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In Reply to: RE: Quick-fix: will the IIIa ribbon with fit the 3.7? posted by neolith on January 10, 2017 at 06:30:58
The Adyton amp Beluga uses on the tweeter and mids is not going to have a problem with the low impedance of a 3a tweeter. It might be useful to check how hot it is running, but I doubt it is a problem.
I am going to venture a guess as to the cause of the tweeter failure. I would guess that it is related to the adoption of high rez DSD and 24/96 and 24/192 files. These are commonly mastered with 6db higher levels of deep bass and the transients are recorded more fully so that the average mid content level is going to be 5-10db lower, resulting in the listener raising the volume setting.
This has two results, 1. there is more bass content that can leak into the tweeter despite the 2nd order XO. 2. the transient content becomes far more powerful with a transient crest factor up to 10 db greater than on CD.. This increases the burden on the tweeter. People playing vinyl on idlers and high torque DD turntables already face this issue.
The solution is to watch your listening level with a fast response peak SPL measurement with a hold function. If you are getting peaks on high rez content (drum and rim shots, brass) at 110 db then you should tamp down the volume. The second solution is to raise the crossover HP for the tweeter up to above 4 khz.
The old 2 ohm 3A/ T IV tweeter can be used and it is more resilient - perhaps a better choice for you. It is a 55" tweeter whereas the 3.7 has a 60" tweeter inherited from the 20.x models.
Your HP cap for the tweeter is likely going to need a change in order to retain the same -3db point with the lower impedance tweeter. Measure and fix the cap value accordingly.
Follow Ups:
neolitb wrote:
"The IIIa ribbon is wider than the newer models (everything after the IIIa) and may not fit the cage.
However I think you could easily swap in the IIIa tweeter (cage and ribbon) into the 3.7 since the width of the cages are the same. The IIIa cage is 0.6" shorter than the 3.7 so it will fit in the "groove"."
The cages that I have, have the same measurements, not sure about the screw holes. Mine are Tympani IVa and 3.6. Both cages are 60 inches long, 1524 mm. Width are also the same.
The 60" v 60.6" length info came from Magnepan and I did have to rout out the tweeter channel so the 3.7 ribbon would fit in my IIIa's.
I married the perfect woman. The downside is everything that goes wrong is my fault.
The manuals up to the 3.6 indicate a 55" ribbon, I don't know at which point in the course of the 3.6 model's life they started using the 60" ribbons. It has been quite a while that when they get both speakers of a 3A or 3.3 and 3.5 for restoration they swap the origina short tweeters for the long one, at least if asked to.
How about we all measure the length of our ribbons and note the model?
55" is what Magnepan say in the manual of the Tympani IVa, that is for the length of the ribbon - not the cage which is 60". My T-IVa are from 1986 and my 3.6 from 2003. Both have 60" cages!Cross sectional view:
Edits: 01/11/17
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