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In Reply to: RE: Difference Between Magnepan Ribbon Tweeters: Mg 20.7 vs 3.6 posted by Norman M on April 11, 2015 at 22:15:32
With fuses. The fuses did not break. This is NOT the first time that I have had a broken tweeter and the fuse did not break. I have asked Magnepan why the fuse did not break and never got a good answer.
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I'm indeed sorry to learn about your ribbon loss. Instances of the ribbon having saved the fuse have been mentioned here many times. In all such instances in which I switched into a source set to play at a too high level, the fuses always blew before ribbon damage occurred. And this happened innumerable times in the 20+ years I've had my Tympani IV-As. So my fuses must have indeed saved both my ribbons and my ass. Magnepan must have heard about instances in which the ribbons died before the fuse many, many times.
I suppose Magnepan didn't have a "good answer" to your question because in reality it amounts to a design flaw on their part, (especially because 'Davey' in this thread describes a way to avert this from happening). I'm not a techie, so I don't know how practical Davey's solution might be. That is to say, can it be built into their ribbon speakers, or is it an after market solution which can only be done on a user's part?
I wouldn't blame Magnepan too much for lack of an explanation. Fuses are just not an optimum solution for this application.
This isn't the first time the ribbon protected the fuse and it won't be the last. :)
The right set of transient conditions can easily create a situation where the fuse element can not heat/react fast enough.
There are better solutions for this issue nowadays....resettable poly-switch being one....but Magnepan seems slow to adopt something else.
Dave.
We have discussed this before and I think it is "safe" to conclude that a fuse will not protect a tweeter and a polyswitch is a better solution. However, I am wondering if the tweeter blew not because of high current but rather because of extreme excursion. In other words, did the ribbon rip apart or did it melt. I suspect it ripped and, if so, then a polyswitch may not help either. Perhaps a close look at the tweeter will reveal signs of melting or signs of a rip.
Since most tweeters fail because of clipping, the best approach IMO is go to a line-level XO (biamp) since this isolates the tweeter from the clipping amp (much more likely to be the bass amp than the tweeter amp). Again however if the tweeter ripped because of large excursion, then this approach will not work either.
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Whether a ribbon melted or ripped (from excursion) wouldn't a lot of 'juice' have been sent its way before either took place? It doesn't appear that too many listeners here might ruin a ribbon as a consequence of clipping, at least not during the course of normal music listening. It's unfortunate that fuses can't break before either 'melting' or extreme excursion happens. While it might not be terribly expensive to get a new ribbon, it wouldn't be easy for me to install one. I have in fact watched my dealer's workers doing that (while in my home) and to top it off on one occasion a soldering joint eventually failed and they returned and did it a second time. If they didn't get it right the first time around, my chances are next to zero.
Actually clipping is probably the main reason why tweeters (cone or ribbon) go bad. Clipping causes a lot of high frequency noise (current) that causes the tweeter to fry. Ideally the fuse should limit the current but in reality that does not happen. You might want to read the article by Rod Elliott (http://sound.westhost.com) on why tweeters blow. My point about excursion of the ribbon is that it may not be a thermal event that afflicted the OP and in that case a thermal protection device, either a fuse or polyswitch, will not help.
As far as replacing the tweeter, it really isn't that hard but good soldering techniques are needed. Try practising your skills before you tackle the ribbon. It really won't take that long to get adept. Remember to use an iron that is hot enough and make sure the surfaces are clean. Also create a mechanical connection before making the electrical connection.
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Yeah, I think it's possible it was excursion related. There are usually capacitors in the signal path, but it's still possible to create decent currents at low frequencies with the odd transient from the power amp. In this case he's noting a sudden resumption of the signal at high volume caused the issue. :)
I'm not meaning to suggest that polyswitches will guarantee tweeter longevity in all cases, but I think they will perform better, relatively speaking, than fuses in both the protection aspect and the "sound" aspect. Fuses are a fairly non-linear device and not what I would want in series with my drivers.
However, these ribbon drivers are fragile and power amps can store/deliver a lot of energy. There is some inherent risk no matter what type of safety approach is taken. :)
As has been noted previously, it's not that expensive to replace one of these tweeters and some users see an occasional failure as the price of doing business.
Dave.
A resettable poly-switch with a markedly faster reaction time will need an adjusted current rating. I experimented with pico fuses, which open very fast, and using the stock fuse current rating resulted in nuisance opens once every month or two. As I have no concern over going fuseless, I upped the rating an ampere and didn't have one blow over a three month period (not conclusive, I know). By that time, I was tired of fiddling around and stripped the connector plates down to just the binding posts.
I guess I'm about to have plenty of time to fiddle with them now. Although, I'm currently very busy with other, more pressing projects.
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Yes, sorry for not elaborating. You would probably not select an equivalent current rating if replacing a fuse with a polyswitch or different type of protection device. An alternative would be a much faster reacting device and higher "rating" would be appropriate.
Dave.
This has happened to me more than once also. If there is an alternative as Davey mentioned with a poly switch which would eliminate this problem I am all ears.
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