Crossover advice - Vifa P22 + Seas 27TDC

[Arctophile]

Hi all,

Since a lot of you here have been raving about the 27TDC tweeter, I thought I'd check if I could get them locally - and hey presto they're actually available at a reasonable price!

So now I'm planning to mate them to My Vifa P22s (yes I know, not ideal for dipoles, but as I've said in another thread finding replacement 8 inch drivers at a reasonable price in Oz just isn't happening).


My problem is I'm a complete noob - a friend designed my current xover for me and I'm not sure how to go about modding it to suit the new tweeter. I suppose I might have to start from scratch.

So if anyone knows of a xover design using the 27TDC in a dipole which I coul d use as a base, I'd be pathetically grateful.


Thanks,
//Adam F

[JonMarsh]

Davey has a design a bit like that- check out some of his posts/threads, you'll find a link to his home site. The search engine is your friend...


~Jon

[Arctophile]

I searched before posting - promise!



Davey built his with a surprisingly simple xover - I used SL's PMTM1 prototype (http://www.linkwitzlab.com/proto.htm#PMTM1) xover as a basis for my current one, and as you can see it has a zobel in the mid section and a steeper (25dB) slope for the tweeter (apparently to "provide additional phase shift in the crossover region to delay the electrical signal and to bring the acoustic output in phase with the midrange" - I'm struggling with these concepts ).


I had my friend design something similar into my current xover - is it superfluous?

Also Davey is using the TDFC rather than the TDC - going by Mark K's tests, the TDC should be just as good if not better at 1.5KHz (the xover point I'm hoping to achieve), but in the official Seas spec sheets it says min. xover point for TDCs is 2KHz (1.5 for TDFC). Would you say ignore the official specs and just buy the TDCs or try to source TDFCs?


Thanks,

//Adam F

[Arctophile]

Oho I should have read more sorry, his EQ and notch filter are built into his active xover.

My main problem is that I can't afford a mic, that's why I used tried and tested baffle dimensions (from these tests: http://home.pacbell.net/donwm/PrototypeTestResults.htm) and relatively predictable woofers (or so I thought - a response curve for the P22WP has eluded me so far).

Still, I would like to know what you think about the TDC vs TDFC question.


Thanks,
//Adam F

[Davey]

Adam,

That's not a Zobel in the mid section of SL's PMTM1. It's a series notch filter that creates part of the dipole equalization and attenuates the peaking created by the rear output adding with the front.

If you want to shoot for a 1.5khz crossover frequency with your 27TDF(C) setup and you'd like to play the system fairly loud you'll need to use a crossover with steeper slopes much like SL's. The simple tweeter crossover I used in my dipole system is the weakest link in the chain by far, but since the xover frequency is a bit higher and the SPL capability of the rest of the system is somewhat limited it works okay.

I haven't used the TDC version of the Seas tweeter, but I suppose it would work okay...... However, I think you'd need at least a 4th-order electrical crossover or there would be quite a bit of distortion.

If you don't have any measurement equipment or capability you're pretty much shooting in the dark trying to design a system in this way. The MTM baffle shown in your avatar looks much too narrow to me, but it's hard to determine whether this will work or not without further measurments.

Cheers,

Davey.

[Arctophile]

Thanks Davey,

I must admit I was having trouble trying to work out how SL's low pass section resulted in a 2kHz xover point and a 16dB/octave slope! I'll simply have to do more reading I guess....

As for my baffle, the other link I posted above is a table of a tonne of baffle sizes and those guys found 11" W x 2.5"-2.75" D x 26" H gave the most uniform polar response overall with this driver layout. Perhaps their testing method was flawed, obviously I don't have the expertise to judge


//Adam F

[Arctophile]

I've decided I need to get a mic and do some testing, but in the short term could someone help me with a simple mod?

This is my crappy MS Paint rendition of the xover which I am using (like SL's PMTM1 xover but with 4ohm vifa woofers):


IIRC the xover point is about 2500Hz, but I can't figure out how this is reached from the given values. It seems to be more complicated than a 4th order on top and 2nd order on the bottom, so I was wondering if someone would be kind enough to tell me which values I need to change to shift the xo point down to 1500Hz (I've decided to go with the 27TDFC to fit the existing xover topology better).


TIA,

//Adam F

[Davey]

Hmmmm.

Adam,

Well, it's not going to be as straightforward as scaling a few values to move the xover frequency down 1000Hz. The mid-drivers are totally different so you've created a much different sensitivity matching situation...in addition to other changes. I don't think any of the component values shown in SL's crossover will be correct and not only that, the topology may not be correct either. It will be a challenging project for a newcomer.

I think you'd be better off getting your feet wet with a much simpler project and later moving up to something like this.

Sorry I can't help you more.

Cheers,

Davey.

[Arctophile]

Well....in my defence the available specs on the P22WP say that it is about 90dB 1W/1m, almost the same as the P21WO. I have also talked to people who've measured it and said that the FR curves of the two drivers are very similar (although I've not been able to get my hands on a graph).


//Adam F