{"id":3807,"date":"2013-07-15T10:51:39","date_gmt":"2013-07-15T10:51:39","guid":{"rendered":"http:\/\/www.audioexcite.com\/?page_id=3807"},"modified":"2020-11-28T14:36:13","modified_gmt":"2020-11-28T14:36:13","slug":"concentric-three-m5","status":"publish","type":"page","link":"https:\/\/www.audioexcite.com\/?page_id=3807","title":{"rendered":"Concentric Three \u2013 M5"},"content":{"rendered":"

Concept:<\/span><\/h3>\n

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Concentric Three – M5<\/strong><\/span> is a large sized 3-way stand-mount loudspeaker or if preferred, it can be built as a small sized floorstander. It\u2019s suitable for small to medium sized listening rooms. Several tuning options are available to fine tune it according to personal sound preferences.<\/p>\n

Loudspeaker driver units:<\/span><\/h3>\n

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Concentric Three – M5<\/span><\/strong> uses a concentric driver (co-axial) unit from KEF. The KEF SP1632 co-axial Uni-Q driver unit is a 5\u201d 4\u2126 mid-range driver paired with a 4\u2126 1\u201d tweeter placed in the centre of the cone. The SP1632 is used by KEF in the \u201cR Series\u201d loudspeakers. It has a very sturdy and nice build quality, but this driver can\u2019t be used for mid-woofer duties, since it\u2019s a pure mid-range driver and consequently it has to be used in a three way loudspeaker design.<\/p>\n

The stiff magnesium\/aluminium mid-range cone has special molded braces in the cone in order to control and suppress the mid-range\u2019s cone break-up. The tweeter is mounted in the throat of the mid-range driver and has a \u201ctangerine\u201d wave-guide in front of the dome.<\/p>\n

The shape of the mid-range cone, Z-flex surround and driver design ring together with the \u201ctangerine\u201d technology makes a wave-guide for the tweeter, which in turn gives a super smooth tweeter frequency response with a very wide dispersion.<\/p>\n

Note!<\/strong> The KEF SP1632 driver isn\u2019t generally available to DIY:ers. Either it has to be stripped from a loudspeaker in the \u201cR Series\u201d or ordered as a spare part, but this requires a valid \u201cR Series\u201d serial number in order to be able to obtain the driver.<\/p>\n

The woofer section consists of two SB Acoustics SB15NRXC30-8, 5\u2033 woofers. By using two 5\u201d woofers we have about 25% more cone area compared to a single 7\u201d woofer.<\/p>\n

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.<\/p>\n

For further details see:<\/p>\n

KEF SP1632<\/a>
\nSB Acoustics SB15NRXC30-8<\/a><\/p>\n

Cabinet:<\/span><\/h3>\n

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Concentric Three – M5<\/strong><\/span> uses a 28 liter \u201cDayton Audio\u201d enclosure. This is a very nicely built curved cabinet with a 25mm (1\u2033) 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.<\/p>\n

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Cabinet drawing:Dayton 28 liters Curved Cabinet #302-751<\/a><\/p>\n

As an option Concentric Three – M5<\/span><\/strong> 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.<\/p>\n

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The mid-range enclosure is made of 10mm MDF with the dimension 17x17x13cm. The internal mid-range enclosure net volume is 2.7 liter.<\/p>\n

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 \u201cbreathe\u201d properly, don\u2019t forget to chamfer the baffle.<\/p>\n

Box simulation:<\/span><\/h3>\n

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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\u00a0 about 24 liters.<\/p>\n

24 liter bass-reflex box with Fb = 37Hz (anechoic response)
\n-6db = 33Hz
\n-12db = 27Hz<\/p>\n

24 liter closed box with Qt = 0,78 (anechoic response)
\n-6db = 48Hz
\n-12db = 31Hz<\/p>\n

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.<\/p>\n

Cross-over design:<\/span><\/h3>\n

 <\/p>\n

The Concentric Three \u2013 M5<\/strong><\/span> uses (acoustically) a second-order Linkwitz-Riley (LR2<\/strong>) filter topology for the mid and woofer section and a fourth-order Linkwitz-Riley (LR4<\/strong>) topology between the mid and tweeter section.<\/p>\n

The filter layout is very simple and consists of only eleven components:<\/p>\n

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 <\/p>\n

The woofer cross-over filter section is very simple and consists of a large coil (L4<\/strong>) and a cap (C5<\/strong>) 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 \u201cBaffle Step Compensation\u201d (BSC).<\/p>\n

Note!<\/strong> The woofers are connected in reverse polarity.<\/p>\n

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

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

(L3<\/strong>) can be adjusted to fine tune the mid-range level and response. See the cross-over simulation section for more details.<\/p>\n

(R2+R3<\/strong>) 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.<\/p>\n

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

(R1<\/strong>) 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.<\/p>\n

Note!<\/strong> Both the mid-range and the tweeter are connected with the same polarity.<\/p>\n

Cross-over simulation:<\/span><\/h3>\n

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Top:<\/strong> Tweeter-axis, 15deg off-axis response.<\/p>\n

Bottom:<\/strong> Tweeter-axis, 15deg off-axis response, mid-range reverse polarity.<\/p>\n

The reverse polarity simulation shows deep reverse nulls indicating an excellent phase tracking between the driver units at the cross-over points.<\/p>\n

Mid-range tuning options:<\/strong><\/p>\n

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Above:<\/strong> Mid-range level.<\/p>\n

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

I personally think the \u201cReference\u201d value of 0.82mH for the (L3<\/strong>) 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.<\/p>\n

Tweeter tuning options:<\/strong><\/p>\n

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Above:<\/strong> Tweeter level.<\/p>\n

The recommended value for (R1<\/strong>) 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 \u201cReference\u201d value yields the best tweeter response without being too bright, but at the same time give a very nice resolution and \u201cair\u201d to the soundstage.<\/p>\n

Due to the position of the (R1<\/strong>) in the tweeter circuit, changing the (R1<\/strong>) value also affect the mid-range response slightly. By omitting the (R1<\/strong>) completely it actually smooth out the \u201cBBC\u201d frequency dip between 2-4kHz to some extent.<\/p>\n

Tuning option summary:<\/strong><\/p>\n

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Above:<\/strong> Reference level v.s. L3<\/strong>=0.56mH and R1<\/strong> omitted.<\/p>\n

By changing the (L3<\/strong>) and (R1<\/strong>) 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 \u201cReference\u201d values.<\/p>\n

I strongly recommend using the \u201cReference\u201d values as a baseline and then try to change (R1)<\/strong> and possibly the (L3<\/strong>) and listen to how it changes the sound and see if it fits your own personal taste.<\/p>\n

Impedance measurements:<\/span><\/h3>\n

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Blue<\/strong>=Left loudspeaker<\/p>\n

Green<\/strong>=Right loudspeaker<\/p>\n

Very small phase shifts and a 3 Ohm impedance minimum @ 120Hz.<\/p>\n

Frequency measurements:<\/span><\/h3>\n

 <\/p>\n

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.<\/p>\n

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

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Frequency response 15deg off-axis:<\/strong><\/p>\n

Blue:<\/strong> Left Loudspeaker<\/p>\n

Red:<\/strong> Right Loudspeaker<\/p>\n

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.<\/p>\n

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\n(click on picture to zoom)<\/p>\n

Top left:<\/strong> Tweeter-axis @ 2m, 0, 15 and 22.5deg off-axis.<\/p>\n

Top right:<\/strong> Tweeter-axis @ 2m, 30, 45 and 60deg off-axis.<\/p>\n

Lower left:<\/strong> Mid-range reverse polarity.<\/p>\n

Lower right:<\/strong> Tweeter level. Blue:<\/strong> R1 = omitted, Red:<\/strong> R1 = 0.82Ohm, Green:<\/strong> R1 = 1.5Ohm<\/p>\n

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Vertical frequency response @ 15deg off-axis:<\/strong><\/p>\n

Above:<\/strong> Vertical frequency response: Blue = +5deg, Red = +10deg and Green = +15deg above tweeter axis.<\/p>\n

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.<\/p>\n

Distortion measurement:<\/span><\/h3>\n

 <\/p>\n

Measurement setup:<\/strong><\/p>\n