KEF SP1632

Overview:

 

The KEF SP1632 co-axial Uni-Q driver unit is a 5” 4Ω mid-range driver paired with a 4Ω 1” tweeter placed in the centre of the cone. The SP1632 is used by KEF in the “R Series” loudspeakers. It has a very sturdy and nice build quality, but this driver can’t be used for mid-woofer duties, since it’s a pure mid-range driver and consequently it has to be used in a three way loudspeaker design.

The stiff magnesium/aluminium mid-range cone has special molded braces in the cone in order to control and suppress the mid-range’s cone break-up. The tweeter is mounted in the throat of the mid-range driver and has a “tangerine” wave-guide in front of the dome.

The shape of the mid-range cone, Z-flex surround and driver design ring together with the “tangerine” technology makes a wave-guide for the tweeter, which in turn gives a super smooth tweeter frequency response with a very wide dispersion.

 

 

Here’s how KEF R Series describes the new “R Series” Uni-Q driver unit:

“The new Uni-Q MF/HF driver array is a masterpiece. Midrange response is amazingly fast and clean, thanks to a braced magnesium/aluminium alloy cone, with KEF’s Z-flex surround smoothing the transition to the front panel and a sturdy die-cast aluminium chassis that prevents resonance being transferred to the cabinet.

At the acoustic centre of the midrange cone is the sophisticated vented tweeter that endows KEF Blade with such startlingly pure HF response. With a large neodymium magnet and a computer-optimised dome structure so stiff that it operates pistonically over its entire working range, the upper registers are articulated with flawless precision – and to capitalise on Uni-Q’s point source characteristics, KEF’s ‘tangerine’ waveguide helps to disperse the higher frequencies evenly across a wider angle to flood the room with an intricate and refreshingly natural soundfield.

Operating together as a single, perfectly integrated voice, the array as a whole delivers a sweet, accurate sound of unprecedented clarity.”

KEF certainly has the market lead in this segment and by using cutting edge technology it has for a co-axial driver “world class” performance.

Note! The KEF SP1632 driver isn’t generally available to DIY:ers. Either it has to be stripped from a loudspeaker in the “R Series” or ordered as a spare part, but this requires a valid “R Series” serial number in order to be able to obtain the driver.

T/S Parameters:

 

A fairly sensitive mid-range, mainly because of the low Dc resistance.  About 8% difference in Fs for the mid-range and about 5% for the tweeter, but overall a good consistency between the driver unit samples.

Impedance:

 

(click on picture to zoom)

Left: Mid-range free-air impedance sample 1 vs. sample 2.

Right: Tweeter free-air impedance sample 1 vs. sample 2.

Slightly different impedance profile between the tweeter samples. Based on the impedance response, it looks like the tweeter uses ferro-fluid in the magnetic gap.

Frequency:

 

Frequency measurement conditions:

The mid-range is measured mounted on a baffle in an 2.5 liter (net volume) closed mid-range enclosure with the following conditions:

Baffle size (WxH): 22,86×55,88cm with 12mm round-over on the baffle edges (r=12).
Driver position: Mounted on center-line with driver unit centered in the middle of the baffle.
Mic position: 1m distance, on mid-range/tweeter-axis. Measurement valid down to 275Hz.
Smoothing: 1/24 octave smoothing applied.

 

Mid-range:

(click on picture to zoom)

Left: Frequency response on-axis (Blue=sample1, Red=sample2).
Right: Frequency response 15deg off-axis (Blue=sample1, Red=sample2).

The two mid-range samples measures virtually the same, which indicates good consistency in the manufacturing process.

 

0deg = Blue
15deg = Red
22.5deg = Green

 

30deg = Blue
45deg = Red
60deg = Green

Summary:

Very smooth frequency response up to 2kHz where it gradually starts to smoothly roll-off. No visible sharp cone break-up effects due to the hard stiff mid-range cone.The mid-range has an usable frequency response up to about 3kHz.

 

Tweeter:

(click on picture to zoom)

Left: Frequency response on-axis (Blue=sample1, Red=sample2).
Right: Frequency response 15deg off-axis (Blue=sample1, Red=sample2).

The two tweeter samples measures virtually the same, which indicates good consistency in the manufacturing process. Note the sharp aluminium cone break-up around 39kHz. This break-up is about 10-15kHz higher up in frequency than usual for this size/type of dome tweeter. The break-up influence is well outside the tweeters usable frequency range.

 

0deg = Blue
15deg = Red
22.5deg = Green

 

30deg = Blue
45deg = Red
60deg = Green

Summary:

The “tangerine” waveguide works as advertised and the tweeter has a very extended smooth frequency response, well above 30kHz with great dispersion. It has no signs of any sharp dips in the frequency response between 10-20kHz, which is very common for waveguide mounted tweeters. For a co-axial driver unit, this is simply an outstanding performance.

Distortion:

 

Measurement setup:

  • Mid-range\tweeter near-field measurement at 15cm
  • Frequency Range Mid-range: 200-10000Hz
  • Frequency Range Tweeter: 1000-10000Hz
  • Baffle size WxH: 22,9×55,9cm

 

The distortion measurements are done in near-field and the amplifier output level was adjusted for each driver so that the fundamental is 90dB and 95dB at 1m. This setting simulates medium-high and high listening levels.

 

Mid-range:

90dB @ 1m (click on picture to zoom)
Left = Sample 1
Right = Sample 2

95dB @ 1m (click on picture to zoom)
Left = Sample 1
Right = Sample 2

Summary:

Good distortion performance in the lower mids. Below 300Hz the distortion increases rapidly. Average distortion performance in the upper mids. The odd-order distortion is elevated at higher frequencies, but without sharp spikes of distortion, which indicates an effective treatment and suppression of cone break-ups.

Tweeter:

90dB @ 1m (click on picture to zoom)
Left = Sample 1
Right = Sample 2

95dB @ 1m (click on picture to zoom)
Left = Sample 1
Right = Sample 2

Summary:

Good distortion performance above 3kHz. Below 2.5kHz the distortion increases rapidly. This tweeter shouldn’t be crossed-over to low. A cross-over frequency around 3kHz and with rather steep filters (LR4, acoustical) should keep the distortion out of harm’s way.