simontaylor Posted October 7, 2014 Share Posted October 7, 2014 Hi all, Would like to get your collective advice! I look after the AV for two lecture theatres - each theatre has Tannoy i12 Dual Concentric speakers powered from QSC PLX 1202 amplifiers. The speakers and amps have been in use for probably at least 10 years. Recently, within the last couple of months, we've had 4 of the tweeters blow, and we are struggling to understand why. The main use for the PA is for tie-mics, and the odd bit of audio from YouTube videos or videos embedded within presentations. The system doesn't really ever get driven very much. The amps are on 24h a day, though. Any idea why all the tweeters keep blowing? Or is it just because they are getting old? Thanks in advance! Simon Link to comment Share on other sites More sharing options...
MarkPAman Posted October 7, 2014 Share Posted October 7, 2014 The system doesn't really ever get driven very much. Are you sure? When I lost a several drivers in a classroom within a month or so, I tracked it down to a new cleaner who at 6:00 in the morning, plugged his phone or mp3 player into the system and turned it up loud enough to hear in all the surrounding rooms as he vacuumed! Link to comment Share on other sites More sharing options...
simontaylor Posted October 7, 2014 Author Share Posted October 7, 2014 Hmm, an interesting thought! I'll have to review the CCTV footage in the meantime! Link to comment Share on other sites More sharing options...
sleah Posted October 7, 2014 Share Posted October 7, 2014 Hmm, an interesting thought! I'll have to review the CCTV footage in the meantime! You can borrow my T-shirt ;) We resorted to putting CCTV in a classroom looking at the IT/AV kit to find out who was disconnecting the sound from the back of the PC, with the subsequent distruption to lessons when the teacher couldn't get sound to work Cleaners. Link to comment Share on other sites More sharing options...
pisquee Posted October 8, 2014 Share Posted October 8, 2014 The amps are on 24h a day, though. Is there a 13amp socket powering the amp (or other equipment - mixer?) which could be unplugged by above mentioned cleaners to plug in hoovers, floor polishers, causing a big bang? Link to comment Share on other sites More sharing options...
simontaylor Posted October 9, 2014 Author Share Posted October 9, 2014 The amps are on 24h a day, though. Is there a 13amp socket powering the amp (or other equipment - mixer?) which could be unplugged by above mentioned cleaners to plug in hoovers, floor polishers, causing a big bang? All the amps are in the (locked) control room, the cleaners don't go in there. We get notifications if the power is lost to the amp rack as well, which there have been none. Someone suggested it could be the crossover components values may have slipped, since they are are 10-15 years old. Is this a possibility? Meaning low frequencies get sent to the tweeters? Link to comment Share on other sites More sharing options...
Peter18 Posted October 9, 2014 Share Posted October 9, 2014 The amps are on 24h a day, though. Is there a 13amp socket powering the amp (or other equipment - mixer?) which could be unplugged by above mentioned cleaners to plug in hoovers, floor polishers, causing a big bang? All the amps are in the (locked) control room, the cleaners don't go in there. We get notifications if the power is lost to the amp rack as well, which there have been none. Someone suggested it could be the crossover components values may have slipped, since they are are 10-15 years old. Is this a possibility? Meaning low frequencies get sent to the tweeters? The capacitors may has degraded in the 10+ years but for two (or more?) different units to have both degraded significantly seems less likely. The lamp limiter / fuse has not been shorted out by some previous 'repair' has it? That might well not help the longevity of the tweeters!In view of the cost of replacement tweeters (!) in might be worth removing the crossover from the cabinet and testing the capacitors out of circuit with a capacitance meter. If one is not available then you could try feeding the tweeter output into something full range and listening to s see if anything below about 2.5kHz is getting through. Link to comment Share on other sites More sharing options...
McIvy Posted October 15, 2014 Share Posted October 15, 2014 Sorry, but quite late to the party... I recall that the Tannoy I - range speakers use liquid Ferrofluid in the sealed magnet gap to help cool HF coils. Just a thought, but these ten year old units left powered 24/7 might well have dried out, causing the coils to meltdown under normal operating conditions? Also, if any of the coils had been replaced without fluid, then failure is guaranteed! Made that mistake many moons ago. Lovely sound though when they are working!!! McIvy Link to comment Share on other sites More sharing options...
mvgear Posted October 15, 2014 Share Posted October 15, 2014 Hello Simon, I use to replace the HF on the i6's and i12's for fun years ago, they aren't great for PA because the peak to peak is so wide so if you get feedback on the radio mics it will kill them very easily. They are really best suited to background music so the mastering process of a CD or MP3 will get rid of the dangerous frequencies. Are you using any kind of processing before the QSC amp? like a 'Driverack' or another generic limiter system? having a limiter before the amp, as long at the tolerances are set correctly, should plug the gap and limit the HF from overcooking the diaphragms. One post did mention the crossover could deteriorate, but it's very unlikely in my opinion. It will most likely use polypropylene capacitors and these are very robust but tend not to deteriorate over time like aluminium capacitors. For what it's worth I would say the amp is very capable of delivering the more than enough power to overdrive the speakers it's more likely that the HF is being over-driven and could be misuse of a microphone feeding back... I've seen it many times in schools when the teachers wave the mics about and usually the speakers are on a back wall behind the mics which is not ideal, is this true of your setup?. If you went down the driverack setup then I'm sure someone on here has experience in setting it up, I've never set one up on an install as I'm tied to my workbench these days. Hope that helps. Link to comment Share on other sites More sharing options...
Solstace Posted October 16, 2014 Share Posted October 16, 2014 How many speakers, on how many amp channels? Is there any pattern to the failures? I'd perhaps be looking at the output of the amplifiers. Is each channel *really* (near) silent when nothing else is playing, or is there some (inaudible) HF output coming out that might be frying the tweeters? Wouldn't be the first time I've heard of amps going into oscillation at ultrasonic frequencies, or at least at frequencies beyond what the speaker drivers are capable of reproducing, and doing so at levels high enough to fry coils that way. Even if that's not the cause, it can't hurt after 10 years to gently have the lids off the amps while you're there and clean out the fur and dust that's likely collected inside them! ;) Link to comment Share on other sites More sharing options...
Stuart91 Posted October 16, 2014 Share Posted October 16, 2014 Wouldn't be the first time I've heard of amps going into oscillation at ultrasonic frequencies, or at least at frequencies beyond what the speaker drivers are capable of reproducing, and doing so at levels high enough to fry coils that way. When I had that happen to me on an installed system, it was because the customers were using a bodged Y-cable to run a stereo L&R down into a mono input. There was no audible sound, but the protection bulbs in the speakers started illuminating. Link to comment Share on other sites More sharing options...
simontaylor Posted October 21, 2014 Author Share Posted October 21, 2014 I recall that the Tannoy I - range speakers use liquid Ferrofluid in the sealed magnet gap to help cool HF coils. Just a thought, but these ten year old units left powered 24/7 might well have dried out, causing the coils to meltdown under normal operating conditions? Thanks for all the responses, some good stuff to check out. These speakers do indeed have the ferrofluid, you get a small syringe with a bit more when you buy a new tweeter (which we've done) so we are going to top up the rest of them! There's no real pattern to the failures, only that they've all started failing very recently. The only processing before the amps is a Nexia Biamp that does all the routing. I think there is some low and high pass filtering going on. Maybe some compression/limiting. And for those interested, here is one of the tweeter voice coils with a very obvious burnt bit! http://s7.postimg.org/4sw2ld7pn/tweeter.jpg Link to comment Share on other sites More sharing options...
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