[cporada]

Several years ago I built the ACI alpha limited kit, and I am having some trouble now with one of my tweeters producing some “fuzziness”, so I would like to replace the tweeters.  I checked with Mike Dzurko, and he suggested the Vifa H26TG35 as being a very close match to the one originally in the alpha, but I am wondering (seeing as how I have to pull the speakers apart anyway)  if anyone else has tried substituting a different tweeter in this kit, preferably one with a bit better off-axis response, better imaging, and a somewhat mellower sound.  The sensitivity of the alpha is somewhere around 93 or 94db, so it would have to be a very high sensitivity tweeter to work in this kit.  If anyone has tried anything, what mod’s did you need to make to the crossovers to incorporate the new tweeter (sorry I don;t know much of anything about designing crossovers)?  Any help would be greatly appreciated.
Thanks in advance
Chris Porada

[cporada]

If this helps anyone to respond, some of the tweeters I was looking at were the Vifa D26TG35, D27TG35 or D27TG45, as all of these would probably have a bit better off-axis response, better imaging, a somewhat mellower sound, and a more extended high frequency response than the horn-loaded H26 (rolls off at 18kHz, kind of nullingfying the benefit of SACD). The low frequency response on all of these others seems compatible with or better than the H26 (so the frequency crossover from mid to tweeter should be fine), but the problem is the H26TG35 has a sensitivity of around 95-96db, so replacing it with one of these others that have sensitivities of around 91db will require me to do something to lower the overall sensitivity of the mids and woofers, since the overall sensitivity of the alpha is somewhere around 93 or 94db.  According to the guidelines on Parts Express, +/- 3db from the overall system sensitivity (93db) should not be noticeable, so perhaps I need not be too concerned since 91db is only 2db below the system sensitivity of 93db?
Any help would be greatly appreciated, since I know nothing about crossovers.
Thanks
Chris

[Mike Dzurko]

Chris:

The only way you can effectively use a tweeter with lower sensitivity is if you bi-amp the system. Using passive means to cut the sensitivity of the woofers is usually not a good idea. Essentially you'd need to add resistance into the woofer circuit and that will degrade damping and transient response. I'd stick with the high-efficiency tweeters as the system was designed. If you want to gain treble extension and perhaps dispersion, you might want to do some research on the concept of adding a supertweeter.

[cporada]

Hi Mike
Thanks for the helpful response.  I had actually toyed wityh th idea of adding a supertweeter, but wasn't sure how to begin making one of those.  Any suggestions in that resepct would be great!  Also, someone from the Parts Express tech talk forum suggested I try the the MG27TG39-04.   He says it'll directly bolt in the H26's hole and it's rated at 95dB (instead of the H26's 96dB). However, it's a 4 ohm tweeter, whereas the H26 is 6 ohm nominal, so I would probably have to change some things in the crossovers.  Are you familiar with this tweeter?  Do you think this would be an improvement, since the efficiency is about the same, but the high frequency response and dispersion are probably better?  Please let me know what I'd have to do to try this tweeter in the Alphas.  I still have the crossover schematics if it would save you the trouble of digging for them, I could e-mail them to you.
Thanks
Chris

[Mike Dzurko]

Hi Mike
Thanks for the helpful response.  I had actually toyed wityh th idea of adding a supertweeter, but wasn't sure how to begin making one of those.  Any suggestions in that resepct would be great!  Also, someone from the Parts Express tech talk forum suggested I try the the MG27TG39-04.   He says it'll directly bolt in the H26's hole and it's rated at 95dB (instead of the H26's 96dB). However, it's a 4 ohm tweeter, whereas the H26 is 6 ohm nominal, so I would probably have to change some things in the ...

Chris:

Sorry, I'm going to have to pass on this one. My design process is pretty detailed . . . measuring/modeling/proto/testing/listening. We're talking a LOT of hours. No question, the crossover will need to be re-designed. Some of the DIY folks are quite good and may be able to put something together for you . . . I 'd  throw out what you want to do on the DIY forums and see if they can help. Unless they do it right, you may make some gains in extension, etc. but lose cohesiveness. Let us know how it turns out!

[cporada]

Hi Mike
Thanks again for the good advice.  I think I'll simply replace the H26's, since the sound was great when they were working correctly.  If I went ahead and added a supertweeter (I was looking at the ones from Fostex which are linear out to 40 or 50kHz), what frequency do you recommend I crossover to mate with the alpha's?  I was thinking around 15kHz, since I think the alpha's are down 3db at about 18kHz.  Does that sound about right as a starting point?  Do you know what type of crossover would be necessary?
Thnaks
Chris

[Mike Dzurko]

Hi Mike
Thanks again for the good advice.  I think I'll simply replace the H26's, since the sound was great when they were working correctly.  If I went ahead and added a supertweeter (I was looking at the ones from Fostex which are linear out to 40 or 50kHz), what frequency do you recommend I crossover to mate with the alpha's?  I was thinking around 15kHz, since I think the alpha's are down 3db at about 18kHz.  Does that sound about right as a starting point?  Do you know what type of crossover would be necessary?
Thnaks
Chris

I think you're doing the right think by replacing the H26s.  As far as adding a supertweeter . . .  you're going to have to either do some experimentation, or find someone on a DIY forum who's willing to do some modeling, etc. for you. Even then, the final tuning will likely need to be done by ear.

[cporada]

Dear Mike,
Last week I received the two Vifa H26TG35 tweeters I had ordered and finally got around to installing them in my alpha's.  I am writing to see if you could tell me how long I need to allow for break-in and if you have any suggestions for the most expedient manner of breaking in the tweeters.  I would also like to thank you for your advice of sticking with the H26's rather than trying to switch to something else.  The sound is phenomenal, even without break-in.  Are these a big step up from the older H25's that were in the alpha's?  The amount of detail is staggering, and the imaging seems to have a taken a quantum leap forward, but I'm not sure why.  While I had the cabinets open, I went ahead and installed Deflex panels in addition to the Dacron fill, so perhaps this helped a bit too?

Thanks for your help

Chris Porada