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 Nimiel
speaker project..
Selection of
drivers..
Nimiel addresses the acoustic
environment of a normal listening room. 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 two pairs of 6.5'' woofer cones can drive in the low frequency
range, is about 17-25 m2.
Nimiel has been designed to
deliver ultimate sonic reproduction throughout the sound spectrum without any compromise.
Surely not a modest statement at all ! Well, what we really wanted was to make a speaker
that could only be outperformed by a three-way design.
At that time (spring 2004) MTZ
Audio staff decided to select the popular aerogel cones of Audax drivers for these
difficult tasks. Nimiel was designed after the 6.5'' HM170Z18 driver.
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 found aerogel drivers 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..
Nimiel enclosure adopts extensive
bracing and a net volume of some 47 litres for deep but well controlled bass reproduction.
Almost 1 m2 (!) of tar sheets take care of wall oscillations and an equal amount of
'ricofon' (TM) absorbing material provide more than adequate suppression of internal
standing waves and resonances.
The reflex-port is tuned at 45Hz
and the resulting cutoff frequency (-3dB) is at 42 Hz.
The enclosure material selected,
is a high-density 22mm thick MDF.
picture of a Nimiel speaker at early development
stage.
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. Large enclosures as such, exhibit severe internal resonances that manage
to be emitted by the vent.
Resonance frequency and intensity
vary with driver and port position on the baffle. Optimum positioning leads to minimal
(not zero) vent emissions. This is the task that LFAv5 software provides through
simulation. In the case of Nimiel vent emissions were kept to a minimum outside the tuning
frequency as illustrated in the following picture:
We did our best to enhance
Nimiel's sonic clarity in the frequency range 120-800 Hz where enclosure standing waves
and vent resonances dominate the vast majority of other speaker designs.
What is also interesting is that
enclosure resonances are rarely discussed in the audio community, the audiophile magazines
included !
A near field SPL measurement in
the port's mouth verified LFA prediction with an acceptable degree of accuracy:
It is true that Nimiel enclosure's
bracing is actually changing the standing wave maths and should be held responsible for
the differences between simulation and actual measurements.
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 picture above depicts the
anechoic SPL response of a Nimiel speaker on tweeter axis, at a distance of 1m, for
2.83Vrms input (Valid measurement frequency range 200Hz-40kHz). No smoothing is applied to
this measurement.
Obviously the upper cutoff (-3dB)
frequency of Nimiel is above. 30kHz !
The small 'valley' in the 1-6kHz
range compensates for early reflections in typical living room environment and enables
really loud listening sessions. People that own Nimiel have verified this approach.
As far as Nimiel speakers'
similarity is concerned, the very low production tolerances of Audax drivers and our
careful selection of crossover components merge to give an excellent result.
A detailed circuit schematic
follows right below:
A few words on
impedance..
MTZ Audio staff has got an
extensive experience on amplifier design hence finds 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 Nimiel's 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
(200Hz to 25kHz) a nearly flat impedance for the Nimiel speaker was achieved.
At lower frequencies where large
impedance resonant peaks occur to 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. Nimiel's impedance correction circuit is surely a decisive
step towards sonic perfection, especially for those who do not own highly expensive
amplifiers.
Some useful links (in Greek) in
AVforum's discussions concerning the Nimiel project:
http://www.avforum.gr/showarticles.php3?topic=11&subtopic=19085&st=0
Nimiel / Technical
Specifications
two-way,
three-drivers' design,
vented-box
low frequency alignment, port tuned at 45Hz,
high freq.
driver: TW025A20, alum. 1'' dome by Audax,
bass-mid
driver: HM170Z18, aerogel 6.5'' cone by Audax,
enclosure
volume: 47lt net,
speaker
ext.dimensions in mm: 244x1169x254 (WxHxD),
SPL
response cutoff frequencies (-3dB): 42Hz-30kHz,
sensitivity:
88dB SPL / 1m / 2.83Vrms input (anechoic),
power
handling capacity: 200W rms per amplification channel,
impedance:
4 Ohms,
crossover
frequency 2.7 kHz.
Drawings and Assembly
Instructions..
Hint : images below, link to
downloadable pdf files.
