Concentric Three – M5 Construction Update 1!
For you who are interested in the KEF concentric drivers!
I’ve now finished the cross-over simulation and I’m moving on to the next stage of building the first cross-over prototype and starting the listening tests.
The cross-over:
Picture1:The cross-over is very simple and uses a
LR4 filter topology (acoustical) between the tweeter and mid and a
LR2 topology (acoustical) between the mid and woofer. The cross-over consist of only 12 parts and all sections are second-order electrically, besides the tweeter section which is third-order electrically in order to achieve an acoustical
LR4 frequency roll-off.
Note! The woofer’s are connected with reverse polarity.
The tweeter level can be adjusted by changing the value of the single resistor
R1.
R2 and
R3 can be replaced with one resistor if it can handle >20W.
I don’t reveal the actual component values until I’ve evaluated the cross-over prototype. The cross-over schematic is only used here to inform how the cross-over looks like. Depending on the listening tests and further measurements it can be changed little or completely.
Frequency response:
The cross-over simulation is based on the drivers 15deg off-axis measurements.
Picture2: 15deg off-xis frequency response (no smoothing applied).
Picture3: 15deg off-xis frequency response with mid-range reverse polarity.
The mid-range driver has a built in “BBC” response dip between 2-3.5kHz. The tweeter has a very extended and smooth frequency response, especially considering that this is a co-axial driver unit. The loudspeaker has a system sensitivity of about 88dB @ 2.83v/1m.
When reversing the mid-range polarity we can see very deep notches around 590Hz and 2.8kHz at the cross-over frequencies. The deep notches indicate a very good phase response tracking and summation between the driver units around the cross-over frequency.
Picture4: Frequency response 0-60deg (no smoothing applied).
Picture5: The frequency response curve is an average of four measurements (
blue): on-axis, 15, 22.5 & 30deg and six measurements (
red): on-axis, 15, 22.5, 30, 45 & 60deg.
The loudspeaker has a smooth power response with a slightly downwards tilting response at higher frequencies. The tweeter has an extended frequency response with very wide off-axis dispersion.
Picture6: Four different tweeter levels.
Picture7: System impedance simulation.
The resistor
R1 can be changed to fine tune the tweeter level. The level 2 is the “reference” level used in the above frequency and cross-over simulations. The minimum impedance occurs @ 120Hz and is slightly below 3 Ohms. The system sensitivity of 88dB @ 2.83v/1m is mainly achieved by the low impedance driver units used in the loudspeaker.
Summation:
As I've written before on my blog, I don’t intend to make a clone of any of KEF’s loudspeakers and I don’t have a clue how the cross-over for KEF’s loudspeakers looks like, even though my chosen cross-over frequencies seems rather similar to those used in the R500 loudspeaker. It just happens to be very suitable frequencies to cross-over at. I would say that a suitable cross-over window is about +/- 100Hz from that I have chosen, at least the drivers seems to be engineered in that way.
With that said, the KEF driver is really great and I’m eagerly waiting for the cross-over components I’ve ordered for the first prototype and I can’t wait to have a listen to the
Concentric Three – M5 loudspeaker!
For further details on the construction and driver unit measurements see:
Concentric Three – M5 Construction Update 1!
Concentric Three – M5 Construction!
KEF SP1632 Measurements!
SB Acoustics SB15NRXC30-8 Measurements!
The KEF R500 loudspeaker using the same mid/tweeter and similar sized woofer’s:
Hi-Fi Speakers - R Series - Fact Sheets - Floorstanding - R500 - KEF International
Independent KEF R500 loudspeaker measurements:
SoundStage - KEF R500 Measurements!
Regards
/Göran
