North Creek Okara II Bookshelf Monitor,

Okara II - C Center Channel,

Eska MTM Mini-TowerOkaraCurlyMaple

Historical Page, retired 2009

 Design Goals:

The Okara II and Eska Mini-Tower are intended for small rooms and rooms in which the loudspeaker positions are close to the rear wall. Unlike our Borealis, Rhythm and Vision loudspeakers, which benefit from placement well into the listening room, the Okara II has been designed specifically to perform at its best when placed either on a bookshelf or stand within two feet of the rear wall. The Eska mini-tower is a floor standing, high impedance MTM. Both are high performance two-way monitors built around the exceptional North silk dome tweeters and the superb Vifa P13WH-00-08 5" polypropylene cone woofer in an EBS aligned vented cabinet. The loudspeaker is voiced in such a way that the combination of low frequency response and low end gain from the room placement constraints produces a low end that is very dynamic and goes remarkably deep, yet still maintains the speed and finesse of a monitor loudspeaker.

We consider the Okara II the world's first truly high end bookshelf-specific loudspeaker.

Photo at right is the Okara II D28/II in light Curly Maple, courtesy of Lee Taylor and Company.

 
 

The Tweeters:D28_Side_View

Both the North D25 -06S and D28-06S were designed by George Short exclusively for North Creek Music Systems.

The North D25-06S  is a 25mm silk dome with an under-hung gap geometry, 1.0mm peak-peak excursion, rear chamber, well controlled resonance and frequency response characteristics, and very light magnetic fluid cooling. The 25mm dome diameter offers excellent off-axis dispersion. Sonically, it is very similar in character to the Scan-Speak D2905/9500 (liquid and wonderfully detailed) but with a touch more sparkle at the top end. The North D25 is widely considered the most naturally musical, affordably priced tweeter on the market.

The D28 takes the refinements of the D25 several steps further. By going to a slightly larger dome diameter (28mm), a more powerful magnet and an advanced motor constructed with a Faraday sleeve, the North D28 excels in every aspect of tweeter behavior. Higher maximum SPL, lower intermodulation distortion, and frequency response contouring lead to an extremely clean, unfatiguing, transparent top end that easily rivals many of the most expensive tweeters in the world.


The Woofer:

The woofer selected for the Okara II is the P13WH-00-08 5" polypropylene cone from Vifa.Vifa_P13

The Vifa P13WH-00-08 is a classic example of a small woofer where resonance frequency, motor strength and cone excursion are perfectly balanced to maximize the low frequency extension from a small vented box. A fundamental resonance in the mid-sixties and 8 mm of linear excursion (14 mm peak) allows this woofer to reproduce convincing bass into the upper 50's, yet stay remarkably clean.

The cone is a strait-sided, mica-filled polypropylene, with a nylon dust cap and Dr. Kurt Muller rubber surround. This combination yields one of the flattest frequency response curves we have ever seen, with a smooth high frequency roll off and no indication of cone break up. It is a delicate balance that emphasizes the smoothness of a poly cone, but without the softness and loss of detail common with larger cones.

The frame is cast magnesium, extremely rigid and strong enough to secure the massive 14.6 oz. magnet without vibration.


The Okara II Cabinet:Image88

To perfectly match the Vifa P13WH-00-08 to the listening environment, we selected an EBS (extended bass shelf) vented alignment. The cabinet volume required is 6.44 liters, and port tuning is set at 63 Hz.

The cabinet measures 12 1/2 tall by 7 1/2 wide by 9 5/8 deep, and is constructed precisely as described in the North Creek Cabinet Handbook. The back is 1 1/2" thick and the front 1 7/8" thick, both constructed with and MDF-plywood laminate. There is a single 3/4" brace. Both the brace partitions and woofer egress are aerodynamically flared to improve system performance at high volume levels.

The port is rear mounted, easily removed to adjust the tuning, and combines a flared egress and forced laminar flow throughout the body to reproduce bass that is remarkably clean and tight even at very high power levels.

Although this cabinet has about the same profile as an 8 1/2 x 11 sheet of paper, it is incredibly rigid and weighs in at a hefty 17 pounds.


The Crossover:

Crossover design is more art than science, and no loudspeaker has exhibited this more strongly than the Okara II. The placement constraints and room size restrictions made the design process incredibly difficult. The prototype was in the small North Creek listening room for over two years, and went through literally thousands of design iterations and months of voicing. Yet in early 2000 the perfect balance of low end finesse, midrange clarity and top end speed was achieved.

Okara_D28_Spread
Okara II Driver, Close Mic and Port Frequency Response

The Okara II network features a simple second order high pass network on the tweeter and a cascaded third order on the woofer. The woofer network topology was developed specifically for the Vifa P13WH, and like the tweeter network it has only a single reactive component in the signal path. Although proprietary, the crossover topology is incredibly simple and showcases the excellence of crossover component quality.

Okara_D28_Full_Range

Okara II - D28 Summed Frequency Response Spread

The overall acoustic network is a symmetric third order at 1.8kHz, the antiphase null is 26dB, and system frequency response is a very smooth 59 Hz to 20kHz 2dB. Reference sensitivity is 85.5dB for 2.83V, 1 meter input. Input impedance is 8 Ohm nominal, 6 ohm minimum. As this speaker was designed specifically for near wall placement in small listening rooms, minimum amplifier power only 30 watts.

 
The Sound:Leeokara1right

Of course, this is all that really matters.

The Okara II is just extraordinary. The system takes the low end finesse and midrange clarity of the small, light 5" woofer cone driven by a massive magnet and combines it with the remarkable speed and resolution of our silk dome tweeters.

The bass is very fast and articulate, yet packs a punch that belies the Okara II's small size. The tempo is also very good, with a lower midrange that is robust but never chesty.

The upper midrange is very clean, and the 1.8kHz crossover frequency brings the tweeter in as early as possible. Female vocal harmonics are thus carried by the tweeter; incredibly lifelike and very accurate.

The top end is typical of silk domes; smooth, sweet, detailed; never bright or hard. The North D2506-S is very musical here.

Overall, the Okara II is one of the most accurate, yet listenable, small loudspeakers ever produced.

At right is Lee Taylor's floor standing optional cabinet in rosewood.

 

Center, Surround and Subwoofer Applications

Because of its modest size and excellent dispersion characteristics, the Okara II naturally lends itself to most audio-video applications. The voicing is very similar to our Borealis/Rhythm/Vision systems from the lower midrange on up, and when configured "small" via the processor they make superb surrounds.

In small AV rooms, when placed at the optimum 8" from the wall and operated in full-range mode, a 5.0 family of Okara II's has a remarkably punchy low end and creates a convincing illusion that it goes much deeper.

For 5.1 and 7.1 or .2 applications, one should remove the deep bass from the Okara II's via the processor and steer it to the subwoofer(s). For optimum results, the Okara II port tubes should either be sealed with a cover or "stuffed" with tightly rolled felt or foam. The North Poseidon subwoofer, with its built-in crossover set to 85 Hz, is the perfect match for an Okara II family in this configuration.

North Creek can provide any or all channels magnetically shielded for a small additional charge. Single channels are also available.

 

The Okara II - C (MTM) dedicated center channelD28-C-Cherry_3

Voiced identically as the original Okara II, and with the same sensitivity, the Okara II - C (Eska-C) is constructed as a shielded, horizontal MTM that specifically matches Okara II mains and surrounds. The -C is best for listening positions a minimum of seven feet (2 meters) from the front of the loudspeaker, and it also performs at its best within two feet of the wall. Dynamically, the Okara II - C is very slightly superior to the standard Okara II, due to its increased woofer cone area. Impedance is also very high - well above 8 Ohms for most of its operating band and reaching minimums of 7.78 Ohms at 250 Hz and 5.8 Ohms at 8kHz.

Dual Okara II - C's set up as main monitors are also spectacular. For those that intend to use them as high impedance, MTM main speakers (with SET or OTL amplification), there is impedance compensation "twister" circuit available that adjusts the system impedance to 8.0 Ohms +-2.1 Ohms from 100 Hz to 20kHz. We can also provide this option as a tower.

Photo above right is an Okara II - C - D28 in Cherry. This excellent cabinet was built by Ken Martinez.


The Eska MTM mini-towerLeeEskaMahoganyCroppedLeft

Like the original Okara II and Okara II-C, the Eska mini-tower is a floor standing MTM designed to be placed within one foot of the wall, in small to medium sized listening rooms. Its vertical dispersion has been optimized for seating six to twelve feet from the loudspeaker face.

Because it is built as a tower, mass loaded and spiked to the floor, the Eska has an extraordinarily tight bass line and is remarkably punchy for such a small loudspeaker. By incorporating longer spikes in the front of the loudspeaker, its vertical "sweet spot" can be adjusted to perfectly suit the listening room.

The Eska is also a dream to drive, with the impedance never dropping below 5.5 Ohms and well above 8 Ohms for most of its range. An impedance-optimized "twister" is also available for this loudspeaker, stabilizing the impedance to 7.8 Ohms +-2 Ohms from 100 Hz to 20kHz.

Photo above right is a Eska in ribbon mahogany, built by Lee Taylor.


Availability:

We are releasing the Okara II and Okara II - C in four versions:

Okara II-North D25 "Ikemo" Loudspeaker project: Lee_Spirit_Sapele3

Contains soft-matched Vifa P13's, perfect pair North D25-06S, fully assembled and tested crossover networks consisting of geometrically optimized 16 AWG inductors in the woofer section, 14 AWG in the tweeter section, North power resistors, Zen metallized polypropylene film capacitors bypassed by Harmony metallized polypropylene film capacitors in the tweeter section, and electrolytic capacitors in the woofer section. All crossover components are hand matched to 1%. Single wired. Also includes port tubes, stuffing, binding posts, gasket tape, screw, etc.; in short, everything except the wood.
Okara II-North D25 "Ikemo" Project …$399.00 per pair
retired

Okara II - Ikemo - C …$295.50 each. retired

Rght is a North Spirit in fiddleback mahogany, photo
courtesy of Lee Taylor and company.


Okara II Upgraded -D28/II

A tweeter upgrade from the North Spirit, contains perfect pair, shielded Vifa P13's, perfect pair shielded North D28/II fully assembled and tested crossover networks consisting of 12 AWG inductors throughout, North power resistors, Cascade-Bypassed Crescendo polypropylene film-and-foil capacitors, and electrolytic capacitors bypassed by Zen capacitors in the woofer section. All crossover components are hand matched to 1%. Bi-wired. Also includes port tubes, stuffing, binding posts, gasket tape, screws, etc.; in short, everything except the wood.Lee_Spirit_birdseye2

To download the Okara II cabinet in Adobe PDF,

Okara II D28/II (black face)kit ...$749 per pair retired

Ikemo Birds Eye Maple, courtesty of Lee Taylor and Company.

 

 

Eska MTM Mini-Tower and Center Channel

The Eska MTM Mini-Tower is identical in component quality to the Okara II - C MTM Upgraded but built with the North D28/II tweeter. This vertical floor standing min-tower includes North Creek's Big Toes, Very Big Toes, and longer internal wiring. The floor standing option, properly spiked, provides a few Hz more bass than a standard MT.

  are available through this link.

Eska D28/II (black face)kit ...$989 per pair. retired

Eska - C D28/II ... $539.50 each retiredLeeEskaTower2

Partial Magnetic Shielding add $8 per speaker ($5 per woofer, $3 per tweeter). Magnetic shielded is included with the Okara II - C, Eska-C and Eska mini-tower. Effective beyond about 12".

At right is George's Eska in Cherry, photo courtesy of Lee Taylor and Co.


Comments on the Absolute Sound of the North Spirit Loudspeaker:

We would like to take this opportunity to thank TAS and Sue Kraft for putting the time and effort into reviewing the North Acoustics' Spirit loudspeaker, and for providing this opportunity to comment.

There are a plethora of excellent small loudspeakers available today incorrectly labeled "bookshelf monitors" simply because of their small size, when in fact most were optimized for free-field placement on stands well into the room and with plenty of volume behind them to breathe. When moved close to a wall, a loudspeaker designed in the free-field will tend to suffer mid-bass bloat, a loss of image stability and sound stage collapse.

Not so the North Spirit. This loudspeaker embraces bookshelf placement, and in fact its low end fills in and sound stage "fleshes out" nicely when tucked into a corner, set a ledge, or tossed up top a mantle piece.

It is incredibly difficult to design a high end loudspeaker for bookshelf placement.. which is why there are so few. The voicing is so delicate; drawing the best bass one can get from a small woofer in a small cabinet, a midrange and top end that can never get hard or forward regardless of
whether the listener is sitting or standing, yet maintaining harmonic integrity. Finding the perfect balance in a loudspeaker that assumes so little yet performs so well requires a little bit of magic.


The North Spirit prototype was designed over a period of two years, entirely by ear. When it comes to scaling a sound stage and recreating image depth, measurements are meaningless and only months of listening and fine tuning suffice. The fact that the Spirit measures +-2dB from 57Hz to 20kHz - far better than most loudspeakers five times its price - is rather amazing, and in fact I was quite surprised myself the first time I measured them.Lee_Spirit_Sapele_Back_Small

Thanks again TAS and Ms. Kraft for your fine work and this excellent
publication.

George Short, President
North Creek Music Systems
North Acoustics

North Spirit and Sapele by Lee Taylor, front and back, photos courtesy of
Lee Taylor and company,
.

 

 

 

 

Comments on the Parts Express 302-70x 0.25 cubic foot cabinets

We have often been asked if the Parts Express pre-made 0.25 cubic foot cabinets (available from , shown at right in Cherry) are compatible with the Okara II's. The answer is a reserved "yes"..... provided one is willing to make some small changes to the insides. The PE cabinets were not really designed with monster-sized North Creek crossover components in mind, and the MDF figure-8 brace that is built into the cabinet gets in the way.

For those that are building the Okara II -Ikemo, it is no problem and everything fits with no modifications. For the North Spirit or Okara II - D28, the following mods are suggested:PE_Okara_D28_2

1) Use North Creek Big as Texas binding posts instead of the bi-wired back cups. This allows enough room on the back panel to mount the tweeter crossover. Make sure to inform the North Creek Office at the time of order that the Texas posts are required, not the standard back cups.

2) Be prepared to sawz-all out the cross member of the figure 8 brace in order to fit the tweeter network into the cabinet, which should be mounted on the back of the cabinet above the binding posts and below the port tube.

3) Mount the woofer coil against the back right corner of the cabinet, behind the brace, and use a lot of silicone. Then mount the small woofer board above the bottom "o" section of the figure 8 brace. It will not sit flush against the bottom, so again a liberal amount of silicone is required.

4) Replace the sawz-alled out section of the figure 8 brace with a small piece of plywood, going all the way across the 8 from the left cabinet wall to the right. One can glue this in, then screw it in with drywall screws to hold it tight against the brace while the glue dries.

5) One would be better off machining completely new fascias for these cabinet, as the 1" MDF pre-finished fascias are a bit insubstantial. A thicker fascia made of 1/2" plywood laminated to 1" MDF is suggested. If one builds a thicker fascia, the screws provided with the PE fascias will not be long enough, but good old Ace Hardware will have 4mm machine screws with the correct pitch to attach the new fascia. The way to make sure the grille will fit is to clamp the original fascia to the new one, face to face, make sure it lines up perfectly, then drill the pilot holes into the new fascia through the pilot holes in the old. The unfinished North Creek fascias are made in this fashion.

6) If one uses the original PE Fascia, then be prepared to sand with 220, re-prime, sand with 330 and repaint the original. There is no way to keep the original finish from getting scuffed and scratched while machining the driver countersinks.