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In Reply to: RE: MGIIIa tweeter question posted by Davy on August 09, 2014 at 06:42:54
When you use a PLLXO you have a substantial insertion loss due to the use of resistors in series (for LP) and their need to be an order of magnitude bigger in each progressive filter stage so that you retain Q. So a buffer allows you to have a lower insertion loss by using a lower value resistor in series after the buffer regardless of the values used before the buffer.
A gain stage (preamp) would allow both compensation for insertion loss and buffering and level control. Latency is very small in preamps - a bit longer if they use transformers either at input, interstage or output.
In some tube pre models you can apply part of the filer in the interstage coupling (a smaller cap in the interstage can be used as a HP, a transformer with limited turns can serve as a low pass).
Follow Ups:
Thanks Satie. I understand what you mean now, replicating the Magnepan stock bass LP XO (MGIII/MGIIIA in my case) at line level in a PLLXO, and use the preamp and buffer to counteract insertion loss. That's a really great idea, I'm sure we mentioned that before and I'm not certain why I didn't try it when I have the necessary buffers and preamps knocking around. That certainly should match up with the Mid/Tweeter HP better than a standard 3rd order Butterworth LP XO. Perhaps I didn't do it before as I was more settled on single amp drive. Need to get another power amp before I can try this though.
Can you get away with just using a preamp with gain or do you really need the buffer as well?
I want to try the Anthem Receiver (with NAD 208) with my Maggies first anyway (in single amp drive) to see what sort of difference ARC makes. I'll post the FR graphs from ARC on here when I try that.
Unfortunately I've remembered that in both of my pairs of Maggies the adhesive stabilising beads that hold the ribbon to the cage at various points has failed at many points (completely gone in the MGIIIA) I will have to put new beads in to stabilise the tweeter first before running the test tones as otherwise I am sure the tweeter will shake/buzz/twist like crazy (actually does that more on bass notes!)
Re splitting the XO - you use a gain stage (preamp) or simplify to a buffer. You don't need both for a third order.
IIRC you were going to repair the dots some time ago. Did you not have a chance to do so? Did you get the material shipped from Magnepan?
Many thanks for the info on the 3rd order LP PLLXO. Sounds great must try that sometime. Would be great if I could get very close to the stock MGIII Bass LP alignment. Will have to brush up on crossover component calculations!No I never did the dots in either the 3.3r tweeter (which is only slight) or the IIIA tweeter (which has failed throughout). I got the chemicals for that repair when I ordered the ribbon repair kit for the Tympanis and I still have the stuff somewhere. Need to get around to it soon and certainly before running test tones through the speakers. I think it's more of a issue (in terms of the tweeter failing) on the 3.3rs as the small area where the dots have come away moves/vibrates a lot and could break quite easily. The whole tweeter of the IIIa has lost it's dots so the movement is shared over the whole area.
Edits: 08/10/14 08/10/14
I am rebuilding my Tympani IVa. Wanted to have the 3 ohm ribbon in them. Ordered a kit from Magnepan. What troubles me is that I am not sure I got a 3 ohm ribbon. I have not dared to unpack and measure it.
Sheila at Magnepan wrote:
“The ribbon you received is for the 60" tweeters which are in the MG3, MG3a, T4, T4a, and T3d. The other sizes are the 40" for the MG2.5 and MG2.6 or the 60.6" and that's for the MG3.3, MG3.5, MG3.6, MG3.7, MG20 and MG20.1
The 40" and 60" is wider and the 60.6" is narrow. They fit in their specific tweeter core channels ( they are also different sizes ) and are designed for specific models.
Confusing as some of you mention there exist a wide 3 ohm ribbon too.
Davy wrote:
“The best mod to do with the MGIIIas in my opinion is to cut the tweeter trough a little longer and install the newer 3 ohm narrow tweeter. This is my plan in the longer term. The output from the new tweeter is higher (than 2 ohm wide tweeter with 1 ohm resistor) with more sparkle. It also sounds significantly more refined and simply nicer than the older two tweeters (though the original fragile thin 3 ohm wide tweeter is not too far behind). The 2 ohm wide tweeter sounds a bit rough/course to my ears is is certainly the worse sounding of the 3 ribbon types in my opinion. The invention of the narrower tweeter was clever as it still uses the thicker material (so its still strong) but as it's narrower it twists less, produces less distortion and hence sounds better than the wider tweeters types.”
Satie wrote:
“My T IV ribbons are the "3" ohm - i.e. 2.75 ohm tweeter. They are capable of taking more punishment than you would expect and the test tones will do them no harm well into the 90s db range. More recently I put the XO at the symmetrical point at 5,8khz, but had to add a filter at about the 250hz area (a copy of the mid HP) to protect the ribbon from its new lower freq operating range. So now it is a higher Q filter. But it is entirely coherent with the mids.”
The best option is the narrow 3 ohm ribbon but can I shorten it to fit the T-IVa? Well, I can also take the tweeter from my MG 3.6…
Hi Roger,I assume you have been sent a ribbon repair kit. Magnepan would have definitely sent you the 2 Ohm wide ribbon foils for the TIVa. There are two types of wide ribbon that are 60" long. The first 3 ohm type was discontinued in the 1980s (not sure which year exactly but probably around 1988). It was used in the original MGIII, some MGIIIas and some TIVas. The original wide type was made from a v thin material and measured 3 ohms. It was replaced with the 2 ohm ribbon (made from thicker material) and 1 ohm resistors were installed in speakers to compensate for the lower impedance. This actually messes up the treble output (by lowering it) as compared to the original design (though some people find Maggies too bright so might have welcomed this). There is mention of this tweeter change in the MGIIIa manual and also they quote the MGIIIa serial no when the change was applied. I also suspect the TIVa used both types of tweeter throughout it's manufacture as AndyR bought a set of used IIVa mid/tweeter enclosures equipped with the 2 ohm tweeter and 1 ohm resistors, but I owned some IVas with the original 3 ohm type and no resistors. (I measured the tweeter, and that tweeter was obviously thinner and looked different to the 2 ohm tweeter)
Magnepan have not said why this change was made, but I suspect it was because the original wide 3 ohm 60 inch ribbon was too prone to breaking as it was so thin.
Fast forward to 1990 and the introduction of the 3.3R. This used a new design tweeter that was narrower than the old two types and slightly longer at 60.6", but the foil used was the same thicker material the 2 ohm tweeter, just narrower. This was clever as it increased the impedance back to 3 ohms but retained strength. It also improved sound quality (in my view). This same tweeter was/is used in the 3.5R, 3.6R and the current 3.7R.
I've heard all three tweeters in various Maggies and swapped some of them around for testing. In my opinion the newer narrow 3 ohm 60.6" tweeter sounds significantly better than the older two wide types, - it's more refined, delicate and has less distortion. The worst sounding one is the 2 ohm wide 60" type and the original 3 ohm wide 60" type is somewhere in the middle.
Roger, if your Tympani IVas don't have a 1 ohm resistor installed under the input plate on then its v likely they used the 3 ohm wide 60" tweeter. I had this exact situation and broke a tweeter in one channel . Magnepan could only supply me with the 2 ohm wide foil as a replacement and I did this repair in one speaker (and installed a 1 ohm resistor too), and found the treble quality was not as good but also the output of the treble was significantly less. To try to get the treble output back up to scratch I removed the resistor, and this was better, but alas the sound quality was still not as good. Removing the resistor is a reasonable fix for this lower output issue in my opinion though, if you amp can handle it. For triamping (but not biamping) you can still run the 2 ohm tweeter with the resistor as you have independent control of the output of each three sections.
It is possible to install the newer, better sounding narrow 60.6" tweeters into the older speakers designed for 60" tweeters (such as the MGIII, MGIIIa and TIVa) if you manually slightly lengthen the ribbon trough in the speaker. I really want to do this for my MGIIIas (which have the 2 ohm ribbon at present), but the problem is, if you are abroad Magnepan won't ship fully built new tweeters to you, and if they did the cost would be very high (likely well over $1000). Its a much cheaper option for people in the USA who can take advantage of the tweeter part exchange program (send back their older cages to get some money back). Roger you have this same problem as Magnepan won't ship a new 3 ohm narrow tweeter to you, but you could potentially (as you suggest) take the narrower 60.6" 3 ohm tweeters out of your 3.6Rs and install them in the TIVas, but you would have to manually lengthen the tweeter trough in the TIVas by 0.6" to achieve this. It would however leave you with another issue, - a set of 3.6Rs without any tweeters. You could not even easily put in the wide tweeters from the TIVas even if you wanted to as the older wide tweeter also has a wider cage too, and I don't think that will physically fit into post 3.3R tweeter troughs. Widening the cage throughout its length is obviously a much bigger and harder job than very slightly lengthening it.
Satie is running the newer, better sounding narrower 60.6" ribbon in has TIVs. I assumed he lengthened the tweeter troughs in his speakers to achieve this. Neolith also did the same modification to his MGIIIas and reckons the output of the 3 ohm tweeter is some 3.5dB higher than the 2 ohm tweeter (with 1 ohm resistor installed).Hope this rather long winded explanation helps. Have both your original TIVa blown?
Edits: 08/12/14 08/12/14 08/12/14 08/12/14 08/12/14 08/12/14 08/12/14
Hi Davy,
Thanks!
My T-IVa are still alive, at least when I last tried them last year. I am not sure whether they are 2 ohm or 3 ohm. There were no series resistors installed. Maybe they are 3 ohm? I have not measured the resistance. At least they looked like wide ones.
So, the dimenions of the ribbon cage has changed too? I made a quick measurement last year and as far as I can recall T-IVa and 3.6 had the same outer dimensions of the ribbon cage. Is that not so? Having all in storage right now.
My T-IVa will not use their original baffles. They will be used with the low bass panels against the side walls and the mid/tweeter sections will be on a decoupled baffles. Mids will use quasi ribbon technology (same foil as 20-series). Biamping via an active crossover, low pass 18 dB/octave at 250 Hz. The high pass will take place within the power amplifiers, 6 dB/octave at 370 Hz (not final). Passive crossover for mid/tweeter.
You didn't mention the slopes you intend to use on your internal mid/tweeter crossover but like I say, I would recommend 6dB slopes (inductor in midrange and cap on the tweeter), giving a 18 6 (6 6) crossover overall and linear phase in mid and treble. I would also recommended the external high pass being a PLLXO. That usually sounds great. The only slight trouble is that the active XO on the bass LP might have a very slight delay compared to an external HP PLLXO and the phase might not quite match up. It will however depend on the particular active XO you are using. This would not be a problem if you used the same active XO on the HP. I would try both active and PLLXO on the external HP and see how they sound.It is also possible to make a 18dB Low Pass PLLXO for the bass (like myself and Satie have recently been discussing) but there is a lot of insertion loss so you need to use a buffer stage or active preamp with gain within the crossover components to boost the gain. I intend to try this at some point, not had time yet.
Edits: 08/12/14 08/12/14
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.
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?.
Sorry I misread you post. You actually measured the tweeter cages. Perhaps you missed the newer tweeter being 0.6" longer as that's only slight, but as to the width did you measure the relevant width (which is width of the part of the cage that goes into the speaker hole?). The width of the outside part (that is visible on the back of the speaker) is the same for the different types of tweeter.
Yes, if there is no resistor installed I think its highly likely the wide version 60" long tweeters in your TVIas are the original 3 ohm version. Those tweeters still sound quite good IMO. Not quite as good as the new 60.6" long narrow 3 ohm tweeter but certainly useable and significantly better than the other wide 60" long 2 ohm tweeter. I would treat those wide 3 ohm tweeters with kids gloves, as if a foil breaks then your only choice is to replace both with the 2 ohm foil (must replace both with the same type to get consistent sound between channels of course). Its a shame Magnepan don't make and provide the older, thinner 3 ohm wide ribbon but I guess it was just too prone to snapping. If those are 3 ohm wide ribbons in the Tympanis I would hope they never break and probably not bother replacing them with the narrow 3 ohm ribbon from the 3.6R (unless you are curious/ a tweeker!). If you want ultimate sound though those the new narrow tweeter does sound a bit better no two ways about that.As for for the dimensions of the different cages, did you actually measure the width and length of the cages when you took them out?, or are you just saying they seemed/felt the same?. With the move to the narrower 3 ohm ribbon the length was definitely increased (as we know) by a slight 0.6", but I was also pretty sure the newer cage is narrower as well( this would mean (like I said before) it would not be at all straightforward to equip a post 3.3R Maggie with an older wide tweeter as the trough in the speaker would be too narrow (not sure why anyone would fit a wider tweeter in a newer speaker anyway (unless you were outside the USA and had no choice) as the wide ones sounds worse)).
Interesting what you say about your Tympani set up. - did not quite understand what you mean by "My T-IVa will not use their original baffles. They will be used with the low bass panels against the side walls and the mid/tweeter sections will be on a decoupled baffles"
Wiring the midrange panels with the QR foil is supposed to be a good idea. I have some and intended to do it in the Tympanis I had (and also my IIIas and 3.3Rs), but didn't get around to it. Am curious as to the improvement.
The 18 6 (6 6) crossover you are planning config sounds great in my opinion. It's what I have set up on my 3.3Rs and it sounds way better than stock IMO. I love the linear phase mid and treble compared to the mess I heard before. I also tried 6 6 6 6 but could not get it to work, - too much IMD and just sounded poor compared to 18 dB on the low pass bass, - not sure how magnepan sorted that problem for the 3.7s. Maybe using the foil on the mid and bass instead of wire really helps with that?
I would advise you to use an 6dB high pass PLLXO instead of an active HP on the external mid/tweeter high pass section. It will sound better (in terms of purity/transparency than any active XO and is very easy, - just a simple capacitor (whose value is determined by the power amps input impedance and the XO point you want). You can use a very high quality film cap in this position and it should sound great if you match it up to the Bass Low pass well.
Edits: 08/12/14
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