BAMTM with Vifa PL27TG35-06

[kgy]

i would like to build the BAMTM with the Vifa PL27TG35-06 and want to know if the xover for the da175 can be used and tweeter crossover redesigned for the vifa?

[ThomasW]

Link to BAMTM design?

[joecarrow]

The link is here:

http://zaphaudio.com/BAMTM.html

Sorry, you can't just drop in the tweeter- it would require a redesigned crossover. Considering that the tweeter in question is discontinued, it seems unlikely that Zaph would be interested in creating (or at least posting) a design using it. What's wrong with the Seas tweeter that Zaph used in the design?

[jkrutke]

I'm sure he has one sitting on the shelf he wants to use.

The simple crossover design for the BAMTM isn't completely unfriendly to other tweeters with similar low end response and impedance curves. The bigger issue is that there aren't many tweeters that can cross at 1500 Hz. I've used a D27 down to about 1700 and that's as low as I would have gone with that based on non-linear distortion measurements with a filter in place.

I've never tested a PL27 tweeter. (correct, I'm not interested in one either) The better question might be "has anyone used a PL27 down to 1500Hz?" I doubt it, but if someone has, the simple BAMTM crossover probably only needs minor tweaking.

I'd say go ahead and try it. Just don't call the final result a Zaph Audio design, and be prepared to yank out the PL27 and drop in a 27TBFCG if the PL27 screams distortion.

[dlr]

Not a lot of difference between PL27 and D27

The PL27 looks to have almost the same motor as a D27, but the pole piece is not flat on top, it has an angle going up from outer to inner area. I suspect this was meant partially to tame reflections, but must have some influence on the flux as it nears Xmax. It probably also has a coil wound differently to yield higher sensitivity. I would guess that for a given displacement, the distortion characteristics would be very similar between the two drivers. Just making a guess from taking them both apart.

[kgy]

thanks for the responses. i had originally planned to make Vinces VHT from the PE site but after i bought the parts my room layout changed suddenly and now i have to make good use of the parts purchased and thought the bamtm would be good use of the parts with a new crossover for the vifa tweeter. if i came up with a modified crossover could someone take a look at it and lend some help with it? i plan on using the same alignment and enclosure for the bamtm. thanks for the help in advance.
kgy

[dlr]

You'll have two things to consider

if i came up with a modified crossover could someone take a look at it and lend some help with it? i plan on using the same alignment and enclosure for the bamtm. thanks for the help in advance.
kgy

The PL27 probably has a slightly different SPL response than the D27, aside from sensitivity. Without measurements it will be a bit tricky to do it well. It would be possible to take the raw measurement of the PL27 at my site and add baffle diffraction using the tools at the FRD Consortium to get a simulated baffle response, but the acoustic offset will be approximate, so the crossover area may have some bumps in it. You should be able to get the relative sensitivity adjusted easily enough.

The acoustic center should in reality be the same for both tweeters. However, CAD software uses the frequency response to determine the phase response if not provided or if you generate it using the tools. In either case, this phase response is dependent upon the model you provide, the lowpass being critical due to usually being an approximation. If you set that to the Fc and slope that zaph used, you should get close. This might cause some slight shift in the value you'd need to enter for the acoustic offset, but it may not be far off.

The other issue is, of course, that the actual measurements on the baffle for the other drivers would be needed. If you can get them from zaph, that would simplify things for you. Without the midwoofer measurements, it'll be a lot more problematic. You'll have to simulate them as well. It looks like zaph doesn't provide those curves, nor the phase response in the simulations.