[réservé pour les avantages habituellement cites pour le filtrage actif]
Je repars des avantages listes ici
http://sound.westhost.com/bi-amp.htm:
Adding up the Plusses:
- Effectively (up to) twice the 'real' power of the amplifiers themselves
- Reduced intermodulation distortion
- Elimination of the low frequency passive crossover, its inherent losses, potentially poor linearity and crossover point inaccuracy
- Reduction of the difficulty of the load presented to the power amplifier
- No padding is required to align the driver sensitivities, so we are not simply wasting power
- The damping factor is greatly improved for both the low and midrange loudspeakers
- Complete freedom from any interaction between the loudspeaker driver (and its environment) and the crossover network
- Cost savings, since complex passive crossover networks are not needed
- Bi-wiring is included free!
- The flexibility to choose amplifiers which are at their best within a defined frequency range
- Ability to match amplifier power to the exact requirements of the drivers for maximum overall efficiency
En francais, et en ne gardant que ceux qui concernent les aspects qualitatifs:
besoin de moins de puissanceC'est bien explique sur cette page, deux amplis de 100W fonctionneront aussi bien qu'un seul de 400W.
Mais cela est fonction du type de musique.
On peut mettre de cote ce point en évitant de faire clipper l'ampli.
reduction de la distorsion d'intermodulationelimination du filtre passif a basse frequencePour les basses frequences c'est clair je pense, l'actif est preferable.
une charge plus facile pour l'ampliCela fait partie des points a eclaircir, en tout cas pour moi
amelioration du facteur d'amortissementOn dit souvent que l'ampli etant connecte directement au HP, celui ci peut mieux controler le HP.
Plus de details ici:
http://sound.westhost.com/biamp-vs-passive.htmMais ceci m'interpelle en particulier:
The difference in damping is quite obvious, and although some (very well behaved) drivers will show little improvement, the vast majority will be much better controlled, and this will show in an impulse measurement. Not at all uncommonly, it will also show up on a swept sinewave frequency response measurement as well, with the amplitude of peaks and dips generally reduced (albeit marginally in most cases).
J'aimerais vraiment mesurer pour quantifier le phenomene, et est-ce si audible?
Un autre lien sur cet effet:
http://hddaudio.net/viewtopic.php?id=65The results show that a typical passive crossover has an absolutely disastrous effect on speaker damping. Over most of the audio band, the damping factor is very low, so the amplifier will have almost no control over the cone movement at all. Were it not for the mechanical self damping in the drive unit suspension, the cone would be flapping around wildly. As it is, the cone movement will certainly not be accurately following the applied signal voltage from the amplifier.
On doit donc pouvoir clairement mettre en evidence une difference de reponse impulsionnelle ou a l'echelon?
Mais normalement cela devrait transparaitre sur d'autres mesures comme la reponse en frequence ou le waterfall
Un dernier point a propos des inductances qui sont surement les composants du filtre passif les plus douteux:
http://hddaudio.net/viewtopic.php?id=886I measured a typical 2 Way 2nd order LR crossover designed for a crossover frequency of 3.5kHz using a number of different inductors, using the drive units as loads at 10W the electrical harmonic distortion was typically 1% (-40db) for the iron cored inductor samples and 3% (-30dB)for the ferrite cored samples which was a suprise! Measurement frequencies were 1 octave either side of the crossover frequency. I did not use an aircored sample because it was highly resistive (0.8ohms) compared to the drive unit impedance which in itself introduces response anamolies of the order of 10% (-20db) which , since the drive unit signal is in error compared to the input in the passband of the filters,it can legitimately be called distortion. In practice response anomolies are measured with all the inductor types due to their resistance. (This is particularily noticable in 3 way designs where the lower crossover point requires large values of inductance)
Bon, la je suis ok pour les ferrites mal dimensionnees, mais la resistance serie d'une bobine "a air" peut (doit) etre prise en compte dans le design du filtre non? Qui plus est, la "distorsion" dans ce cas n'en est pas vraiment une.