I’ve started the construction of the Concentric Three – M5 loudspeaker design. The main feature in this design is the KEF SP1632 co-axial driver unit.
The KEF SP1632 is an aluminium/magnesium 5” mid-range driver paired with a 1” aluminium dome tweeter mounted in a wave-guide like setup. There are a lot of co-axial PA drivers available to DIY:ers, but there aren’t many so called HiFi drivers available to choose from. SEAS has a couple of drivers, but in my opinion none of them come close to the features and performance of this new KEF driver.
Since the driver is a pure mid-range and cannot play bass, it has to be paired with a woofer or mid-woofer in a three-way design. In this first setup I chose to pair the KEF SP1632 with two SB Acoustics 5” mid-woofers in a Dáppolito configuration mounted in a Dayton 28 liter M-T-M enclosure.
The KEF driver is mounted in a closed box with a net volume of about 2.5 liters. In the Dayton enclosure this leaves about 12 liters each for the two mid-woofers and is about optimal for these drivers. A port tuning between 38-40Hz seems feasible and will be tested.
This loudspeaker could be built as a stand-mount monitor like I do or as a floor-stander, perhaps paired with an active sub-woofer in the excess volume of the floor-stander.
One could say that this design very much look like the KEF R500, but let me point out that I don’t intend to build any sort of clone of the KEF “R” loudspeaker series. I don’t have a clue how the filter design used in this loudspeaker series look like besides the official specification about cross-over frequencies etc. In fact it’s not interesting at all, since all I want to do is to find a truly nice co-axial driver for my loudspeaker design and in this case the KEF SP1632 was the best driver I could find for my design needs.
I will base my design on the measurements I’ve done on the SP1632 and that alone together with my own preferences will decide the cross-over topology cross-over frequencies and so on. As a DIY:er I have the advantage not having to consider making design choices based on reducing cost for e.g. the filter design. Consequently I will use the quality and the amount of cross-over components needed for the intended design.
I don’t know if KEF has done such design choices with its R series designs, but for a commercial loudspeaker manufacturer like KEF they always have to weigh in production cost and often in this price range at the expense of a non-optimal performance.
The details of the driver unit measurements can be found here:
Currently I’m running the Concentric Three – M5 with my DEQX HDP-3 pre-amp in an active cross-over configuration using 48dB/octave phase-linear FIR filters with a cross-over frequency of 500Hz and 2.8kHz.
I use my DEQX setup to burn-in the drivers and to get a general feeling of the driver’s characteristics and usable frequency ranges etc.
Even in this early stage of construction and evaluation the sound from these KEF drivers is really nice and I’m looking forward to start with the passive cross-over design and simulations.
Regards
/Göran