MTZ Audio - ARCOS X speaker kit project

The idea of the MTZ audio top floor-standing speaker was introduced just before July 2005. Our initial advisors were colleague-diyers, owners of our previous speaker projects (such as Mylo, Nimiel and Arcos with Audax drivers). For all of us it was more than clear that this speaker should outperform the simple notion of a 'massive speaker driving with excessive ease rooms between 25 and 50 sq.m.'. Instead it should bear a unique performance in terms of soundstage.

In fact it should reconstruct musical action with not only the truthfullness of planar speakers but the unbeatable precision of directional sound source. In addition this 'precision' should withstand the deficiencies of any listening room.

Those that systematically deal with speaker design, do know very well that a 'perfect SPL response curve' would not fulfill the initial idea.

A couple of months later we became aware that the flawless mechanical perfection and quality of the top Seas drivers, namely the Excel drivers, would be able to support all our design principles throughout the various project steps.

Thus it was the famous 8-inch W22EX001 woofer that was selected for its magnesium cone and the large voice-coil (39mm).

The main feature of this driver is its 'freely' suspended spider, delivering a symmetrical air load to the voice coil and outstanding linearity to very low frequency reproduction. Seas lab measurement of nonlinear distortion are indeed extremely low for this driver.

The very first listening sessions validated our choice and unveiled unexpected clarity and transparency even at extremely high SPL.

On the other hand the high frequency driver was supposed to be able to maintain linearity and lack of colouration at very high electrical and thermal load. In addition it should exhibit increased directivity, a key to high fidelity sound reproduction in typical residential environment.

It is our belief that all these were luckily 'more than easy' targets for the well known Seas 'millenium' tweeter (T25CF002).

Designing an enclosure for ARCOS X is surely an ambitious process with challenging issues. Let us have a look on them :

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MMT configuration (lower Midbass, upper Midbass, Tweeter) for excellent placement of musical instruments on the vertical plane. For us in MTZ audio this configuration has proved its superiority when stereo imaging comes first,

optimization of front baffle width of better focusing in the listening room,

thorough selection of the height of tweeter driver with respect to the nearby baffle edges for the suppression of diffraction problems and early floor/ceiling high frequency reflectivity,

use of medium width (22mm) and increased particle density MDF for an adequately rigid enclosure, quite capable of handling high level pressure surges of the enclosed sound waves,

precision woodwork for best driver mounting on speaker front baffle,

optimized driver and vent height selection for minimum internal standing waves and 'clean' vent emission,

use of cylindrical rods inside the speaker cabinet leading to an increase in wall vibration damping,

use of 2 different sound absorbing materials acting in different levels : wooden wall 'passivation', effective mid/high frequency absorption and artificial volume increase for an improvement of the low frequency cutoff of the speaker's SPL response,

separate speaker base including highly absorptive mounts.

As in the case of the rest of the MTZ audio speaker projects, the low frequency simulation software LFA was used for the design of Arcos X enclosure. The optimum value of 77lt was deduced for the speaker's internal volume, producing a low freq.cutoff at 35.4Hz.

Usually a cabinet design procedure aims to the minimization of all secondary emissions/colourations from speaker cones and the vent itself. These are actually unwanted resonances formed by the enclosed space formed inside the walls of a speaker. They are always present since they are inherent to all speakers.

This minimization is merely a matter of careful selection of the drivers' and vent positions.

In the preceding figure the near field SPL response of the vent is simulated by this software. Apart from the peak of the main vent resonance at 37Hz, we can observe secondary emissions the level of which is kept as low as possible due to optimized drivers' and vent positions on the front baffle. This simulation has been carried out for a completely empty Arcos X enclosure (no absorbing material). When all absorbing materials are put in place, low freq. colouration will be further suppressed.

In the following picture the measurement of vent SPL response of an assembled Arcos X speaker is illustrated.

As expected the case of the Arcos X crossover network was rather special. The woofer drivers featured a highly rigid magnesium cone (thanks to which musical transients are reproduced in a much better way) having an inherent SPL resonance peak at 5000 Hz.

Thus the crossover filters should not only achieve a smooth cooperation of the phase responses of the acoustical low and high-freq. branches (for a superior soundstage to be attained) but suppress in a selective way the 4-5 kHz range of the low freq branch. Moreover a flat and resistive impedance response should be achieved.

The circuit topology selected was not complicated and when the optimization of component values came to an end, all targets were fulfilled.

No need to say that hundreds of listening hours were finally spent for elaboration and performance optimization.

In the figure below the dashed curve stands for the woofer SPL response. The resonance peak of the driver's magnesium cone is concentrated at 5000 Hz. The normal curve shows the resulting low freq. SPL response as filtered by the low-pass section of the crossover circuit.

The successful suppression of the unwanted resonance as well as the spectral smoothing of the speaker response are more than obvious..

In order for Arcos X to merge its response with the room's reverberant field, the speaker's anechoic response in the 1500 to 5000 Hz range was specially treated. Exhaustive listening sessions at very high SPLs were utilized only to reveal an unexpectedly pleasant experience.

The picture above illustrates the speaker's power response at a distance of 1m for a 2.83V rms electrical stimulus. Power response is said to predict the sonic signature of a two-driver speaker in a typical domestic room in a much more accurate way than the usual SPL response.

Finally the cooperation of the speaker's acoustical branches (i.e. the low- and high-freq. sections) can be easily determined by a close look at the picture that follows. It can be deduced that in the overlapping or 'crossover' region the low-freq. branch exhibits an approx. 15dB/oct. slope whereas the high-freq. one a smaller value of 11-12dB/oct.

Crossover freq. is approx. 2200 Hz. Those who wish to examine the phase response will surely note that the tweeter's phase (continuous curve) moves right in the middle of the phase responses of the two woofers (dashed curves). The superior 3D focusing of the musical instruments and voices becomes merely an inevitable result..

Since noone expects the listening rooms to bear a significant degree of similarity, the Arcos X crossover topology includes a resistor of unfixed value so that the DIYer can get the necessary wide range of adaptation of the speaker's sonic signature to his/her room and/or personal taste.

In the recent years many DIYers have been more and more engaged to the realization of a 'boardless' crossover, the 'hardwired' one. In such projects ground-return paths are far better than those we can get on a pcb. If we keep this in mind when we think of the large instantaneous electric currents of a speaker then 'hardwiring' is definitely justified.

Naturally the details of such a realization are quite many and critical. Yet MTZ audio eagerly provides (oral) instructions that will satisfy all needs.

In addition several elegant housing solutions are also provided. It is the customer to decide..

The expression 'easy/hard' load is very often used among audiophile enthousiasts. Some people define a speaker as an easy load for an amplifier the one with high SPL sensitivity. At MTZ audio we understand that when it comes to 'electrical loading', it is the range of instantaneous currents drawn by the the amplifier and the problems induced to its power supply unit as well as its feedback circuitry that really matter. It is only them that really drive an amplifier outside its nominal steady-state power specs, raising perceptual differences between amplifier brands and models.

Needless to say that this is an issue too big to be discussed within a short presentation text. We therefore prefer to describe our initial target concerning the impedance curve for the Arcos X speaker : flat and purely resistive in a frequency range as wide as possible.

The actual curve obtained in the lab and expressing this design philosophy is viewed in the following picture.

Impedance modulus only slightly deviates from the 4 Ohm value even at frequencies well above the audible spectrum of 20kHz ! Its argument (which decides the achievement of pure resistivity) hardly exceeds the positive or negative 5 degrees' value !

Listening to a pair of Arcos X speakers driven by an amplifier bearing even a 'plain' power supply will certainly lead to very interesting conclusions..

complete speaker kit includes enclosures (unpainted cabinets), screws, damping materials, SEAS drivers, crossover components, vent, cabling, speaker base with vibration-free mounts.

The copyright of the following photographs belongs to the owners of the illustrated speakers.