Saturday, 13 March 2010

Active Speakers - What are They?

There are three types of loudspeaker:
Passive
These are what most hi fi enthusiasts use, they are the cheapest to make and simple to design. Anyone with basic electronic skills can design a crossover, if they can’t there are computer programs that do it for them. Drivers are available off the shelf and The Loudspeaker Cook Book will give them a grounding.
The principle is simple, you need a separate power amp and inside the speaker box is a circuit known as a crossover that sends high frequencies to the tweeter and bass to the bass driver. In a three way it’s more complicated and you have an extra crossover to split the mid off from the lows.
Although they are simple, passive speakers are deeply flawed and introduce enormous amounts of distortion to an otherwise very pure signal from a modern amp and DAC. There are so many different types of distortion that it is difficult to describe them all, however the important ones are: Firstly the crossover must disconnect the amp from the drivers and so relinquish control of them. They are free to overhang and overshoot and secondly the crossover components add more distortion too, so it’s probably a good guess that they are a factor of a at least thousand times worse than the active equivalent. There are other problems too, but you get the picture........
In simple language they boom, tizz and congest compared to a good active speaker.
Powered speakers are simply passive speakers with the amps built in as is typical with computer speakers and the ones to be wary of because many are sold as active when they are not and will not be as good.
Active Speakers
Form very early on it has been accepted that active speakers are best and this was proven in the nineteen seventies when Op-Amps made it practical to build suitable crossovers for Public Address and Studio Monitoring. Active speakers are considerably better than passive ones, so much so that it is hard for many who’ve invested a fortune in piles of boxes to accept that a better result can be achieved with what looks like basic electronics on a plate screwed into the back of a pair of speakers, but it can.

The reasons are very simple. Firstly the amplifiers are permanently connected to the drivers and preventing them from overshooting or overhanging. Secondly the crossovers, which are before the power amps, can be much more accurate, have steeper filters and be more perfectly phase and amplitude correct. In a passive system the drivers are reacting against the crossover, but they can’t do this to an amp. Thirdly amplifiers have less distortion if the bandwidth they cover is restricted as it is in an active system. Fourthly high voltage low current amplifiers have less distortion than high current amps needed to drive passive speakers.
The difference between active and passive speakers is literally enormous and, as you can see from the illustration, this was recognised before 1954, It means that they can compare with the better headphones.
Why isn’t everyone using active speakers?
Everyone in Broadcast, Movie Making, Music and Public Address probably is, but not many hi fi enthusiasts who tend to be late adopters of new techologies, although that surely will change soon.
Hi fi enthusiasts have for years relied on subjective evaluation, they have believed that even cables sound different and they have insisted on auditioning each part of their systems separately. Even if that isn’t really possible, that’s what they think they are doing. They are also conservative and resistant to change, many are still using turntables, still will not accept the quality of MP3 or AAC and they have often spent a fortune, so aren’t keen to know that they can do better for a fraction of the cost, but they can.
There is no doubt also that conventional active speakers were more expensive than an Integrated amp and passive ones, because you still need a preamp and a CD player etc. However computers have changed all that by being the best source and cheap and we’ve (AVI)  put the DAC and preamp into remote controlled active speakers and so reduced the cost considerably. AVI has changed everything.

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