High Definition Towers Mark II
by Dorwin Gregory

HDT MK II Loudspeakers

Looking through the pages of Sterilized and Absolute Nonsense, one can't help feeling that speaker design has changed very little since the basic principles were laid down in the 1940s. Yes, we are seeing the use of molded compounds and the rounding of sharp edges and corners into Shmooaform contours. Progress seems to be coming from the driver designs, computer modeling and the use of exotic materials  but beneath the refinement and tweaking there is the basic base reflex, air suspension systems, horns and planar systems. There has been very little innovation in the design of the speaker enclosure itself.

A person looking at the High Definition Towers Mark II for the first time might assume that it was just another of a long line of rectangular box designs. It has side firing woofers or passives so it must be a sealed box. It probably has the standard Swiss cheese shelf type bracing. And what's that stack of latticed panels on top behind the tweeter supported by little bowling pins that looks like the top of a 1930s refrigerator?  Probably catches the back wave of the tweeter. But a closer examination of the internal structure of this speaker reveals that it stands beside only a handful of others as one of the truly unique and innovative approaches to speaker design in the last 70 years. Steve Deckert has been working on and perfecting this design for almost three decades.

On the inside of the HD Mark II are five interconnected chambers one of which is a transmission line. The front firing 8 inch woofer and side firing 8 inch passives present a surface area at low frequencies equivalent to a 24î speaker presenting as much surface area as a Hadly! The woofer runs full range and the top mounted ribbon tweeter is filtered in first order with a single capacitor.

Dave Wilson once told me while he was demonstrating the WHAM in his home, that a little understood distortion in speakers was their tendency to go into a kind of harmonic resonance where the whole cabinet vibrates back and forth and from side to side. This is different that the front and side panels moving in and out independently and acting like diaphragms which can be addressed through bracing. Do you treat that with diagonal bracing?" I ask. "No" he said. He explained that in his speakers he adds and subtracts mass from all of the panels so that the front and back and sides and top all have different densities. Steve Deckert addresses this problem another way so that the right front corner is braced against the left rear one and the left front to the rear right. This is achieved not with mere struts but with entire panels that run from the top most of the way down dividing the box in the upper part into four triangular shaped, tuned chambers, the rear one of which making up a transmission line.

Cross sectional diagram of the HDT  MK IIConfused?  Imagine you are looking at the cabinet from above with the top removed.  Imagine a rectangular panel of mdf that runs from the right front corner to the left rear one dividing the box diagonally in half. Now imagine another board crossing the first from the left front corner to the right rear. Now you have an "X" cross section forming four triangular chambers. Triangles are very rigid structures and resist harmonic resonance in the first place. Further more triangular chambers have no parallel surfaces so sound doesn't bounce back and forth like it does in rectangular  boxes.

Standard front to back and side to side bracing invites the kind of vibration Wilson was talking about. When the front speaker moves backward the back panel also retreats at the same time. The same for the sides, especially if side firing speakers or passives are involved. With diagonal bracing, the movement of the front of the speaker is carried to the back corners making the transfer of energy non symmetrical. The result of this approach is much less sound coming from the cabinet which contributes to the Mark IIs remarkable ability to disappear. More about this later.

On the bottom of the tower is a single chamber called  the summing chamber. Each of the three front triangular columns has a tuned tube attached to its bottom connecting it to the lower  box which receives and blends the three Helmholtz resonances. The front tube is tuned differently that the ones on the side. The combining chamber opens into the back to the transmission line which carries the sum of the three tubes upward, the rear triangular transmission line opening completely on the top.

But somehow the sound has to get from the front column where the woofer resides into the side columns in order to excite the passive radiators and load the tubes. This is achieved by oval ports at speaker level, one on each side. Why oval? So any sound bouncing back and forth from the parallel surfaces of the ports would be different any where on the ellipse. But Steve defeats this problem even further by scalloping the edges of the port so there are no parallel surfaces at all. In fact, the only parallel surfaces in these speakers at all is the top and bottoms of the triangular columns and the inside of the combining box which has foam egg crate foam damping on its bottom. There is an additional duct in the bottom of the back panel at the level of the averaging chamber. This further tunes the transmission line and provides a way to recover lost articles that fall down it.

But what about the low frequency energy coming up the transmission line and firing upward toward the ceiling. That's where the strange looking diffusion panels on the top comes in. On the top of the HD Mark IIs are two rectangular panels, one atop the other with hemispherical indentations cut out for the tweeter. The diffusion panels are supported by four heavy metal spikes resting on the part of the top above the "X" bracing. Inverted spikes also support the upper diffuser to the lower. The lower diffuser panel has a triangular opening in it positioned above the opening of the transmission line, but the triangle is reversed in orientation to the opening of the T line.  This allows some of the wave to go through to the upper panel and some to into the lower.  This diffusion apparatus dose several things. First it adds considerably more base. With the diffuser panels removed, the speaker sounds very clean and precise and the base has more of a point source quality. When they are added back there is a sense of much more base coming from the speaker and the radial distribution of the low frequencies sounds more realistic and contributes to the ability of the speaker to disappear.

Furthermore, the diffuser panels slot loads the base lowering its frequency, and acting as an acoustic lens, it stops the wave of low frequency energy coming out of the transmission line from firing up at the ceiling and bends it into a three hundred and sixty degree circle coming out at ear level.  Steve Deckert has explored radial sound dispersion in some of his other speakers and he has concluded that the lower and midrange frequencies are best served by this method while imaging is preserved by the use of a point source tweeter.

With the front and side firing woofer and passives, the rear port, and the omnidirectional dispersion pattern of the transmission line the Mark IIs project a quasi radial wave front in the lower frequencies. Because of its radial dispersion in the low frequencies, the Mark IIs radiate base in multiple phase angles and can actually be plaid out of phase without the expected cancellation. How can more than one phase angle be good? Wouldn't there be lots of distortion?  Actually all real sound sources project their sound in quasi omnidirectional dispersion patterns. In the low frequencies this improves the amount and smoothness of the base and there is less awareness of the sound coming from the speaker.

But with all that base firing into the delicately supported structure, wouldn't the diffuser panels themselves vibrate and radiate out of phase energy in all directions, mixing it with the primary wave from the transmission line? Indeed it would, but Steve solves this problem in a most ingenious way. The upper and lower panel are connected within the aria of the tiptoes, by a threaded bolt that screws in to an nut embedded in the lower one.  By tightening this bolt, the upper and lower panels are drawn closer and closer together applying a large degree of torqued pressure on them, tightening them like the head of drum. This not only completely stops the diffusion panels from resonating, but if offers a degree of acoustic tuning where the actual quality of the base can be adjusted from a livelier sound to one more damped. It is very important to tighten this bolt more than you might at first think. It takes a full turn and a half to eliminate the out of phase resonance that is otherwise produced. The adage "do not over tighten" does not apply here.

The same design for the cabinet of the Mark II is used in the other High Definition Tower speaker that Decware produces which is designed for full range single drivers such as Fostexs or Lowthers.  This model uses more internal damping, different passives and a different tuning on the tubes.

The 8 inch Woofer is a Silver Flute. The cone material is made from wool  fiber which while dense and rigid enough to resist breakup is very damped as far as cone resonance is concerned. Lightly tapping on its surface is like tapping of a wool suit.

The tweeter is a transformerles ribbon design. Its narrow width and low mass allow it to be extremely fast, more so even that some larger designs with bigger magnets and larger ribbons. It images extraordinarily well projecting a rock solid, holographically delineated sound stage and when the cap brakes in, is extremely natural. If you hear any roughness or edginess from this tweeter its not the speaker but something in the source material or system.

The Mark IIs have an efficiency of 94dB making them ideal for low power gear. On the other hand, they really respond to higher powered amps of the 8 to 20 watt range creating a larger scale presentation and more base and slap.

What about the sound?

My first impression in listening to the Mark IIs was that they were extremely wide range and very smooth in their execution of the frequency range. Some speakers sound best at certain frequencies and show their limitations in others. The Mark IIs sound good everywhere in the frequency sweep; wherever the music goes the speaker follows. This offers a very full, flushed out sound which while being extremely well delineated, never sounds thin.

The base from the Mark II is absolutely astounding!  It is so clean and tight and goes down so deep that it truly does not require a sub woofer to sound complete. The one exception might be full orchestral classical music where a sub adds a more realistic sense of scale, especially in larger rooms. I find that a sub woofer adds to the effect of home theater movie soundtracks. In some speakers the higher frequencies float on a cloud of diffuses base energy but here the base is part of the structure of the sound-shape. The holographic sonic images have hard edged outlines and the base is contained in the image and does not spill out into the surrounding space. If there is no base in the score, there is no base from the speaker but when a really demanding low frequency passage comes along, it is unbelievable how much these speakers put out.

The tweeter and woofer blend extremely well are very similar in character in the sense of being fast and clean.

The Mark IIs are a very analytical speaker but not in the sense of being cold or aloof or intellectuality detached. With the right source material they are utterly emotionally absorbing in their presentation. They are like microscopes for examining the program material and the component chain. In this capacity they can be rather critical of edgy resonances in some digital components or  recordings. You tend to want to listen to your best recordings. They are very good for evaluating other components in the chain articulately revealing subtle changes in interconnects, amplifiers, cartridges and other devices.

These speakers are designed to play and sound exceptional when driven by components made by Decware which can stand beside some of the best sounding high end gear in the world. They are in no way limited to this Decware electronics, however. This means that no mater how good your equipment is, you will not outgrow these speakers as you upgrade, nor will they ever become the week link in the audio component chain,

The Mark IIs excel in the near field where listening room coloration's are secondary to the direct voice. In this set up they can play to full capacity with amplifiers as small as two watts. Their imaging is so good that they can actually produce sounds that seem to come from behind you!  I never heard that before in a two channel setup. They sound excellent as well in the far field and in larger rooms, but here a larger amplifier improves the presentation. The Mark IIs really excel with jazz, acoustic instruments, string quartet, new age and ethnic music. They love close miked material and they are suburb at reproducing the human voice. In contrast, they coherently resolve the acoustic space of live recordings in large rooms, halls or auditoriums. In the near field with the right recordings they produce a rock solid, four dimensional, holographic sound stage with very precise location of sonic objects. There is lots of back wall and it is easy to visualize details of the recording session such as which way the piano was facing.

They disappear as well as any speaker I have heard. The ability of a speaker to disappear results from the ability of the enclosure to not radiate any sound of its own except for that which is coming from the drives, passives and ports. 

Does it play all of your records?  well no. The Mark IIs are so revealing and peers so deeply in to the deepest layers of the sound pool, that if a recording sounds hard or edgy or has phase anomalies from microphone misphasing or from the mix, this will be very apparent and the listening session. Untamed digital sources and some video sound is ruthlessly revealed for what they are and are not masked by coloration's like some speakers present.

I must qualify these comments by saying that if you want to hear the kind of sound I am describing, the speakers must be braced with small dowels beneath the woofer and the passives extending to the rear corner of the columns. These prevent the front and side panels from acting like diaphragms, as the speaker go back and forth. These braces are supposed to be standard in all factory made versions of the Mark IIs but their have been cases where they were accidentally omitted.

By the way you can buy plans and blueprints for these speakers if you or a friend are handy at woodworking and you can buy parts kits, passives and diffuser panels to finish them off. .

So what do these speakers sound like?  What does a mirror look like?  They sound exactly like the source material and the whole component chain from front end to speaker wire and everything in between. What did the turd say to the mirror? "You look like shit. " if your house is in order and you have your electronic ducks in a row, they you are going to love them. If not, you will hear perhaps for the first time where your problems are.

If you like a speaker that is extremely fast, detailed and transparent with superlative imaging and a wide and deep holographically defined sound stage, this might be the one for you. If you want to really hear what is going on in the recording and in your system, these speakers will excel. It you want a speaker that will reveal the warmth and beauty and emotional depths of music and allow you to loose yourself and sooth your soul and let you get lost in that rock and role, you should audition the High Definition Towers Mark II.

Dorwin Gregory

Associated equipment

Zen Triode Monoblocks model SE84ZSM
Steve Burger Stereo SE 45/2AC  amplifier with Sophia Princes 2.5 volt mesh-plate 300Bs.
Bruce Tilden 300B SE  monoblocks with Sophia Princes mesh-
plate 300Bs.
Prometheus transformer coupled passive line stage.
alternately, Select Zen Preamp model CSP2
Select Zen Phono stage model ZP3
Early Kuetsu Onyx MC cartridge recently refurbished with a new cantilever and stylus by Vandenhul.
Tamura moving coil transformer
Eminent Technology air bareing tone arm with teflon tube.
Wisa air pump with a regulator and Herb Wolf surge tank.
Ultra high mass Final Audio Research (of Japan) turntable on a  Vibraplane Isolation table with a Verus direct drive motor from Teris Audio. 
DSRII 1.0 Silver Interconnects from Decware.




Decware is a trademark of High Fidelity Engineering Co.
Copyright © 1996 1997 1998 1999 2000 2001 2002 2003 2004  2005 2006 2007 2008 2009
by Steve Deckert