BenLaw said:
oldric_naubhoff said:
but I'm sure that in a few days time we're gonna see known "active speakers are better because they have active xover which distorts less and therefore active speakers are more accurate" kind of posting...
What are your thoughts then on page 16 of
this, ATC's view on the benefits of active over passive?
(Also recently posted by Rick.)
well Ben, you gave me some challenge. a lot of writing (I hate a lot of writing) but I'll do it.
firstly I'd like to note that I'm not anti active. there are a lot of scientific eveidence saying that, technicaly, active speakers should perform better than passive speakers. what I'm against are false facts re: active speakers superiority praised by some over here. what bugs me more is that those persons overheard some marketing propaganda on another forum and repeat it ever since without even trying to verify what they learnt.
back to your question. all ATC is claiming is true but there need to be made some notes, as this info only represents one side of the problem.
1. very true. but there's 1st order xover which, regardless if passive or active, will always maintain linear phase (no phase shift) and therefore good group delay characteristic. the advantage of active xover in these respect is that you can design active filters maintaining those properties with much steeper slopes. analog active filters would usually be 4th order and DSP based xovers would easily be 8th order or higher. higher filters are better because they take some responsibility off drivers, which otherwise must be linear over wide freq spectrum.
however, the best result you can obtain is with one driver. no phase shifts, no power losses due to xover network, single sound source. since there are no good quality full range drivers you have to go with second best solution, namely wide bandwidth. my current speakers cross over at 650Hz (quite low) and they definitely image better than any mini monitors I previously owned. I'm pretty much sure it's due to the fact that xover point was shifted lower into the midrange where it's not interfering with spacial clues coming form higher up the freq spectrum.
that's the reason why, when I'll be upgrading my speakers, it's going to be for something with a driver which will be able to cover even wider spectrum, like from 200 Hz or lower up to audibility limits. rather than wondering if I should use active or passive xover.
2. true again. but, again, 1st order passive xover is immune for that.
http://sound.westhost.com/parallel-series.htm
(see graph 1.5 and its description)
furthermore some manufacturers of the drivers deliberately give up on using copper coils in exchange for aluminium coils (for instance Dynaudio). the end result is substituting higher conductivity of copper for better thermal properties of aluminium - slightly smaller efficiency in return for much better power handling and thermal impedance stability.
and it's worth noting that described mechanism plagues only dynamic/ moving coil speakers and ribbons. if you have ESLs or other more exotic designs (piezoelectric, or NXT type drivers) you have no problems with rising temperature of coil with rising power form amp. also ribbons are very much immune to that because voice coil has usually much larger surface (especially in cased of large ribbons) made of aluminium so it can dissipate heat more efficiently.
3. all right, this one stinks of marketing blurb. 20dB lower form what? what is the reference point? anyway, I think it's a very good point nonetheless. IMO intermodulation distortion (IMD) is much more useful type of distortion than THD in terms of evaluating performance of amps. because you can easily squash THD in benchmark test by applying feedback. nobody measures THD for open loop amp where real THD measures are seen. almost every amp manufacturers builds amps with global feedback loop which aim is to compensate for THD (and also linearise frequency response and lowering output impedance). you could find some amps which use no global feedback and then you know that if the amp measures low THD it in fact produces low THD. but I'm slightly drifting off topic now. anyway, global feedback is not so good in repressing IMD figures, so by evaluating IMD you can see if you're dealing with quality amp or piece of c**p (you noticed how every amp on the market measures 0.05% THD or lower and yet not all of them are of equal quality. that's the reason why).
so, I asked on the beginning of this point what is the reference point? because quality full range stereo amps can have IMD as low as -110dB - -100dB (for instance Soulution, MF, Mark Levinson, MBL. just as a point of reference Roksan Kandy K2 has IMD at -90dB), which by itself is pretty inaudible. if we assume amps in ATC's active speakers measure IMD at -120dB, will it really make any difference compared to -100dB?
and there's one more point that needs investigating. even in case of active speakers a typical HF amp would have to cover spectrum from around 2-3 kHz to 20 kHz, which still is large. so IMD may still be an issue. I say "may" because I'm not sure. I need to dig more into the problem.
4. this is absolutely true. active bass sounds faster because of that. it's easier to maintain desired system Q. in case of most passive systems Q is too high hence you've got flabby bass, but you could also design passive speakers with low Q to compensate for it. but it'll be always easier to keep Q stable in case of actives.
5. true too. but it should be noted that this statement, although true, stinks of marketing blurb as well. if you have well matched drivers in terms of sensitivity and you're using nice and simple passive xover network you shouldn't be expecting 6dB power losses. in fact there would be hardly any loss at all. in case of one driver being more sensitive than the other you need to compensate for that fact which will indeed introduce serious losses.
but it should be noted here that most modern passive speakers don't exceed a certain efficiency threshold. 85dB would be considered the lower end of spectrum. this in fact is more than enough for most domestic use anyway. most moderately powerful (say 50Wpc), quality amps will have no problems driving such speakers to high volume levels. power losses issue is more important in pro audio world, especially for sound reinforcement gear. you know that for every 3dB volume raise you need to apply 2* more power. when you operate at kilowatts levels this makes a difference.
so, my point is not everything is so black & white as some may describe it. and active technology definitely serves well dynamic drivers, but with different driver technologies gains are not so clear cut.