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In Reply to: RE: Difference Between Magnepan Ribbon Tweeters: Mg 20.7 vs 3.6 posted by Davey on April 12, 2015 at 06:39:58
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.
Follow Ups:
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.
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.
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