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In Reply to: RE: MGIIIa tweeter question posted by Davy on August 12, 2014 at 07:17:49
Well, I measured the ribbon cages at the back of the 3.6. From what I recall they had the same dimensions as the cages of the T-IVa. I have a drawing of the IVa ribbon cages somewhere at work. From your writing I take it the distance between the magnets is different for the narrow ribbons.
All the drivers of the IVa are out of the original baffles. Mids have new 6µm Mylar and soon the foil conductors. At first, I got the wrong foil from Magnepan. Thought I needed the foil of the 3.7 mids, 0.001"x0.075", but the correct one was as used in the 20-series, 0.0005"x0.1". All drivers are 60" tall. Low bass is 13" wide, mid bass is 12" and mids 6". Mylar is 11"x58", 10"x58" and 3"x57". The ribbon cage is 2.25" wide.
Maybe I was not clear about the crossover. Only low pass for the basses will be active. The mid high pass will be done inside the input stage of the power amplifiers (for mid/tweeter), just a switch to a smaller value series capacitor. Of course, the type of capacitor will be better too.
The passive high level crossover mids to tweeters will be a second order filter, more like in the MG 20. Will measure responses with the drivers in its new baffle. The drivers will also sit closer to each other, center-to-center will decrease. The mids of the IVa are allowed to operate with less roll-off towards higher frequencies, lower mass diaphragms helps. The 3-series is rolled-off early, the ribbon need to go low.
Follow Ups:
Well the channel in the newer tweeter is clearly narrower so I assume the magnets are probably closer too , - not checked for sure. I am pretty sure the width of the whole cage is narrower on the newer tweeter.I had previously toyed with the idea of trying to install the newer narrow 3 ohm ribbon foil in an older type wide cage (previously used for a wide foil) but the problem with that is that the tweeter is usually glued to the side of the cage in various places to damp it and stop twisting and distortion, and you would not be able to do this. Also the magnet gap might be different too.
Wow that's a big rebuild job you are doing and sounds great. So your mids were originally shot? Sounds quite hard to use new mylar how do you know when the tension is correct? Using thinner mylar might indeed improve the mids, the only thing I might worry about is that there is then a even bigger speed difference between the mid and bass (the bass is already quite slow in relative comparison to the mid and tweeter)
Sorry I see what you mean now about the crossover, - you are actually intending to use a HP PLLXO on the external mid/highs crossover (i.e. a capacitor), not active.
Yes the post 3.3R 3 series speakers are rolled off a bit early in the mids and the tweeter has to go low. In the older MGIII and MGIIIa it's not as much for some reason. Its possible Magnepan wanted to take the midrange lower in the post 3.3R speakers to improve the lower midrange suckout that can occur in many rooms.
So it's a 18 6 12 12 crossover you intend to implement that is like a MG20 or MG20.1 crossover configuration. That can work quite well, - tried it before, but the 18 6 6 6 crossover sounds much better in my opinion due to the linear phase of the mid and tweeter and the whole things sound much more coherent and seamless. I strongly recommend to try it out. Once I heard it there was no going back. It's also cheaper as you are using less components. You can used a spaced 6dB crossover between the mid and tweeter if necessary to protect the tweeter and it still sounds v good as there is a good overlap between mid and tweeter due to the mild 6 dB slopes (I am using -3dB points of 3 Khz on the mid and 5.3 Khz on tweeter).
Edits: 08/12/14
Magnepan have not changed much over the years. My first set is from 1975 and they are very similar to the set from 2003. They still seem to use the same basic ingredients like magnets and wiring. I am really not sure they changed that much of their ribbon drivers. It can still be same magnets and ribbon cage....
My mids were damaged in shipping. There is a german company that has suitable streching jigs and they can give the new Mylar the tension of the original.
http://www.forumbilder.se/DDN65/img-0349.JPG
The mids of the T-IVa are the only Magnepan with 6µm Mylar diaphragms. Compared to the 3-series, the diaphragm is divided into smaller sections, giving each a very small moving mass. Bass will not pollute these mids as in models that share bass and mid on the same sheet of Mylar. Tympani basses have less moving mass than the 3-series. Up to 250 Hz is really not much of a problem.
You are right in that Magnepan speakers have not changed that much, - particularly in terms of the main drivers, just making subtle changes to foil instead of wire etc (and some crossover changes).
As for the tweeters, the wide and narrow ribbon cages are v similar in dimensions (and the magnets are likely the same) but there are other obvious differences between them. The channel the ribbon foil goes into is obviously a different size between the two. Take a look at yours. The wide foil will not fit in a narrow tweeter cage channel and although the narrow foil will theoretically fit in the wider cage channel it will not work optimally as it's not possible to put in the adhesive securing dots needs.
Yes sorry I forgot the Tympani IVa midrange is different to the 3 series. Sounds nice. Having the midrange on a separate sheet of mylar should give less IMD and the two sections should not pollute each other like you say. I hope they sound great when fixed up. I bought a set of dead IVas a few years ago and repaired them, but they were way too big for my room so I sold them on. Am quite interested in used MG20s or 20.1s but they are risky buy as its a huge problem if they need repair (due to the sealed push pull design of the drivers) and shipping to Magnepan to get them fixed from abroad is not viable. At least the single ended Tympanis and 3 series are DIY repairable.
Thanks for the info on the German company that can repair the drivers with new Myalr,- great to know. Was it expensive to get them to put in new Tympani Mylar?
Dave,
You can look at www.shackman.deReiner Römer does the new Mylar. The re-wiring is done by a friend of mine.
Prices are higher than at Magnepan but no problem with customs as it is within EU. In the end cost is similar to sending them to Magnepan. I visit Germany regulary (my wife is german) and that make shipping easier.
Tympani I - IV, Bass, new film and wire panel 300.0 €
Tympani I - IV, MT/MHT, new film and wire panel 200.00 €
MG I/II. Bass/MHT, new film and wire pair 790.00 €
MMG/SMG, non quasi ribbon, new film and wire pair 650.00 €
MG 0.5 - 12 pair 286.00 €
MG3.x Series, Bass, MHT, new film and wire pair 1250.00 €
Sheila at Magnepan wrote:"The 40" and 60" are 2 ohm and the 60.6" is 3 ohm
I don't know about a 3 ohm for the T4a, they use a 60" tweeter."I think even a 3 ohm ribbon would work in the older ribbon cage. Could not be all that important if the gap between the magnets is a bit wide, it is a matter of about 1 mm each side of the ribbon.
Edits: 08/13/14 08/13/14 08/13/14
Thanks for the info. Well its great to know someone can replace the mylar with the same spec and tension for a cost. I can rewire myself but it's a bit of a slow process so rewiring as well is welcome. Seen many different Tympanis before for sale with broken mylar. If this had been available then....
I think you are right it's actually well worth trying an narrow 3 ohm ribbon foil in an older wide cage. The gap is wider but not vastly. I thought about it before but was put off as it's is very important to get those adhesive dots down to secure the ribbon and I didn't think it would be possible (discussed it with Satie). If it however viable to put those dots in (might be a bit tricky because of the extra space) then it might work just fine. I might try it soon in my MGIIIa tweeters cages, as like I say, that narrow ribbon sounds a lot better than the 2 ohm wide type.
I'd still bet on your Tympanis having the older discontinued 3 ohm wide ribbons in them though, as there are no resistors in the crossover. It's still a good tweeter, just not exactly replaceable if it breaks. Its a very thin material so I'd be careful moving them around.
Its a bit weird that Shelia is not aware of the older 3 ohm wide tweeter that is 60" long. It's likely a common issue that comes up with people break their MGIIIa ribbon as both the wide 3 ohm and 2 ohm ribbon was used in that tweeter, and if supplying a new 2 ohm ribbon to someone that had an early MGIIIa (with the 3 ohm ribbon) they need to be told to install 1 ohm resistor in the crossover. Info about that is written in the MGIIIa manual. The Tympani IVa obviously also used both types of ribbon too throughout its manufacture.
Magnepan is clear on when to install a resistor in series with the ribbon. They just do not mention why. My T-IVa are early ones, serial no. is 020838. They can have 3 ohm ribbons as there are no resistors.
http://www.forumbilder.se/DDN94/60-inch-ribon.jpgNext step will be to investigate the ribbons on the 3.6 to see if they really have different ribbon cages.
Edits: 08/13/14
Thanks for the link. That confirms the TIVa (like the MGIIIa) also used both types of wide ribbon during it's manufacture. Those instructions are for installing the current 2 ohm wide ribbon, - which is why they are saying you need to install a 1 ohm resistor in the earlier models (as it was not present already because the theearlier speaker used the 3 ohm wide ribbon).
Yes from your serial no (and lack of resistors) I'd almost bet my life on your TIVa ribbons being the discontinued wide, very thin 3 ohm versions. Why did you buy more ribbon foils?, just for backup/spares?
Let me know what you see when you have a look at the 3.6 ribbon vs the IVa ribbon. The channel the ribbon goes in is definitely a different size between the two (newer version is narrower). If you take them out I would also be interested two know whether the cage is the same width (the part that hides in the speaker) or different (I think the newer cage is likely a less wide, but I could be wrong). I could look in my two pairs but no time at the moment.
At some stage I might buy some of the new thinner 3 ohm ribbon foils and try installing in the older wide cage. It might be a bit tricky but still possible to glue the foil down with the necessary adhesive dots. If the magnet gap is also different between the two cages it might not quite work as intended though, - perhaps it might affect the output a bit?.
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