Concentric Three – M5

Concept:

Concentric Three – M5 is a large sized 3-way stand-mount loudspeaker or if preferred, it can be built as a small sized floorstander. It’s suitable for small to medium sized listening rooms. Several tuning options are available to fine tune it according to personal sound preferences.

Loudspeaker driver units:

 

Concentric Three – M5 uses a concentric driver (co-axial) unit from KEF. 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.

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.

The woofer section consists of two SB Acoustics SB15NRXC30-8, 5″ woofers. By using two 5” woofers we have about 25% more cone area compared to a single 7” woofer.

The SB15NRXC30-8 woofer has a 30.5mm voice-coil paired with a healthy 10mm (p-p) linear coil travel. It has a very low voice-coil inductance and is fitted with a copper sleeve on the pole piece to ensure a low distortion performance. For its size it has relatively low resonance frequency and is capable to deliver a powerful low frequency response.

For further details see:

KEF SP1632
SB Acoustics SB15NRXC30-8

Cabinet:

Concentric Three – M5 uses a 28 liter “Dayton Audio” enclosure. This is a very nicely built curved cabinet with a 25mm (1″) MDF front-baffle. The enclosure walls are made of 18mm MDF as well as a 15mm brace from the bottom to the top. The baffle edges have a 12mm round-over.

 

Cabinet drawing:Dayton 28 liters Curved Cabinet #302-751

As an option Concentric Three – M5 can be built as a floorstander (see right baffle drawing). As long as the baffle size and driver unit lay-out are made as described here. The residual box volume of the floorstander can house the cross-over and/or be filled with e.g. sand.

The mid-range enclosure is made of 10mm MDF with the dimension 17x17x13cm. The internal mid-range enclosure net volume is 2.7 liter.

The internal cabinet walls are taped with strings of bitumen pads to reduce resonances. The enclosure is lightly filled with sheep wool. All drivers are flush mount and in order to let the mid-range driver “breathe” properly, don’t forget to chamfer the baffle.

Box simulation:

This is the anechoic response when using The 28 liter Dayton audio enclosure. If the volume for the mid-range enclosure and port are subtracted we have an effective net volume of  about 24 liters.

24 liter bass-reflex box with Fb = 37Hz (anechoic response)
-6db = 33Hz
-12db = 27Hz

24 liter closed box with Qt = 0,78 (anechoic response)
-6db = 48Hz
-12db = 31Hz

The closed box version could be an option if the loudspeaker is placed very close to the rear wall or actively crossed-over and used together with a sub-woofer.

Cross-over design:

 

The Concentric Three – M5 uses (acoustically) a second-order Linkwitz-Riley (LR2) filter topology for the mid and woofer section and a fourth-order Linkwitz-Riley (LR4) topology between the mid and tweeter section.

The filter layout is very simple and consists of only eleven components:

 

The woofer cross-over filter section is very simple and consists of a large coil (L4) and a cap (C5) which shapes the cross-over slopes to a LR2 roll-off with a targeted 570Hz cross-over point. The woofers are connected in parallel and the design is tuned to a full “Baffle Step Compensation” (BSC).

Note! The woofers are connected in reverse polarity.

The mid-range cross-over filter section is also very simple and the high-pass section consists of a cap (C3) and a coil (L4) which shapes the cross-over slopes to a LR2 roll-off with a targeted 570Hz cross-over point.

The low-pass section consists of a coil (L3) and a cap (C4) which shapes the cross-over slopes to a LR4 roll-off with a targeted 2.8kHz cross-over point.

(L3) can be adjusted to fine tune the mid-range level and response. See the cross-over simulation section for more details.

(R2+R3) are two 5.6 Ohm resistors connected I parallel for a total resistance of 2.8 Ohm and is used to adjust the mid-range level to match the woofers response.

The tweeter cross-over filter section consists of a (C1+L1+C2) which shapes the cross-over slopes to a LR4 roll-off with a targeted 2.8kHz cross-over point.

(R1) can be adjusted or omitted in order to fine tune the tweeter level and to some degree the mid-range response. See the cross-over simulation section for more details.

Note! Both the mid-range and the tweeter are connected with the same polarity.

Cross-over simulation:

 

Top: Tweeter-axis, 15deg off-axis response.

Bottom: Tweeter-axis, 15deg off-axis response, mid-range reverse polarity.

The reverse polarity simulation shows deep reverse nulls indicating an excellent phase tracking between the driver units at the cross-over points.

Mid-range tuning options:

Above: Mid-range level.

Even though I recommend the mid-range level as described in the cross-over section the (L3) coil can be changed to fine tune the frequency response in the mid-range. By using a smaller value for the (L3) the frequency response can be changed about +/- 2dB between 1.5-3kHz.

I personally think the “Reference” value of 0.82mH for the (L3) gives the best balance between sound stage depth and up-front presence. The option of 0.56mH can sound impressive at first with a huge presence and soundstage, but can be a bit too forward sounding and tiresome in the long run for my taste.

Tweeter tuning options:

Above: Tweeter level.

The recommended value for (R1) is 0.82 Ohm, but it can be changed to fine tune the tweeter frequency response and it can in fact even be omitted completely if desired. Personally I think the “Reference” value yields the best tweeter response without being too bright, but at the same time give a very nice resolution and “air” to the soundstage.

Due to the position of the (R1) in the tweeter circuit, changing the (R1) value also affect the mid-range response slightly. By omitting the (R1) completely it actually smooth out the “BBC” frequency dip between 2-4kHz to some extent.

Tuning option summary:

Above: Reference level v.s. L3=0.56mH and R1 omitted.

By changing the (L3) and (R1) values you can fine tune the sound to fit your HiFi equipment and your personal preferences. I spent many listening hours trying to find the best balance between resolution details, soundstage and depth and the result is the “Reference” values.

I strongly recommend using the “Reference” values as a baseline and then try to change (R1) and possibly the (L3) and listen to how it changes the sound and see if it fits your own personal taste.

Impedance measurements:

 

Blue=Left loudspeaker

Green=Right loudspeaker

Very small phase shifts and a 3 Ohm impedance minimum @ 120Hz.

Frequency measurements:

 

If nothing else is noted in the comments, the following frequency measurements of the finished loudspeaker are made at a 2m distance at tweeter height for the “Reference” tuning option.

All frequency response charts are presented in a 50db scale with 1/24 octave smoothing and the measurements are valid down to 375Hz.

Frequency response 15deg off-axis:

Blue: Left Loudspeaker

Red: Right Loudspeaker

A very nice frequency response match between the two loudspeakers. A small “BBC” dip between 2-4kHz, but otherwise a very linear and extended frequency response.



(click on picture to zoom)

Top left: Tweeter-axis @ 2m, 0, 15 and 22.5deg off-axis.

Top right: Tweeter-axis @ 2m, 30, 45 and 60deg off-axis.

Lower left: Mid-range reverse polarity.

Lower right: Tweeter level. Blue: R1 = omitted, Red: R1 = 0.82Ohm, Green: R1 = 1.5Ohm

 

Vertical frequency response @ 15deg off-axis:

Above: Vertical frequency response: Blue = +5deg, Red = +10deg and Green = +15deg above tweeter axis.

If you stand up or sit down the frequency response is virtually unchanged. The “BBC” dip flattens out a bit when you listen above the tweeter axis.

Distortion measurement:

 

Measurement setup:

  • Tweeter-axis near-field measurement at 20cm
  • Frequency Range: 200-10000Hz
  • Baffle size WxH: 23×56cm (baffle edge round-over r=12mm)

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 and high listening levels.

(click on picture to zoom)

Top left: Left loudspeaker, 90dB @ 1m

Top right: Right loudspeaker, 90dB @ 1m

Lower left: Left loudspeaker, 95dB @ 1m

Lower right: Right loudspeaker, 95dB @ 1m

Overall a low distorting loudspeaker with a remarkable low second order harmonics. The third order harmonics is well below 0.4%, even at higher listening levels.

Component list:

 

The cost of the driver units are about 40% of the total cost of the suggested configuration. In this configuration the loudspeaker would cost about 1515 US$, but the cost can be reduced by choosing less expensive cross-over components and by building your own enclosures.

I strongly recommend using a copper foil coil for the L3 inductor. It really makes a positive noticeable effect on the mid-range clarity and transparency.

The L4 inductor can be replaced by a cheaper one. The inductor resistance should be 0.5 Ohm, but anything between 0.4-0.6 Ohm is acceptable.

The C5 68uF cap can be replaced by a simpler one, but make sure it measures 68uF +/- 3%.

Summary:

 

Sound description:

The KEF SP1632 is a very nice sounding driver and it delivers an amazing clarity and transparency, even at higher listening levels.

The mid-range is pure, neutral and natural sounding. The tweeter is smooth, refined and airy without ever sounding harsh or exaggerated.

Considering the loudspeakers size, the two SB Acoustics woofers deliver a bass which is deep and powerful.

The loudspeaker has very good imaging with a very precise focus and a huge soundstage with a good sense of depth.

The crossing between the mid-range and tweeter is seamless and together they deliver a remarkable purity, refined and open sound without any hint of cold analytic sound.

Much thanks to the KEF concentric driver unit, the soundstage is intact even if you stand up at the listening position and it doesn’t collapse as many loudspeakers do when moving from the sweet spot.

 

Recommended loudspeaker placement:

  • No or little toe-in.
  • The loudspeaker can be to bass heavy if placed too close to the wall. I suggest at least 0.5m or preferably 1m from the back wall.
  • 2.5-3.5 meters apart.
  • 2.5-4 meters listening distance

The loudspeaker is optimized for a 15deg off-axis listening angle, but as always it pays off to test different loudspeaker placements and see what fits the room and personal preferences the best.

Technical Specification:

 

Concentric Three – M5
Type: Three-way large sized stand-mount or small size floor-stand loudspeaker
Enclosure type: Bass-reflex
Woofer driver unit: 2x 5” Coated Papyrus fiber cone. SB Acoustics SB15NRXC30-8
Mid-range driver unit: 5” Magnesium/aluminium cone. KEF SP1632
Tweeter driver unit: 1” Aluminium dome KEF SP1632
Cross-over frequency: 570Hz and 2.8kHz
Cross-over function: Woofer-mid, LR second-order acoustically. Mid-Tweeter, LR fourth-order acoustically.
Frequency response: 33-35000Hz -6/+0dB
Sensitivity: 88db 1m/2.83v
Impedance: 4 Ohm
Enclosure volume: 28 liter internal volume
Weight: 20kg
Enclosure dimensions: (height x width x depth): 559x229x365mm (curved cabinet)

 

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Thank you!

/Göran

Author of the “AudioExcite Loudspeaker Design” website