This project would have been
impossible without the help and support of Jo Ki, Andy Whittle and Ken Kessler.
As well as inspiring me to add some subs to my LS3/5as, Jo went way beyond the
call of duty and opened up his AB1s in order to check the dimensions I had
calculated. Andy is the designer of the original AB1 and following an approach
from Ken, very generously agreed to supply details of the AB1's crossover for
inclusion here.
Although I have owned LS3/5as for perhaps 25 years I have to confess that it
was only this year (2000) that I first heard them partnered with the Rogers
AB1s. I was very fortunate to receive an invitation to visit Joseph Ki when I
was in Malaysia during August. Jo's system really knocked me out, it sounded so
good that I was very tempted to buy a pair of AB1s and bring them back to the
UK. The price of AB1s there was very good compared to the UK and finally it was
simply lack of sufficient luggage allowance and the risk of a nasty excess
baggage charge that stopped me doing that.
On my return to the UK, without the AB1s, it dawned on me that I might try building some subwoofers based on the concept of the excellent AB1s. I already had a spare pair of the B110 drive units and so I decided to find out as much as I could about the Rogers' design.
Quite a lot of information about the AB1 is available from reviews and from Rogers' own literature . The design is a fourth order bandpass subwoofer with -3dB point at 55Hz. The LS3/5a is fed from the subwoofer through a second order high pass filter which takes a lot of the strain off from the LS3/5a and as well as extending the frequency response downwards also allows a couple of dB extra spl from the combo.
An email to KEF resulted in the data sheet
for the B110 arriving next morning in the post (thank you Ron). It's great to
know that KEF still provide excellent service for amateur constructors.
Design information for sub woofers including Fourth Order Bandpass systems is available from the DIY Subwoofers pages which provide an excellent resource for sub woofer builders. From this site a design spreadsheet written in Excel called "Bandpass" can be downloaded.
"Plugging in" the values for Vas, Fs and Qts from the B110 data sheet and using a value of -3dB at 55Hz into "Bandpass" gave me values for Vf, (the net volume of the vented section of the enclosure) and for Vr (the sealed section).
The external dimensions are 570mm high (the original AB1 has a 25mm plinth
bringing the height up to 595mm), 190mm wide and 160mm deep.The cabinet walls
are made from 18mm MDF (also own as Medite) and the internal baffle for the B110
drive unit is made from 12mm MDF.
A local timber merchant cut the panels from a single sheet of 4ft x 4ft MDF
and the panels (with the exception of the back) were glued and screwed together.
All joints were sealed using frame sealant, it is vital they are airtight.
The AB1's port is 4.2cm in diameter and 8.5cm long, I hunted round for some plastic tube 4.2cm in diameter before realising that the plastic tube that the frame sealant had come in was 4.6cm inside diameter. Re-calculating the port gave a new length of 10.5cm for this diameter, an excellent result given that the frame sealant tube was exactly 21cm in length!
Car body damping pads were applied to all internal cabinet walls and on top of these 1/2" (12mm) high density foam was glued. The sealed section of the cabinet was stuffed with foam and the drive units were mounted before screwing down the back.
Andy Whittle, the designer of the AB1 kindly supplied the circuit of the AB1 crossover for publication here. Thanks both to Andy and to Ken Kessler for their help.
I was unable to obtain 16mH inductors off the shelf so I used 15mm Ferrite Cored inductors obtained from Wilmslow Audio and increased the value of the capacitors to 240uF to keep the crossover turnover frequency the same. Wilmslow Audio also sell B110 drive units, damping pads and suitable non-polarised electrolytic capacitors.
The crossover components were mounted on a
simple PCB. Unwanted areas of copper can be removed with a craft knife.
Components are mounted on the copper side of the board and the ferrite inductor
is secured to the PCB using cable ties through 4 small holes drilled in the
board. I mounted my crossovers in small ABS boxes on the rear of the cabinets
because in the future I want to experiment with bi-amping and active crossovers.
In the real AB1 the crossover PCBs are mounted in the sealed section and a
second set of terminals feeds the LS3/5as.
I tested the subs with my LS3/5as to ensure they were working. Special attention was paid to make sure there were no air leaks. I then veneered and varnished them. The results are excellent, the subs allowing the LS3/5as to go deeper and louder whilst retaining the excellent midrange and stereo image of the LS3/5a. I own a pair of the Grade 1 BBC LS5/9 monitors and was able to compare the combos with them directly. The bottom end compares very well with the LS5/9 (although this speaker goes louder) and I find I much prefer using the LS3/5as and subs.
If you cannot get hold of a pair of real AB1s, want to save some money, or like me simply enjoy a constructional project, then these are well worth making. If you have to buy everything from new, including the B110 drivers, the total cost is around £120.00 for the pair.