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DIY: Rogers AB1

   Being really impressed with the sound of Chartwell LS3/5A and being aware of their limitations at the same time, I started looking for solution that would keep the magical midrange of LS3s but add a bit of weight to the bottom end. As many other people with the same problem, I have come across Rogers AB1 subwoofers - dedicated set for LS3/5A. Mixed opinions could be found on the internet. Some people liked them a lot, whereas others, did not at all. Being an inquisitive person, I have decided to give it a go and build a set myself. Enjoy! I did.


Planning
   From what I found on the Internet, the original Rogers AB1s were made from 15mm MDF, with an exception of the back panel, which was made from 12mm MDF. Purely for economical reasons, I decided to make all panels from 15mm MDF. To compensate for the volume loss, my AB1s are 15mm taller than the original ones (see dimensions on the image below).
To test my table saw prototype, I decided to build enclosures using 45 degree joints. The wood was initially cut by the wood merchant to slightly oversized pieces. More precise 45 degree cuts were made in my workshop.



Crossovers & Connectors
   Crossover design is fairly basic. It features 16mH iron core inductor and 220uF electrolytic cap in parallel with 4.7uF film capacitor. Design was based on the schematics from the ls35a.com website (provided by a gentleman called Andy Whittle, who is the designer of the original Rogers AB1s). Precise specs of the components are listed in the table at the bottom of the page.

 

Enclosure Building
   I started my project from cutting the oversized wood pieces to the desired measurements and 45 degree angles. The prototype table saw turned out to be more precise than expected. Rear sides of all the panels where then put on a router table where 10mm of MDF thickness was taken out. This was done to create a recess for the back panel (clearly visible on the image 13 below). Grooves in the side and front panels were routered out, to create recess for the driver unit shelf (visible on image 11 below).

   Centre of the driver unit shelf was predrilled, to allow for the router on the home made compass to be used. Driver holes were then cut out to accommodate the KEF B110 unit to be mounted from the top. Please note - because I have decided to mount the unit in this particular way, it was necessary to chamfer the driver hole, to allow the driver to 'breathe'. Unfortunately I have forgotten to do it at the early stage, and had to do it after the enclosure was glued together (see image 32). As you can imagine, this takes a lot longer, so it is much better to do it in advance. Luckily, I remembered to drill the driver unit mounting holes.
It was also necessary to create a small grove on the inside of front panel, just above the driver unit shelf (visible on image 19). Without this groove, basket of the KEF B110 unit will not fit.
All the panels were put together to ensure that they fit properly. The panels were then glued using PVA wood glue, and once dry, sanded down starting with 80 and finishing on 150 Grit.
Spare rear panel was then used as a master template, to drill the rear panel mounting holes.

   Upholstery foam was cut to correct sizes using electric kitchen knife. I've experimented with different cutting methods and this by far gives the best results and flexibility. 2mm neoprene sheet was used to cut out one-piece rear panel seals. Seals were glued using contact adhesive. The rear panel screw holes were countersunk. Speaker terminal holes were enlarged to allow for slightly too short speaker terminals to be used. Rear panels were the spray painted back.

   Two grooves were made on the inner side of real panels. One to accommodate basket of the KEF B110 unit and one to accommodate the wires going to the LS3/5A output terminals. Previously cut foam pieces were glued to the rear panel using spray contact adhesive. Crossovers where mounted just above input terminals. The wires going to the LS3/5A output terminals were placed in the grooves and sealed with silicone, to ensure that they are airtight. Rogers AB1 labels were mocked up in Photoshop and printed on good quality paper. Labels were then cut out, terminal holes were punched out using hole punch. Labels were sealed using clear car lacquer and glued to the back panels.

 

Veneering
   For this project I have decided to try traditional veneering method - hammer veneering. People have been using this method for thousands of years and there were plenty of video tutorials on YouTube, so it sounded like a good idea at the time. There are number of pros to this method, main ones being: reversibility, glue not affecting veneer finish and glue pulling veneer down while drying (no need for clamping). There are also some cons, like terrible smell of the glue, the fact that it can be dissolved in water and need of hot application. I did not want to spend a fortune while trying this method, hence, I have bought a wax heater from eBay instead of professional hot glue pot (9.99 instead of 99) and I have also made my own veneer hammer from a piece of wood and a small sheet of 3mm brass (5 instead of 30).
   Having done all the preparations, work was started from cutting up the veneer into smaller pieces. Pieces were marked on the reverse to indicate which enclosure and side they should go onto. This was done to ensure continuity of the wood grain when the veneer is attached to the enclosure. Veneer sheets were then glued to the enclosures in the following order a - sides, b - tops, c - fronts. This order provided best visual result as the front sheet covers the edges of the side and top veneers. Once the glue dried, enclosures were sanded, starting with 120 and finishing on 180 Grit. Masking tape was applied on the veneer, and the bass reflex holes were cut out. Enclosures were then varnished using 4 layers of water based varnish. 240 Grit sandpaper was used to sand between coats of varnish.
   The process resulted in fairly good looking enclosures, however, if you look closely at the finished enclosures, you can notice 'stain' patterns on the fronts - I was probably too enthusiastic with my hammer or the heat that I have applied. I find hammer veneering a lot more difficult than it looks on YouTube videos. I probably need more practice, nonetheless, I find the PVA glue and clamps a lot more user friendly than hammer veneering.

 

Port Tuning & Listening Tests
   Initial port calibration was done during the building process (see image below) and it resulted in perfect vent tuning. Once these subwoofers were ready, they were measured again to ensure that they match initial measurements, and they did. The time has come to do first listening tests. Much to my surprise, these DIY Rogers AB1s produced way to much bass. I have tried different port lengths and different amounts of damping material inside the enclosures, with no success. Even with the foam plugs in the bass reflex ports, these subwoofers provided too much bass when coupled with my Chartwells LS3/5A. I have also tried using drivers within the LS3/5A specification and the results were exactly the same. At this point my only conclusion was that AB1s are too sensitive in comparison to my 15ohm LS3s.
   Number of listening tests were conducted with the L-Pad attenuator, with the aim of limiting sensitivity of the AB1s. Eventually the perfect setting was found - sensitivity was reduced by 7.5dB.
L-Pad attenuators was replaced with good quality, high power resistors.
   I'd like to emphasise how important matching sensitivity actually is. Before installing permanent L-Pads the amount of bass was too overwhelming, and it made the whole set (AB1 + LS3s) sound disjointed. It was only since L-Pad installation when the subwoofers started properly blending with LS3s, making the whole set sound coherent. It was only at this stage where I was actually able to appreciate how much improvement carefully tuned AB1s can bring to any set of LS3s.
   Another thing that is worth mentioning is the measurement visible below (Image 67). Based on this graph, one may think that my AB1s do not add enough bass and that I should not have installed the L-Pads. One would be very wrong as often things that look good on the graphs do not always sound as good as the graphs would suggest. Also, sometimes speakers that measure not very well, sound surprisingly good.
What I am trying to say is that measurements are important but there are number of other factors that need to be taken into consideration, including how much you like what you are hearing...

 

Enclosure Finishing
   Finally, time has come to start putting things together. Previously cut foam sheets were glued into enclosures using spray contact adhesive. Drivers were mounted in the enclosures, all connectors were soldered and insulated using heat shrink tubing. The plinths were cut out of 25mm MDF and spray painted black. Spikes were attached at the bottom of the plinths and the plinths were attached to the bottom of the Rogers AB1 enclosures.

 

List of Components & Prices
   Table below shows list of the parts together with prices, necessary to build the enclosures as described above. Please note - some of components that were already in my possession are not on this list (i.e. gasket, screws, PVA glue, etc.)

Project Summary:
Total Time: 35h
Total Cost: 296.15

 

Item Item Specific Unit Quantity Price Per Unit Total Price
KEF B110 Driver SP1003 Piece 2 60.00 120.00
MDL Electrolytic Capacitor 220uF, 100V, 10% Piece 2 4.22 8.84
Jantzen Cross-Cap Capacitor 4.7uF, 400V, 5% Piece 1 3.06 6.12
Jantzen Ferrite Core Inductor 16mH, 0,82Ω Piece 2 16.99 33.98
OFC Wire 1.5mm 1m 3 1.00 3.00
Speaker Terminals Gold Plated Pack 6 2.00 12.00
Brass Spikes & Pads M8, 11mm Long Pack 1 21.99 21.99
Brass Spikes M6, 27mm Long Pack 1 14.99 14.99
Upholstery Foam 25mm, Low Density Sheet 1 14.95 14.95
MDF 2440x1220x15mm Board 1 18.00 18.00
Bass Reflex Port 42mm, 130mm Long Piece 2 3.98 7.96
Cherry Veneer 3150x210x0.6mm Sheet 2 13.60 27.20
Ronseal Interior Varnish Satin, 250ml Can 1 7.12 7.12

TOTAL

296.15

 

Copyright 2014 by Brast.