well its actually technically twin 12 db /oct filters so it adds up to 24db oct
Er, um, ... it doesn't work out quite that way. If your say bass driver is cut by 12 dB/oct on top, and your say mid is cut by 12 dB/oct down, what you end up with is a 12 dB/oct, or second order filter.
You have 24 dB/oct only if your filters are in a straight series, one after the other.
In my view, 24 dB/oct is excessive. While it will of course filter very well, it will also tend to introduce quite a bit of phase shift and could well inhibit dynamics (sort of slowing down fast passages).
though i suspect i need this phase issue thingy explained somewhat!
Well, in very loose terms, each and every filter ever made by anybody will, by default, introduce some phase shift. As you know, phase shift is changing the time relationships of the output signal as compared to the input signal. Essentially, capacitors tend to do that rather well simply because their impedance is non-linear (VERY non-linear), but they could also be introduced by poorly designed electronics (essentially, electronics with a small open loop response, which need much feedback to linearize them).
Subjectively, we tend to hear phase shift as a blurring of the sound, as its going out of focus, and the first to die is low level spatial information, what usually gives us what we call "air". If really overdone, we will tend to say something is wrong, the sound is unreal, even unnatural, but that bad is rare these days.
The greater the filter cut rate, the more capacitors you need to use, and consequently, the greater the opportunity for more phase shift. Also, the charge and discharge time of more capacitors tends to lengthen, which then tends to slow down the delivery and mess up the sound dynamics.
Please understand, this is all in very loose terms, I'm trying to describe what is essentially very subjective - however, also quite measureable.
Which brings us back to filters. Obviously, one is faced with a dilemma: use steeper slope filters for better driver coupling (less driver extension outside its designated range) with phase and dynamics problems, or use less steep filters with better dynamics and less phase shift, but also more driver range overlapping.
In my view, 12 dB/oct represents about the best overall compromise between the two, but I must add I have heard some really good electronic XOs which used 6 dB/oct for driver top cutoff and 18 dB/oct for mid lower filtering - in other words, a combination of two different slopes.
This is where the art part of the deal comes in. The designer must have an excellent practical knowledge of the drivers he is using to be able to make the appropriate choice, and using 3X drive also helps because the critical 800-3,000 Hz range is usually covered by a dedicated driver. My choice were drivers from Son Audax, but there are other drivers from other manufacturers which are just as good, and some probably better. The reason why I opted for Son Audax is because I happen to like their arguably softer sound than most others, but another reason was because of their uncommonly linear impedance modulus, which makes them easy to drive and allows me to concentrate on other aspects of the amps more than on sheer brute power. Lastly, I have yet to hear bass drivers made as well as Son Audax makes them (this one at least, 10" long throw, plastic over paper, 7 lb magnet, die cast frame, real rubber surround).
It seems to me this is the key part of designing a good active system - you need to use just as good drivers as you would in a passive design (i.e. no cost-cutting because you have direct amp drive with no passive XO), you need to know the drivers you work with and you need to select them very carefully. The electronics part is then fairly straightforward, but not cheap if you go for the max.
For example, you need to decide how to supply your amps with power - use one single supply line for voltage and current gain stages, or use separate (slightly higher for voltage gain stages to compensate for natural voltage drops) lines, regulated or not, and, most important, will you regulate the output stages as well? This will lock them into an almost ideal operating state, but it practically doubles your outlay for the power amps. And it could well open the door to pure class A amplification, done by the book.
Remember, your mid and tweeter drivers will be very happy with just 30-40 watts, since they are by default efficient and there is no power-robbing passive XO in between.
Decisions, decisions, ...
Cheers,
DVV
