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History : the Arcos experience..

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A bit of history..

In the past MTZ Audio had the opportunity to gain a first-hand experience on Audax driver units. Three speaker projects, sold as DIY kits, were designed and implemented with the famous aluminium dome TW025A20 tweeter driver and the HM170Z18, HM210Z12 woofer units.

tw025a20v.jpg (25227 bytes)

copyright AUDAX, www.audax.fr

Many greek DIYers constructed these speakers. Their satisfaction was more than rewarding for the people of MTZ Audio. Since September 2004 Audax has stopped OEM driver sales.

 

The Arcos speaker project..

picture courtesy of an MTZ Audio friend who has built an Arcos pair.

 

Selection of drivers..

The idea behind Arcos was to give robust, undistorted bass reproduction for relatively large listening rooms. Although there are a lot of opinions on the room area a speaker can 'load' effectively, MTZ Audio people have come to the conclusion that the optimum room area a pair of 8'' woofer cones can drive in the low frequency range, is about 27-50 m2.

Generally speaking the acoustic cooperation of an 8'' woofer with a dome tweeter in the mid-frequencies is questionable. Apart from an acceptable SPL graph which is easily achieved by crossover network manipulation, such driver combinations usually fail in many fields of sonic reproduction.

At that time (early 2004) MTZ Audio staff decided to select the popular aerogel cones of Audax drivers for these difficult tasks. Arcos was designed after the 8'' HM210Z12 driver.

The uncomparable aerogel cone of this driver managed to produce a flawless breath in the demanding mid-frequency region. Classical orchestral music will tell the truth !

The aerogel cone material made Audax drivers famous for their naturalness, openness and transparency in the mid-frequencies.

Many things are subjective in audio; in that sense MTZ Audio people has found that aerogel drivers are much better than many popular and unexpectedly expensive kevlar- or carbon-fibre drivers manufactured by other well-respected brands.

 

copyright AUDAX, www.audax.fr

 

The enclosure design..

Arcos was given a 84lt net volume. The reflex-port was tuned at 37Hz (initial calculation with no damping material inside the speaker cabinet) and the resulting  cutoff frequency (-3dB) is  at 38 Hz approx.

The enclosure material selected, was a high-density 22mm thick MDF.

arcos_enclosure_2sv.jpg (110125 bytes)

arcos_enclosure_3sv.jpg (99864 bytes)

arcos_enclosure_4sv.jpg (125371 bytes)

arcos_enclosure_5sv.jpg (109155 bytes)

pictures courtesy of MTZ Audio friends who has built Arcos speakers..

Driver and cabinet data were input to our simulation software (LFA v5.0) to derive the optimum driver and port positions over the baffle board. Arcos enclosure is large and therefore extremely prone to unwanted vent emissions. This analysis helped us in keeping them to a minimum outside the tuning frequency.

 

lfa_screen_shot_1.jpg (119731 bytes)

 

This simulation screen shot for an undamped Arcos enclosure is provided for reference and has been verified through a near field SPL measurement in the port's mouth : 

 

MYLO#1_vent_near_field_SPLs.jpg (73910 bytes)

As soon as Arcos enclosure is damped with absorbing material linings a comparatively excellent sonic purity in the frequency range 120-800 Hz is introduced providing very good rejection of the above illustrated enclosure resonances. (Note : this lab measurement was made at early development phase of Arcos speaker and features a somewhat lower vent tuning frequency due to the larger vent length used at that time)

The crossover network..

We at MTZ Audio, believe that crossover networks are intended to manipulate, compensate and align driver responses, not just decorate the interior of a speaker's cabinet. We certainly do not promote minimalism in this area of electronics. Although we respect other design approaches, we definitely do not encourage the excitement of driver's nonlinearities and cone break-up waves that many DIY designers induce through their oversimplified crossover network topologies.

The idea is simple : Intermodulation distortion artifacts never appear in SPL measurements. Crossover networks are not merely 'frequency dividing' circuits. It takes more than a desired SPL curve to really design a speaker system.

The case of Arcos is difficult. In the crossover frequency region there are three (instead of two) co-emitting drivers with sparsely located acoustical centers. Their phase alignment can only be achieved by meticulous steps during the crossover network design.

The target is not merely a good SPL curve. It is sound coherency and soundstage that will be endangered if phase alignment is not absolutely perfect.

MYLO#1vs#2_SPLcor_s.jpg (104725 bytes)

The picture above depicts the anechoic SPL response of an Arcos speaker on tweeter axis, at a distance of 1m, for 2.83Vrms input (Valid measurement frequency range 400Hz-48kHz). No smoothing is applied to this measurement.

The solid line stands for the power SPL response and dashed for the usual (pressure amplitude) SPL response. The range 20Hz-2.5kHz of the dashed SPL response is only valid on a confined sub-space in front of the speaker baffle. We at MTZ Audio prefer to draw our results from a combined inspection of (solid) pressure power and (dashed) pressure amplitude level graphs.

On the other hand the upper cutoff (-3dB) frequency of Arcos is approx. 30kHz !

The small 'valley' in the 1.5-5kHz range compensates for early reflections in typical living room environment and enables really loud listening sessions ! People that builded Arcos are really aware of it !

Exceptional similarity of Arcos speakers SPL response is fulfilled by the very low production tolerances of Audax drivers and our careful selection of crossover components.

A hardwired sample crossover network is shown in the following picture:

 

A detailed circuit schematic follows right below:

 

A few words on impedance..

MTZ Audio staff has got an extensive experience on amplifier design hence regards the nature of a speaker's complex impedance very critical for amplifier feedback operation. We all know that amplifier specs are measured across laboratory resistive loads of 4 or 8 Ohms. IEC 268 standard was never revised despite the strong criticism by many researchers and designers involved in the amplification case.

A special impedance correction circuit was engaged in parallel to Arcos' crossover network, to render the latter's impedance as resistive as possible in a wide frequency range. This sub-circuit is shown within the crossover network schematic just above. Its 'healing' properties can easily be understood in the following impedance graph:

In a very wide frequency range (100Hz to 30kHz) a nearly flat impedance for the Arcos speaker was achieved.

At lower frequencies where large impedance resonant peaks occur in absolutely all loudspeaker systems (up to 40 or more Ohms !) such correction circuits demand extremely large inductance and capacitance values and therefore are not feasible. Arcos' impedance correction circuit is surely a decisive step towards sonic perfection, especially for those who do not own highly expensive amplifiers.

 

 

P8280034s.jpg (124309 bytes)

picture courtesy of an MTZ Audio friend who has built an Arcos pair.

 

Some useful links (in Greek) in AVforum's discussions concerning the Arcos project:

http://www.avforum.gr/showarticles.php3?topic=11&subtopic=17808

some useful technical tips :
http://www.avforum.gr/showarticles.php3?topic=11&subtopic=18829&st=0

http://www.avforum.gr/showarticles.php3?topic=11&subtopic=18969&st=0

and some listening reviews :
http://www.avforum.gr/showarticles.php3?topic=11&subtopic=18297&st=0

http://www.avforum.gr/showarticles.php3?topic=11&subtopic=19346&st=0

http://www.avforum.gr/showarticles.php3?topic=11&subtopic=15955&st=0

http://www.avforum.gr/showarticles.php3?topic=11&subtopic=20639&st=0

http://www.avforum.gr/showarticles.php3?topic=11&subtopic=16046&st=0


Arcos / Technical Specifications

two-way, three-drivers' design,

vented-box low frequency alignment, port tuned at 37Hz,

high freq. driver: TW025A20, alum. 1'' dome by Audax,

bass-mid driver: HM210Z12, aerogel 8'' cone by Audax,

enclosure volume: 84lt net,

speaker ext.dimensions in mm: 264x1169x384 (WxHxD),

SPL response cutoff frequencies (-3dB): 38Hz-30kHz,

sensitivity: 88dB SPL / 1m / 2.83Vrms input (anechoic),

power handling capacity: 200W rms per amplification channel,

impedance: 4 Ohms,

crossover frequency 2.5 kHz approx.

 


Drawings and Assembly Instructions..

Hint : images below, link to downloadable pdf files.

Mylo_Final_Assy_Y12-324_00-005vA_page2s.jpg (50144 bytes)

Mylo_Final_Assy_Y12-324_00-005vA_page3s.jpg (61282 bytes)

 

Copyright 2005-2007, MTZ AudioTM , website v.7.1, last update : January 27th, 2007

PLEASE READ CAREFULLY THIS DISCLAIMER : MTZ Audio staff do their best to ensure that all material -technical or not- presented in this website is accurate and correct; however MTZ Audio holds no responsibility for this. In a similar manner, should anyone claim to have been harmed directly or indirectly by the use of technical information presented in this website, MTZ Audio will not be liable for such claims.