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TNT Primaluce

Three-way floorstanding de-coupled cabinet loudspeaker

[Italian version]

Product: TNT Primaluce - 3-way floorstanding loudpeaker
Producer: not for sale, TNT-Audio DIY design
Adopted drivers: Hertz HS 200 (woofer), HV 1600 (mid-range), HT 250 (tweeter)
Drivers made by: Hertz Audio - Distributed worldwide
Approx.cost: 800 Euros
Author: Giuliano Nicoletti
Date of design publication: September 2002

[TNT Primaluce loudpeakers]

After a long wait, here we have, finally, on TNT's pages, a new loudspeaker design. Primaluce is a three-way system sporting three drivers, a sensitivity of something like 89 dB, and a 4-ohm nominal load.
Its scale is still human, and its price is reasonable (about 750-800 euros), with more than sufficient power handling. I was being pressed more and more by receiving requests for a high-level design (within limits - after all we're speaking of DIY stuff), and, having published cheap loudspeaker designs (available on this website, only in Italian), that sector was well looked after.

Some clarifications: to set up a DIY design means, IMO, to balance performance with cost and implementation difficulties. Although Lucio didn't limit the budget for those speakers, I just can't ignore the cost factor, and for several reasons.
First, I don't think that you can automatically reach first-class results by paying megabucks for exotic drivers or luxurious cabinets; instead, my experience taught me that a good design - and a good sound - is mainly the fruit of harmony and synergy between the loudspeaker's different elements. When you get this kind of fusion you get something whose overall value can surpass the mere sum of its constitutive parts. Moreover, I think that, in any case, the cost of a DIY design shouldn't reach the stratosphere - anyone who decides to invest an insane amount of money in building audio components by him/herself, would usually rather take the full responsibility for the results than follow a prescribed design.
In this case however, I have to guarantee our readers that these loudspeakers will definitely be satisfactory, and, with so many variables playing a role, I don't think it's responsible to require an excessive investment. As well as that, I don't think it makes any sense to design a too-hard-to-do cabinet, thus automatically excluding those who don't have long-term experience in woodworking. In other words, it's not difficult to design a woofer cabinet sporting three layers of different materials with sand-filled spaces, with a concrete base and twenty internal braces. While it's much more complex to design a simple system that is just as effective - be it technically or aesthetically.

[The drivers]

In our PrimaLuce (which, in Italian, stands for "FirstLight") the low range is handled by a 20-cm woofer; the midrange is a sealed-volume 16-cm driver, and the 1" tweeter sports a soft dome. All the drivers are car audio models made in Italy by HERTZ.
I decided to use those drivers simply because I played an active role in their development (actually, my primary job is precisely in the HERTZ R&D division); needless to say, I know their voice very well, and I think they are high-quality drivers.
The midrange and the tweeter belong to the top series, the Mille (1,000) line, and offer refined and effective technical solutions. The HV 1600 sports a cellulose membrane, fixed aluminium phase plug, copper magnetic extensions, aluminium short-circuit ring, generous magnet, long drive-excursion and low distortion.
The tweeter HT 250 offers a 1" soft dome membrane, with a 10 mm high neodymium magnet (huge! This feature gives the driver tremendous dynamics, with energy and finesse), a rear decompression space and no of ferro-fluid (this also contributes to better micro- and macro-dynamics). The HS 200 woofer is a driver specifically designed to be reflex loaded. It has long drive-excursion, high power handling and low distortion. All these drivers respond perfectly to the project specification, and turning them to my needs was basically an easy task.

[Side view of the TNT Primaluce]

For the Primaluce I used a rather peculiar decoupling system that allowed me to simultaneously obtain several results:

[Rear view of the TNT Primaluce]

Mechanical decoupling is achieved thanks to a holding frame that individually isolates the mid and high frequency units, suspended by O-rings. This system very effective and easy to implement, leading to really encouraging results. Tests performed during set-up confirmed the importance of a mutual mechanical insulation between the different drivers of the loudspeaker for a substantial improvement in the finesse, in the transient definition, and of all the micro-detail of the sound information.
It seems obvious that only the membrane must vibrate, yet it's amazing to observe how good is for the tweeter and the midrange to be completely insulated from woofer-generated vibrations.

Acoustic phase alignment of the drivers is made possible by the ability to move the drivers relative to each other, without being limited to a single mounting surface. It must be underlined that we're not speaking of a mere mechanical alignment, where the single emission centres of the different drivers are made coplanar (in relation to the listening position). In our Primaluce we align single drivers' acoustical phase, which is also strictly dependant on the type of crossover used. Acoustical phases of the drivers in the cross-frequency zone are practically overllappable (except for little adjustments derived from listening sessions, to perfectly shape the acoustic image rendered by the loudspeaker in the desired way), and this is an evident benefit for the sensation of coherence of the entire system.

The last point is no less important than the former ones: the ability to optimise size ratios of the system's different boxes allows us to obtain a valid compromise between inner resonance and external diffractions. These are particulars too often underestimated. And the classic leaning rear panel is not a panacea, just as it's not enough to move a driver from the centre of the front panel to get a good linearity of the frequency response and a correct step response. As usual, it's a matter of blending different technical solutions (often practically contrary to each other), and to obtain the correct size ratios depending on the mounted drivers and their sonic character. Every Primaluce's box is made to compensate its driver's response, just as if were tailor-made. Obviously, it is a benefit for the crossover network, too, which need not contain the electronic elements you usually have to use to compensate for system's acoustic alterations, that here are minimised at their very source.

The crossover filter has a rather simple structure; acoustic crossover frequencies are placed at around 250 and 3000 Hz, and the acoustic slopes are about 18 dB/oct. Sensitivity, as stated, is slightly higher than 89 dB (2,83V-1m), and the low range -3 dB point is placed at around 35 Hz.

Some notes about electric parts: choose inductors offering the lowest possible eddy resistance - they may be core wound (but the core must be made of high saturation threshold stuff). Speaking about capacitors: in the woofer section you can even use non-polarised electrolytics (better if bypassed by better caps of a few microfarad), while all other capacitors should be of the polypropylene or polyester type, with a minimum voltage of 100 V.

Resistors must be able to dissipate at least 10-watt.

Those are general indications. Everyone can choose his/her favourite technologies and materials, also depending on the maximum budget.


[midrange detail] The Primaluce consists of three interconnected boxes. The frame holding the midrange unit and the tweeter is mounted on the woofer box, being bolted to its rear wall at three points (an interface like blue-tak should be used there). The midrange box is literally suspended, using three hooks, each one with a pair of o-rings. Those o-rings (I personally checked) are available in many hardware shops and in plumber's supplies stores: the ones I chose (perfectly fitting our needs) have an external diameter of 15 mm and a cross sectional diameter of 2.5 mm.
These rings have a rather limited durability, indeed because they undergo a noticeable stress; in my prototypes, I had to replace them about every two months, as well as the tweeters' rubber bands. Considering their very low cost, I think it's a minor negative; simply keep an eye on 'em, or you could find your midrange box down on the floor, with all the ugly consequences of such an occurrence.

[tweeter detail] The tweeter is mounted into a standard hydraulic plastic pipe about-75 mm-long (the orange one, of 50 mm external diameter). The empty space at the back of the tube is blue-tak filled. To mount the tweeter into the pipe, you could use the adapter that comes with it for 100 mm holes, cutting away the two little side wings. The opposite end of the pipe from that in which the tweeter is mounted must be closed with a wooden hemisphere (Obviously, I bought it already turned), on which we are also going to place the pair of connectors. The tweeter bearing is made of elastic bands that pass first into four little hooks placed in the inner sides of the holding frame, and then around the cylindrical structure. They can be the ordinary rubber bands you find in a package of 50, and you just have to experiment.

[upper rear view of TNT Primaluce] Midrange and woofer boxes must also be loosely stuffed with acrylic absorbent (like the stuff that is used in anallergic pillows). The internal walls of the midrange boxes should also be covered with 2 cm thick felt panels, or that classic blue-spotted compressed waste wool.

The crossover filter should be kept outside the boxes, safe from unwanted vibrations. If you can, house it in a little wooden box, leaving only the connectors outside. I don't suggest tri-wiring. IMO, the best solution is the shortest mono-wire connection between power amp and filter, and three dedicate cables from there to the loudspeaker. Here you can try different conductors, starting from the classic Star and carrying on with the experiments (TNT offers designs for every taste). Take into consideration that the cables for the tweeter should not be heavy (not to unbalance the structure) and flexible, to avoid vibrations running through them to the component.

Plywood must be used for woofer and midrange side walls, and for the holding column, and MDF for the remaining panels. This for two reasons: using materials of different mechanical characteristics allows us to benefit from the pros of both of them, while minimising their defects (MDF tends to resonate quite strongly, and plywood fails in stiffness; combining them you get a very damped and rigid structure). Moreover, this solution makes the finishing work much easier, since you can leave a lot of natural wood uncovered and that can be treated in many ways (as always, this is up to you). Of course, the plywood can be replaced by a well-seasoned hardwood (walnut, beech...); just consider that the cost rises, an not by a little. It is also suggested to use damping panels. For the mounting, please refer to my previously published article, presently available only in Italian (building up a loudspeaker: woods and boxes ); I reccomend you avoid using silicone, nails or screws. PVC glue on its own is the best way to stick wooden panels together - classic joiner's clamps can help with this.

We're speaking of a loudspeaker that is quite delicate to build, so I suggest you don't start if you have any initial doubts about carrying it through. You will have to adapt some components (the hooks, for instance), and a minimum background of experience and imagination is also required. Remember to respect the relative distances between the emission centres of the single drivers, as shown in the construction plans.

Once you finish the construction, burn 'em in strongly for some days, and later you can start to fine tune the system; a small variation in tweeter offset (that is, moving it outward or inward) causes a slight variation in timbre and acoustic imaging. Then, you can try to stuff the reflex port with absorbent material, thus modifying the low range behaviour, to synergize with the room.

Some thank-yous: first, to Riccardo Abbatelli, who puts up with me every day, giving me wise and useful hints; to Bebo, Lorenzo, Daniele, Roberto, Fabio, Marco, to my brother Luca (and his patient wife Emanuela), to Ilaria and to everyone who contributed to this project. To Cristiano and Federica Ardau, who patiently translated my obscure drawings into CAD design construction plans, to Fabio Genovese who managed their conversion into JPG files.
Finally, to all who, during these years, constructed from my designs, and offered their just critiques and invaluable support.

To Virginia, lastly; to her I dedicate the Primaluce.

Go to the Detailed construction plans (enclosures and crossover network).

© Copyright 2002 Giuliano Nicoletti - http://www.tnt-audio.com

HTML editing: Paolo Saggese - Translation: Carlo Iaccarino - Supervisor : Peter Janssen

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