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- rjj45 8:15AM
Do less harm (GD & Phase) filter
Just at initial stage in playing with crossover for a coaxial. And, I was looking at making it as simple as possible, but no simpler... The thought being if I needed / wanted to do an active overlay it would be to the coax as a unit and not individually to the mid and tw.
So, initially looking at a 1st order vs. a Weems special: a notch on the mid with a 1st order HP on the tweeter, with a goal of keeping the phase somewhat rhyming (running ~parallel at least through the 1kHz thru 6kHz high ear sensitivity range, or better).
While a first order may be doable (top graph in attached), the Celestion FTX mid has a nasty peak btwn 3 and 6kHz. (The driver is uniquely designed as a common motor coaxial, so I'm trying to maintain some resemblance of phase coherence).
Interestingly, the Weems notch creates a similar phase inversion as do higher order filters (which I'm looking to avoid). However, it does look possible to get this inversion a) out past 16kHz with the drivers actually crossing around 5 to 6kHz, and b) with the midrange being down 12dB or more at 16kHz. Second graph in attached.
The third graph shows the problem (per my goal) with a 2nd order LP for the mid. (with the advantage of course of dropping down the mid's peak much better than via a 1st order LP). But, toss in a resistor in parallel with the inductor, forth graph, and I get something that looks very good relative to the phase of the drivers, and attenuates the mid's peak.
In each graph, the green line is the phase of the tweeter with a 1st order HP.
Thoughts ?
Comments
Watching this ^.
Seems like the the GD is pretty high and these graphs are only showing us the Tweeter data, right?
It would be helpful to see the mid GD & Phase and the FR data as well . . . for both . . .
Sorry for any confusion: graph from VituixCAD titled ‘GD & Phase’. Red lines are the mid, green lines the tweeter. In each I kept the tweeter with a 1st order HP, and then its the mid driver’s filter that changes.
FYI, I have the FTX0617, which is, I believe a bit different from (if I remember correctly) your FTX0615s. Close and similar issues, looks like the 615’s might be a bit better behaved, with the 617’s able to take a bit more power. The data sheets shows both mids as Kevlar loaded paper, yours with VC winding height 13.5 and Qts 0.376 vs mine at 10.0mm and Qts 0.738, you have a silk tw diaphragm w stated pwr rating 10w, vs polyimide w pwr rating 40w.
In any event, it will be interesting. Don’t know if you are set up active / passive with ability to actively eq the coax as a unit- but it provides a lot of flexibility with these low order filters.
Unfortunately my ‘We need to do’ spring list around the house and yard seems to be growing exponentially.
Right now I’m just focusing in on the phase in combo with the ability to attenuate the mid rise. Actual Xover components/ design will be done after actual measurements in box (after my hunny 2 do list gets rapped up - which are actually mostly my 2 do list - clean up yard, cut down a few trees, stuff etc.)
I’m looking at a vented design for the coax (addressing reflective back pressure and waves, not for low end support), along with a highly damped / stuffed aperiodic vented W.
OK, thanks - that cleared up the previous description for me.
I can't speak to the GD/Phase preferences but your 1st and 4th graph look almost identical while unable to see the FR change mentioned - I would go with the 4th graph (2nd order w/parallel resistor) and EQ the devil out of it.
Most car audio coaxes have either Zero order (run wild) or 2nd order woofer LP filters while the tweeters are 1st order HP - but they are made to be inexpensive rather than perfect.
Hybrid XO = get it close with passive and EQ actively.
Concerning spring - completely understand - My internet had been out for the past 25 days straight and just got back on-line yesterday but I have done a lot of yard work and shop reorganization in the mean time.
Best!
Just an fyi at this (playing around) point regarding apparent phase effect (per VituixCAD) with and w/o resistor across inductor???
When finally built (drivers placed and measured), the values of the components will expectantly need adjusted to get the drivers' frequencies in sync, but just tossing this out as I had not really looked at the phase chart much (primarily focused in the past on frequency). Its probably not unexpected for many of the experienced folks here, but something I just stumbled on.
as above: green is tweeter with a 1st order HP, the red is the mid per filter design shown (phase only shown)
Phase of a crossover filter is directly proportional to the amplitude response...
So, which is the preferred configuration for the mid?
I’ll be giving them each a go once built. Currently giving the nod to the 2nd order with the resistor across the inductor. But I’ll have to see how actual measurements pan out. 2nd preference looking at the Weems notch.
While I haven’t had time to make sawdust for speakers (still cutting down trees) and get actual net driver measurements, I’ve been playing with a potential series 1st order passive (in VituixCAD) for the common motor Celestion FTX0617 coax. This should allow for the most contiguous relative phase relationship between the tw and mid. Of course, while phase shifts per frequency, and equalization, my thinking is by applying an active to the coaxial as a single unit it should not affect the relative phase relationship of the drivers (but only as a coherent pair). I.e. it’s when individual drivers phase/frequency shifts relative to each other that cause constructive and destructive interference / lobing issues through the crossover range, as seen with higher order filters.
Another benefit of active / passive provides a bit of flexibility as shown in the attached; implementing a broad notch eq ~3200Hz to the coax, bringing down each driver’s level across the pair. Thinking this solution will be much more ‘sound’ than a higher order passive only crossover, as per Celestion’s recommended crossover (shown on the bottom of the attachment).
This approach can of course be done / tried via an all active design (notch before the mid/tw crossover) for anyone currently working on a coaxial, just saying….
Following up on looking at a 1st order series crossover for the Celestion (previous posts), I've come across a way to potentially adjust / steepen the rolloff of the tweeter (for protection at lower frequencies), with the addition of a capacitor, C4.
In the attachment compare tweeter green line below 2kHz towards frequency down @ -35dB. Top graph series 1st order, middle graph with C4 30uF, bottom graph C4 at 10uF.