idc said:
So putting the amp inside the speaker is supposed to have major advantages. What happened to the benefit of putting amps on racks to ensure they are isloated from vibration? How can it be ggod for an amp to be vibrating away in time to the music?
Hi IDC
I see you're genuinely interested in this active speakers phenomenon. but your thinking is wrong. sorry if I sound preachy for you but I just want to explain the difference between active speaker and powered so you know in the future. and it doesn't lie in just adding a power amp inside speakers enclosure.
so, as mentioned before active speaker has active crossover. active crossover is working with line level signal which means it lies just after preamp in components' chain (hence you must connect an interconnect to active speaker and not speaker cable). what crossover does (active or passive regardless) everybody knows; it splits full range signal into streams suitable to be reproduced by different drivers (woofers and tweeters). so if you have 2-way speaker there would be 2 streams and if you 3-way speaker there would be 3 streams and so on. after being split in crossover the signal is sent to power amps for amplification and then power amps are connected directly to drivers (this implies that if you have 2-way active speaker you'd have 2 power amps inside the box, for 3-way active speaker it'll be 3 power amps and so on; one power amp for each driver). when power amps are connected directly to drivers it has this advantage that every Watt of power being produced is spent to drive the driver and not to drive passive crossoer as it's the case with passive system or just powered speakers. to give you rough idea how much more efficient this solution is; if you have a 2-way active speaker with 50W power amps for each driver (100W in total for speaker) it will give you dynamic capabilities of a 200Wpc power amp connected to the same speakers but in passive version. you might argue if you really need all that power and dynamic range in home use but fact is fact... there's more. each power amp in active speakers is employed to amplify only narrow part of full range music spectrum. depending on crossover point it would be DC - to say 2000HZ for woofer amp and then 2000HZ - 20.000Hz for tweeter amp. this reduces intermodulation distortion becouse amps are not forced to amplify very low frequencies and very high frequencies simultaneously. it should be noted that well designed power amps will not have problems with intermodulation distortion even if asked to amplify full range signal but well designed means higher cost to customer...
another important matter for active speakers vs. passive speakers is crossover design. if you use a passive crossover of 2nd order or higher you'll most likely have problems with electrical phase shift meaning poor imaging and not linear frequency response. if you ever get a chance to see a frequency response graph of a passive speaker with 2nd order crossover you'll see a dip or bump of about 3db in crossover region. it's because frequencies from woofer and tweeter either cancell themselves out or strenghten themselves up depending if drivers are conneted in positive or negative acoustic polarity. also the sound from both drivers should reach listener at the same time which is not always the case. this can be entirely eliminated when using active crossover and even steeper crossover slopes can be used. it's impossible to design a good passive crosover of steep slope because of variable and unpredictable impedance from the drivers, hence the problems with taking phase shift under controll. the only situation when a passive crossover is as good as active is when it is first order (most shallow slope). but to be able to use it you have to have drivers of outstanding linearity and wide bandwidth. I know Dynaudio is using first order crossovers for sure. I guess B&W as well but not sure. few manufacturers of hi-fi speakers boast what crossover types they use. also Monopuloses speakers address the issue of lack of time coherence between drivers.
P.S. I wrote above that you connect an interconnect into an active speaker. in fact you connect an interconnect into a powered speaker too. but the difference with powered speaker is that the signal reaches power amp for amplification and then amplified sygnal is filtered through passive crossover, hence facing the same limitations as traditional apm + passive speakers set up.
hope this clarifies the main differences a little