Filtre active versus filtre pasive.

[SIG]

Motto:

"Calitate inseamna,printre altele, mai multa munca :b1: "

 

 

Dupa cele prezentate mai jos,unele spicuiri doar din articolul a carui legatura este redata mai jos,

se pare ca filtrele active scot o calitate mai buna in boxe decat cele pasive.

Si nu este un moft de a avea filtrare activa,chiar are suport tehnic.

Cand ne apucam de construit cate un amplif,poate e mai bine sa ne transformam boxele in unele active.

Cu un drum prindem doi iepuri cum ar' veni (amplificare + filtrare activa boxe).

Ce-i rept trebuie un amplif in plus,dar daca nu te multumeste filtrul activ din boxa faci compromisul.

Ce parere aveti?

Passive Crossovers When an amplifier reproduces the entire musical range, coils (inductors) and capacitors are used in the speaker cabinet to separate the high and low frequencies so that each may be supplied to the appropriate loudspeaker driver. A loudspeaker can be a difficult load for any amplifier, but when additional inductance and capacitance enter the equation, this only makes matters worse. Add to this the fact that all passive crossovers introduce some degree of loss (in some cases as much as 3dB - which means that they are "stealing" half the available power), and one can see that getting rid of them cannot be such a bad thing.

 

Look at the impedance graphs for almost any speaker system, and it will be seen that there is almost always a dip in impedance (sometimes severe) at the crossover frequency. This is caused by the interactions of the loudspeakers and their inductor/capacitor networks, and in some cases can cause amplifiers considerable grief - especially at high power levels. Although few amps will fail, one can expect a reduction in effective output power as the protection circuits limit the maximum power available due to the loading of the crossover network.

 

These vague thoughts are brought into stark reality when one learns that the inductors and capacitors needed for the low frequency crossover are quite large values, which leads many speaker designers to compromise in the interests of economy. The inductors may have an iron or ferrite core - which improves its inductance, but ruins its linearity. So now the crossover behaves differently depending upon the amplitude of the signal. High value high quality capacitors are expensive, so again, bi-polar electrolytics are often used. It is often stated that these sound awful, although this is a somewhat contentious issue, but without any doubt their characteristics change with temperature and age. They also have rather mediocre accuracy against their claimed value (+20/-50% is typical), so a 10uF crossover cap may be 12uF, or as low as 5uF. This does not make for an accurate crossover network, and most reputable speaker manufacturers will not make this sort of compromise, at least not for their top-of-the-line models.

 

In addition, at high powers, the impedance of the voice coil rises because of the temperature rise in the voice coil. This is not stable, and varies widely with the music. So with loud passages, the voice coil temperature might rise significantly, which will severely impact the performance of the crossover - relying as it does on the load impedance being a constant. A loud bass solo followed by a relatively quiet but complex passage might create an interesting shift in the crossover frequency and phase response as the voice coil cools, which is unlikely to enhance the listening experience.

 

Electronic Crossovers In contrast, the electronic crossover uses active filters in the low-level signal path. These suffer none of the power "eating" problems of the passive variety, and are far more easily tuned to exactly match each other - both within the same unit (between low and mid+high), and from one unit to the next. Indeed, it is so easy to tune an electronic crossover that they could be (and often are) set up individually for the exact loudspeaker drivers installed in a given cabinet.

 

In the sound reinforcement industry, crossover frequencies may (or should) be changed to suit the type of music, or even to suit the acoustics of a particular venue.

 

There are no issues with the crossover frequency shifting, since it is stable and not at all reliant on the voice coil impedance. This will still change with the power level, but the effects are unlikely (certainly less likely) to be audible.

 

Now there are another two plusses to add to the list - elimination of the low frequency passive crossover, resplendent with its inherent losses, potentially poor linearity and crossover point inaccuracy (either as manufactured or with time, or both), and the reduction of the difficulty of the load presented to the power amplifier. Both of these result in more effective available power, ensuring that transients are preserved and overall linearity is improved markedly.

 

Later edit:

Sursa

 

http://sound.westhost.com/bi-amp.htm

[donpetru]

In aplicatii unde este nevoie de o putere foarte mare utilizati intotdeuna incinte acustice active avand amplificatoare audio cu intrare balansata.

[Barbos Radu]

Cam complexa problema !

 

1- E greu sa realizezi sa zicem pentru trei cai cate un amplificator,si nu ma refer la munca in sine,ci la calibrarea acestora , in sensul ca nu cred ca avem garantia suta la suta ca nu vor aparea tot felul de defazaje si inegalitati intre amplificatoare,e greu la cele stereo pe o singura banda dar mai ... multiamping !

 

2 - Aici problema este sa zicem putin mai usor de rezolvat in sensul ca iti poti permite sa faci mai multe teste si masuratori intr-un mod mult mai usor decat la cele de tipul multiapming!

[Vizitator vtp2]

Este lipsita de sens o abordare de genul pasiv versus activ!!!

Orice solutie reprezinta totdeauna un compromis, compromis facut pentru a atinge un scop in anumite conditii de calitate, cost, etc. Daca in discutie intra incintele de uz domestic, cele active necesita investitii mai mari, si, necesita in utilizare cunostiinte si posibilitati de masurare serioase, altfel totul devine o joaca dezordonata! Solutia este uzuala in domeniul pro unde costurile, cunostiintele tehnice si aparatura de masura sunt la un nivel corespunzator.

Pentru uz domestic, filtrele pasive sunt mai ieftine, mai usor abordabile, si, ridica suficiente probleme pentru o rezolvare onorabila a situatiei.

[Vizitator spirtaru]

Eu folosesc biamplificarea pentru simplul motiv c? m-a costat mai pu?in dec?t dac? foloseam filtre pasive. Nu mai pun munca implicat? de realizarea ?i m?surarea bobinelor, ?mperechere condensatoare etc.

[mmc^2]

Filtrele passive pentru trei cai sunt foarte greu de dimensionat , cele mai multe boxe HiFi de calitatae raman la doua cai .

 

Apar probleme cu defazajul de grup al componentelor armonice .Se pot vedea unele boxe cu "corectia spatiala" a fazei pozitionand diful de bas in fata celui de medi care si el este mai in fata decat tweeterul.

 

Alta problema ar fi puterea nefolosita judicios ( in caz de instalatii mari ptr. discoteci-concerte )

 

La o instalatie fullrange pe un singur etaj final basul moduleaza mediile si inaltele si le impinge in domeniul nelinear de clipping chiar la amplitudinea basului de numai aprox 60% din puterea nominala /Asa pierdem 40% din puterea basului.

 

Daca se face un calcul se vede ca este mai calitativ si mai ieftin sa avem o instalatie pe doua cai cu crosover electronic la care fiecare banda de fercvente foloseste la maxim etejul final dedicat.

 

Fara a mai umple pagina cu cacule ..de ex..la pasiv am avea nevoie de aprox 200W statie fullrange ca sa avem aceiasi putere in bas numai cu 100 W statie ptr bass si 50W ptr medii inalte si suna si mai curat nemai avand IMD ( intermodulatie )intre bass si medii.Neluand in calcul pierderile pe filtrele pasive care la bass au cel putin 1 ohm cea ce poate sa insemne 20% pierdere de putere.

 

Filtrele active mai au un avantaj , mai bine zis filtrele electronice dar tot pasive , care au un raspuns "plat" fara supracresteri deci nu coloreaza tonul se apropie de filtrele "ideale" cea ce coresounde cerintelor HiFi.