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In Reply to: here's a crossover question for the EE's out there.. posted by stuck.wilson@gmail.com on March 26, 2007 at 12:44:03:
der..it's a linkwitz.. about 14-ish ohms on the driver at XO-- not that it's that pertinent for a general answer to the question 'what does the cap do in a second order crossover'.. but for the math folks out there!
thanks!
Follow Ups:
I'm not too sure how much math you want. The formulas for capacitive and inductive reactance are widely known. You know, the 2-pie stuff. Or worse yet, that funny w, omega:)Suffice to say that a cap’s, or an inductor’s, AC resistance (called impedance) is a function of frequency. In your second order low pass crossover the inductor blocks the highs and the cap shunts any of the highs that get past the inductor to ground. The reactance of the 4uF cap at 1k5 is 26.53 ohms and the 5uF cap is 21.22 ohms. The reactance of the 3mH inductor would be 28.27 ohms.
These parts form a frequency dependent voltage divider with you woofer. Just consider that the cap is in parallel with the woofer and look at the reactance ratios to see how much power is going where.
3.7uF looks about right assuming 14 ohms for the driver. But is your 14 ohms for the woofer telling the whole story? What of its rising inductance (Le)? Do you have a zobel network in place? Remember you need to account for how many ohms that woofer is an octave or two past the crossover point also.
The 5uF cap would have been rolling off the highs that sneaked past the inductor a little more aggressively, and earlier, than the 4uF caps does. The quality of the cap might be a factor but I'd also think (okay big wild ass guesss) maybe you like your woofer running up higher or the treble driver is a little harsh pushed down that low.
alrighty..i get a lot of what you said in theory- and thanks for the math lesson!
sonically-- i like what's happening more NOW than i did previously.. 1 uF in there made a big difference-- the 5uF cap seems to've made mids a bit strained, for lack of a better word-- and what i'm SPECULATING was going on, is that to get the mids right, i had to boost the horn-- which made the top end too hot by a bit. now she's right in the zone-- neither too bright, or too recessed-- which has been one of the toughest struggles: balance! then.. i guess that's the difference between a good crossover and a bad one, i s'pose!
in any case-- yep-- gotta run the bass driver further into the mids (it's an altec 414b) to get the right midrange. i used to use a 1k cross.. but it was too 'detaily' sounding.. the altec's a lot smoother in there, to my ears. it DOES rise-- to about 25 ohms an octave up, and it's really just on a gentle slope from about 12 ohms an octave lower. 414's run pretty well up for a 12" driver- which is why it is where it is, and from what i've been told-- it's a good place for dispersion too- if my ears are any indication, i believe!
the horn above it is set on a first order, and they're blending nicely right now!
thanks again!
You might want to try crossing over even higher. In terms of "beaming" 13" is right about 1,000 hertz. So a 13" cone would beam there. But what actually happens is the cone edges start decoupling from the center of the cone so your 12" woofer isn't really 12" at 1500 hertz because the outside edges are hardly moving compared to the dust cover/center of the cone. IIRC some speakers had that woofer running up near 3khz. Now that is too far IMHO but 2khz might be worth trying. I mention this because reading between the lines I feel you enjoy the mid range warmth and tonal qualities of the woofer and find the treble horn a little taxing.You are smart to use a 1st order electrical on the horn as it has a first order acoustical already. Don't be afraid to stagger the crossover points a little. Say 2khz on the woofer but 2k2 (or higher) on the tweeter. Many woofers have more output as the hertz goes up so it will play higher up than the crossover would dictate (unless you put a zobel on it). Plus it is not unusual to have a little extra energy at the crossover point as both drivers are combining to the output.
For me, trying to get a crossover right with only my ears is an act in frustration and a huge waste of time and parts. You really have to measure the driver parameters, do a spot of modeling, and then measure in room response.
But I know you are enjoying what would be frustrating to me:)
yep- definitely staggered crossovers!i found that if i push them too far out though-- the gap in the middle causes those same balance issues-- right now, i think my horn crosses in around maybe 1600 hz-- but i like it a lot. i had a little hot spot in there (where hard piano transients'd 'buzz' a little bit)-- but the new cap on the altecs made a big difference there. i used to run the horn out higher (bigger gap in the middle)-- but balance got squirrely-- if i had good mids, too much on top, if i had the top.. dead mids. i
right now-- i'm pretty psyched with the tonal balance.. it's kind of a miraculous transformation.. i dunno how i botched THAT.. but i did! regardless-- i whipped in a few capacitance values on the horn-- and pretty close to 'textbook' is working (i'm down a few tenths).
honestly though-- the BMS 4552 i'm using isn't too taxing-- adjusted wrong.. holy cow.. yes sir! but, with this iteration.. it's nice and smooth up top. i know everybody is pretty anti- ears, or anti-measurement.. but i definitely enjoy it. i use caps from a pretty big collection of old oil cans (all scrap parts and ebay finds.. not $100 in the lot, but literally 200 of 'em, mostly vitamin q's, etc.), and i've got about 5 sets of madisound sledgehammer inductors from about 1.8mH to 3.0.. again, less than $100.. so it's cheap fun, more or less! things getting close.. i'm spending less and less nowadays too.. thank god.. less time soldering too..
much more listening anymore.. thank goodness..
thanks again!
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