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.
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.
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.
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 :
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.
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.
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.
