Here is a picture of the finished enclosure:

And in another likely location alongside the AV cabinet:

The other two sides of the enclosure and the top:

Alongside one of the E83 front speakers to give some sense of scale, showing just how relatively compact the 2 cu ft sealed enclosure is:

A close-up of the oak veneer:

And one to show the final finish:

Lining the enclosure

Rythmik Audio recommend the use of 2” polyfill to cover all walls of the enclosure. The closest I could find to this was 40mm BAF Wadding from Wilmslow Audio. I briefly considered getting some polyester wadding from a local fabric outlet, but in the end ordered some proper stuff as it wasn't worth the risk of spoiling the ship at this stage. The wadding was cut to size and I then used a couple of blobs of contact adhesive to hold each piece in place.

Here is a pictures of the inside with the wadding in place:

Assembling the kit

The first thing I did was mount the driver. The kit came complete with self-tapping screws to mount the driver and the amp plate which had a 2.4mm shank. I drilled a 2mm pilot hole using with the driver in the required orientation in the base. Here is a picture of the driver from the inside of the enclosure:

There was quite a large amount of wiring between the plate amp and driver, so to avoid any risk of the wiring vibrating on the inside of the enclosure I fixed it in place with a couple of cable ties:

And then drilled pilot holes for the plate amp and fixed it in place:

And finally a picture of the driver:

References

Ilkka's Subwoofer Tests – Autumn 2007

Taking a BK Monolith-DF as a benchmark for performance around the £400 mark, note that the DIY Rythmik Audio Servo 12” Sealed 56l entry has lower SPL than the Monolith around 20Hz (which is where the Monolith is tuned) but has greater SPL above and below this point with better low frequency extension.

Detailed Rythmik Audio Servo 12” Sealed 56l Test Results

Note the 12.5Hz to 109Hz frequency response (-3dB) at 90dB and the lower group delay than the benchmark Monolith. The affect of the servo on power compression below 30Hz is clear to see.

Costs

Project costs to date:

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