Kensho Two – M2

Concept:

 

Kensho Two – M2 is a small sized 2-way stand-mount bass-reflex loudspeaker. It’s ideal for small to medium sized listening room. The tweeter level can be fine-tuned according to personal preferences and taste.

“Kensho” is Japanese for “seeing nature.” It means seeing one’s true nature; perceiving the self, but can also in Zen Buddhism mean “a brief experience of enlightenment”.

Loudspeaker driver units:

 

Kensho Two – M2 uses a 5″ mid-woofer from SB Acoustics and a ribbon tweeter from Fountek.

The SB15MFC30-4 is a low distorting 4Ω mid-woofer with a mineral filled polypropylene cone.

The Fountek NeoCD3.5H is a horn-loaded true ribbon tweeter that uses a reinforced sandwiched-aluminum ribbon diaphragm together with a strong neodymium magnet and it has a wide horizontal dispersion.

For further details see:

Fountek NeoCD3.5H
SB Acoustics SB15MFC30-4

Cabinet:

 

The Kensho Two – M2 uses an 8 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 enclosure net volume is around 7 liters (enclosure volume – drivers, filter and port).

Cabinet drawing: Dayton 8 liters Curved Cabinet #302-701

 Baffle drawing and driver unit layout:

KT-M2 -- Baffle drawing

It’s a part of the design that the ribbon tweeter shouldn’t be flush mount, which in turn simplifies the loudspeaker build a lot.  On the other hand, the mid-woofer is flush mount and in order to let it “breathe” properly, don’t forget to chamfer the baffle.

If you are building the 11 liter version you must keep the baffle width and driver unit lay-out, but you can make the loudspeaker taller by adding a couple of cm below the mid-woofer and make the enclosure deeper.

I recommend using a separate compartment for the Fountek ribbon tweeter if you are building your own enclosure and not using the Dayton enclosure option.

Box simulation:

KT-M2 -- Sim Box

Bass-reflex 8 liter Fp=46Hz

-6dB = 44Hz
-12dB = 35Hz

Bass-reflex port: 30x90mm

(Optional) Bass-reflex 11 liter Fp=43Hz

-6dB = 38Hz
-12dB = 32Hz

Bass-reflex port: 45x155mm

Make the bass-reflex port about 25% longer than the above example. Measure, cut, measure, cut and so on until you get the desired port tuning frequency.

Impedance measurements:

 

KT-M2 -- Imp 1L vs 2R R2=6.8

(click on picture to zoom)

Blue: Left loudspeaker

Green: Right loudspeaker

Minimum impedance of 3,55 Ohm @ 200Hz.

Cross-over design:

KT-M2 -- x-over

The Kensho Two – M2 uses an acoustical fourth-order Linkwitz-Riley (LR4) filter topology.

The mid-woofer cross-over filter section is an electrical fourth-order and consists of two coils and two caps (L2+L3 and C3+C4) that shapes the cross-over slope to a LR4 roll-off with a targeted 3kHz cross-over point.

The L2 coil and L3 should have a total series resistance of 0.4 Ohm +/- 0.1 Ohm.

The tweeter cross-over filter section consists of two resistor (R1) and (R2), together with a third-order electrical filter (C1 + L1+C2) that shapes the cross-over slope to a LR4 roll-off with a targeted 3kHz cross-over point.

Tweeter level options:

R2 can be changed in order to fine tune the tweeter level according to personal preferences. Any value between 4,7-8,2 Ohm is valid.

Tweeter level

R2= 8.2 Ohm +0,5dB

R2= 6.8 Ohm Reference level

R2= 5.6 Ohm -1dB

R2= 4,7 Ohm -2dB

Warning! Never connect your ribbon tweeter without a protection cap or the tweeter cross-over attached. You will most likely destroy your ribbon tweeter when measured or played without any cross-over.

Cross-over simulation:

 

KT-M2 -- Sim 15deg
KT-M2 -- Sim 15deg RPKT-M2 -- Sim 15deg phase

(click on picture to zoom)

Top: 15deg off-axis frequency response.

Left: 15deg off-axis frequency response, reverse polarity.

Right: Individual driver unit phase tracking.

The reverse polarity simulation shows a deep reverse null indicating an excellent phase tracking between the two driver units. The mid-woofer response is bit elevated in the mids between 0.6-1.3kHz. It rolls off steep above 6.5kHz.

The rising response of the ribbon tweeter between 12-20kHz can be corrected for, but it’s a design choice not to do so. Correcting it doesn’t improve the sonic performance and the rising response only adds more airiness to the sound character.

Frequency measurements:

 

If nothing else is noted in the comments, the following frequency measurements of the finished loudspeaker are made at a 2m distance.

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

KT-M2 -- Freq On-axis 1L vs 2R R2=6.8KT-M2 -- Freq 15deg 1L vs 2R R2=6.8

                 Frequency response on-axis:                                                      Frequency response 15deg off-axis:

Blue: Left Loudspeaker @ 2m

Red: Right Loudspeaker @ 2m

KT-M2 -- Freq 0-15-22.5deg R2=6.8KT-M2 -- Freq 30-45-60deg R2=6.8

(click on picture to zoom)

Left: On-axis, 15 and 22.5deg off-axis @ 2m
Right: 30, 45 and 60deg off-axis @ 2m

KT-M2 -- Freq 15deg system R2=6.8KT-M2 -- Freq 15deg system RP R2=6.8

Left: 15 deg off-axis @ 2m
Right: 15 deg off-axis @ 2m, reverse tweeter polarity

The tweeter is a bit “hot” in the on-axis frequency response, but at 15deg off-axis it’s very linear between 1.5-10kHz. The 15deg off-axis response is the angle the design has been optimized for. The ribbon tweeter has an extended frequency response up to 35kHz with a rising response between 12-20kHz, but this evens out off-axis.

Distortion measurements:

 

Measurement setup:

  • Tweeter-axis near-field measurement at 15cm
  • Frequency Range: 200-10000Hz
  • Baffle size WxH: 19,05×30,5cm

 

The distortion measurements are done in near-field and the amplifier output level was adjusted so that the fundamental is 85dB, 90dB and 95dB at 1m. This setting simulates medium, medium-high and high listening levels.

KT-M2-1L -- 90dB 15cm
KT-M2-1L -- 85dB 15cmKT-M2-2R-- 95dB 15cm

(click on picture to zoom)

Top: 90dB @ 1m

Left: 85dB @ 1m

Right: 95dB @ 1m

At higher listening levels the third order distortion rises between 2.5-4kHz, but is well below 0.4% at medium-high levels. This is a nice distortion performance from such a small low cost ribbon tweeter.

Component list:

 

KT-M2 -- Component list

The suggested components would cost about 650 US$, but the cost can be reduced by building your own enclosures and choosing less expensive cross-over components.

Summary:

 

Kensho Two – M2 is a great performing low cost loudspeaker with higher ambitions than the price tag might suggest.

It can be built as a small form factor 2-way stand-mount loudspeaker with good bass performance or as a slightly larger loudspeaker if bass performance is more prioritized than a small form factor.

The SB Acoustics 5” mid-woofer is a low distorting and bass capable mid-woofer with a nice mid-range presentation. In this price range, it’s as good as it gets and both the SB Acoustics mineral filled PP cone and the NRX paper cone versions are among the very best 5” woofers available, especially considering its moderate price tag.

It’s the ribbon tweeter which makes this loudspeaker design so special. The horn-loaded ribbon tweeter has its unique sound character that brings out the better of two worlds. The nice ribbon shimmer and effortless treble resolution paired with a very good dispersion, both horizontally and vertically. The sonic character of this tweeter paired with my 35W tube amp, makes it almost magical to listen to. A good class A or A/B amp will of course work well to.

Warning though, if you are a person who doesn’t like the sound of ribbon tweeters this isn’t a loudspeaker for you, because you will get a lot of sweet delightful ribbon tweeter sound.

At this price range, It’s not much complain about this Fountek ribbon tweeter, but of course it would have been nice with a molded horn instead of a plastic one and even better low frequency/distortion performance below 4kHz.

The loudspeaker has a very generous sound stage with a nice width, height and depth.

Even though the loudspeaker is designed with a bit of a “hot” tweeter level at higher frequencies, it doesn’t sound sharp or edgy as some dome tweeters could do with same sort of tweeter tuning.

I urge you to test different tweeter levels by swapping the R2 resistor to different values. Together with my tube amp I get the best results with R2=8.2 Ohm, which gives an enormous sound stage, but it can be a bit too much of the “good” when I use my class A amp and I have to reduce the level to R2=6.8 Ohm in order to get the right tweeter balance.

Don’t be afraid to test different tweeter levels that best fits your taste and equipment. There is no right or wrong here. Trust your ears and tune the tweeter to your liking.

Recommended loudspeaker placement:

  •         No or little toe-in.
  •         >0.5 meters from back wall.
  •         3 meters apart.
  •         2.5-3 meters listening distance

 

The loudspeaker is optimized for a listening distance of >2.5m, but everything between 2-3.5m works fine. As always it pays off to test different loudspeaker placements and see what fits the room and personal preferences the best.

Technical specification:

 

Kensho Two – M2
Type: Two-way small sized stand-mount loudspeaker
Enclosure type: Bass-reflex
Woofer driver unit: 5” mineral filled PP cone. SB Acostics SB15MFC30-4
Tweeter driver unit: 9x60mm horn-loaded ribbon tweeter. Fountek NeoCD3.5H
Cross-over frequency: 3kHz.
Cross-over function: LR fourth-order acoustically,
Frequency response: 44-35000Hz
Sensitivity: 86,5db 1m/2.83v
Nominal impedance: 4 Ohms
Minimum impedance: 3.5 Ohms
Enclosure volume: 8 liter
Weight: 9,8kg
Enclosure dimensions: (height x width x depth): 30,5×19,0x26,75cm

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Thank you!

/Göran

Author of the “AudioExcite Loudspeaker Design” website