Hello Endo, congratulations for the purchase.
I have same driver and working on the design of the dipole panel. It will cover 250-1K in my system with the M74B/T25B above it.
This is not a driver to work with the usual DiY methods. I recommend asking Bliesma for the 3D file. It requiere attention, as you already noted.
I recommend precision routing both sides (if you use MDF). I decided to use CNC/solid 6061.
Whether you use OB or closed a very sloped chamfer is necessary at the back to leave the MC as free as possible, this is Bliesma recommendation.
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Sorry, you are welcome to despise me. But I'm not willing to pay that price, and for alleged benefits that I personally don't see.
But everyone is free in his decisions.
Many greetings,
Michael
I'm betting thd rise on the 142a that starts at 2.8k is also on the 142b both spec sheets say 180-2800hz prob for that reason alone I messed around with the 142a running upto 4k without issue but thd was raising but was within avg driver ranges -40db. I need to start a build thread for my constant directivity dumb loud speaker aiming for 20-20k at 115db under 1%thd.
In some respects, Alu/Mg can overall sound more neutral than Be if the alloy is specifically designed to ease the breakup peak,sometimes over 15 dB in level, so a notch is necessary even with steep filters. In some of the smaller Ti and Alu mid domes I've used, the peak around 10k can be audible even if its down 30 dB after filtering and notching. Some people are more sensitive to the peak than others.
I'd be interested in trying the M74A since after hearing the difference between the T34 in both Be and Alu versions. I thought the Alu version was every bit as good as the Be and maybe even a little smoother. I like the fact they kept the flange on the T34 variants at 104mm, which opens up the ability to swap in quite a few HF drivers ie ScanSpeak and Seas. Their Be domes are overpriced IMO and not as good as the T34B overall.
I'd be interested in trying the M74A since after hearing the difference between the T34 in both Be and Alu versions. I thought the Alu version was every bit as good as the Be and maybe even a little smoother. I like the fact they kept the flange on the T34 variants at 104mm, which opens up the ability to swap in quite a few HF drivers ie ScanSpeak and Seas. Their Be domes are overpriced IMO and not as good as the T34B overall.
"The breakup of alu/mag tweeters is out of the band of human hearing. Yet the beryllium tweeters sound far better. With the mids the breakup is in the band of human hearing. So the audible improvement is even larger than with tweeters."
The breakup peak on a metal dome can cause IMD down into the mids if not treated, despite being out of audible band and down in amplitude more than 25 or so dB. There are some people who will swear up and down the HF breakup on a small metal dome isn't a problem, but every time I add a notch at 25 - 27k to a 25 - 28mm dome, it opens up the sound across the entire response range in the HF. The problem is accurately measuring the peak as most mics don't go up that far and flat. You'd have to use the impedance data to find the peak and hopefully get the amplitude close enough.
I don't know specifically about the T25A and B, but alot of other mid and HF domes use a phase disc at the tip of the dome to mechanically notch the breakup. In many cases you're still left with an off axis peak and these phase discs also affect the upper midrange off axis performance. Removing it and adding an electrical notch usually is noticeable. Many older Ti domes used a phase ring which is better, but also messes with off axis FR. The Be domes in the Yamaha NS1000 had these rings and I wonder if they would have performed better without them.
I'd be curious to see how the M74B would respond to a felt disc of varying diameter right behind the grill center as opposed to an electrical notch.
The breakup peak on a metal dome can cause IMD down into the mids if not treated, despite being out of audible band and down in amplitude more than 25 or so dB. There are some people who will swear up and down the HF breakup on a small metal dome isn't a problem, but every time I add a notch at 25 - 27k to a 25 - 28mm dome, it opens up the sound across the entire response range in the HF. The problem is accurately measuring the peak as most mics don't go up that far and flat. You'd have to use the impedance data to find the peak and hopefully get the amplitude close enough.
I don't know specifically about the T25A and B, but alot of other mid and HF domes use a phase disc at the tip of the dome to mechanically notch the breakup. In many cases you're still left with an off axis peak and these phase discs also affect the upper midrange off axis performance. Removing it and adding an electrical notch usually is noticeable. Many older Ti domes used a phase ring which is better, but also messes with off axis FR. The Be domes in the Yamaha NS1000 had these rings and I wonder if they would have performed better without them.
I'd be curious to see how the M74B would respond to a felt disc of varying diameter right behind the grill center as opposed to an electrical notch.
Endo2112 - - We all hear things differently, my friend... I have used beryllium tweeters, and carbon fiber tweeters, and aluminum dome tweeters. In my opinion, most of what we hear (or what I hear) as differences can be traced back to small differences in frequency response. Once these transducers have been adjusted, EQ'd, or voiced to have similar response, then the subjective differences become small and subtle. There ARE definitely differences in how drivers sound, but the differences are small once frequency response has been normalized.
I've worked with a few T34Bs in other people's designs. I was the lucky one to get the only bad pair in existence.
I know those threads have put a target on my back and I know you'd like to see me burn in hell for it. I still like the T34s, just not the ones I have.
Doing some prototypes out of CF reinforced ABS or even CF / PLA would be an option. It sounds like there's a lot of little mexhanical details to consider when adapting the M142 chassis to a bespoke enclosure design. Alot of CAD design help would be needed in this case which isn't cheap. A tapered midrange TL would be nice like B&W does and could likely have big sonic benefits vs a simple stuffed enclosure.
About the only parameter the M74B and Yamaha JA0801 have in common is their diameter.
Motor, surround, dome shape, back chamber, damping are different.
Besides, the M74B dome is a pressed foil whereas the Yamaha dome is vacuum deposited, so the physical properties of these beryllium domes are different.
Therefore, break up modes and Q's will be different.
From my experience your going to need a brick of foam behind the 142b lose fill wool didn't help much while 4 Inchs of foam knocked down that reflection foams king from 250up anyways. I went and found the properties of angel hair I would use both 20g of it and a 4 inch block melamine. Also I think that angel hair would be better suited to my cardioid box then melamine foam I'll need to buy some if anyone knows where I can get some In usa.
Aerogel okay but why yes it's better but it's only like 1.2x better per inch if you really want to destroy the back wave just 3d print a metamaterial wouldn't even take more then a week to design run each tube down the edge of the back wall you would need around 40 tubes of between 1/2 inch and 4mm to cancel from 50hz to 4k.