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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 Nimiel speaker project..

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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.

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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.

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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:

MYLO#1_vent_near_field_SPLs.jpg (73910 bytes)

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.

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

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:



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.

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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.