Chris Adam Posted February 25, 2009 Share Posted February 25, 2009 I've just replaced two 15" drivers in OHM TRS115 speakers that I found in a cupboard in the depths of the venue. Looks like the same old story, run too hot, for too long with no protection.The drivers were definately gone. Crunchy coils are never a good sign.However, the crossover looks in good condition, apart from a bit of melting on one of the coils. (for anyone who doesn't know this model, it's got three coils and fuses)So, the new drivers are in, the fuses look fine, and the speaker sounds like it's doing it's thing across the full range with a quick 1-2-1-2 etc. However, with the melting around the coil on the crossover, will there be future issues if I put them back into use. I actually quite like this box and it could come in handy from time to time.I'll be running the cabs with appropriate amps and limters, but don't want to run the risk of putting a speaker out that will fail after 30 mins. cheers in advance. Link to comment Share on other sites More sharing options...
johndenim Posted February 25, 2009 Share Posted February 25, 2009 If you have gone to the trouble of new drivers, I'd protect them with new crossovers. I take it these boxes are two way?Thomann do one here. Link to comment Share on other sites More sharing options...
Simon Lewis Posted February 25, 2009 Share Posted February 25, 2009 Did you replace the failed drivers with OHM drivers, or consider having the originals reconed? If not, how do you know that the new drivers are correct for the box volume and tuning? The crossover has built in HF protection and phase correction. If there has been thermal damage, it's worth having it checked (and repaired if necessary). A generic replacement from another source isn't a good idea. Of course, you can put any 15" driver in the box, but it won't perform as OHM intended... OHM have good aftersales support. Why not call them? Simon Link to comment Share on other sites More sharing options...
KevinE Posted February 25, 2009 Share Posted February 25, 2009 When you say melting coils, its unlikely the wires themselves have melted much as the wire's very thick. Are you sure it isnt just the plastic bobbin material thats gone a bit gloopy when it got hot? This happens alot and should be ok so long as it still offers mechanical support for the winding. Link to comment Share on other sites More sharing options...
Chris Adam Posted February 25, 2009 Author Share Posted February 25, 2009 Yeah it's new OHM drivers. Chris has been very helpful, but I only spotted the thermal damage tonight when I was replacing the second driver.It's not much, and everything else on the board looks in tip top condition.I really don't want to spend another £70.00 per box on what is essentially a spare set of speakers. I agree though that I'll not be putting thomman crossovers near someone elses cabinets. Not a good idea at all.Now, I'm no loudspeaker expert, so what would I be testing for on the coils? Impedence I'd guess, but have been wrong before. As mentioned in my 1st post, the box sounds like new with a bit of vocals and a quick play of some music so I'm hoping they'll be cool. cheers again. EDIT: sorry, yes should have been more clear that it's just the plastic that has melted. The wire still looks ok. It was more the insulation of the wire I was concerned about it it's shorted inbetween the end point of the coil. (can that even happen?) Link to comment Share on other sites More sharing options...
Simon Lewis Posted February 25, 2009 Share Posted February 25, 2009 Yeah it's new OHM drivers.Good!EDIT: sorry, yes should have been more clear that it's just the plastic that has melted. The wire still looks ok. It was more the insulation of the wire I was concerned about it it's shorted inbetween the end point of the coil. (can that even happen?) Yes, the insulation can burn, thus leading to a short circuit. Either get OHM to check them, or ask them what the DC resistance of the coil should be, unsolder one end and measure it. Link to comment Share on other sites More sharing options...
johndenim Posted February 25, 2009 Share Posted February 25, 2009 I agree though that I'll be putting thomman crossovers near someone elses cabinets. Not a good idea at all. Why not?In my book a crossover is a crossover if the freqs are (roughly ;) ) the same? Link to comment Share on other sites More sharing options...
Chris Adam Posted February 25, 2009 Author Share Posted February 25, 2009 I'd rather the loudspeaker performed exactly how the manufacurer designed it to, not nearly. As simon said (there's a song title there somewhere), any crossover, or any driver would work - but just not exactly how it's been designed. Link to comment Share on other sites More sharing options...
bruce Posted February 25, 2009 Share Posted February 25, 2009 In my book a crossover is a crossover if the freqs are (roughly ;) ) the same? That doesn't sound like a very good book.... there's much more to crossovers than just frequencies... power handling, linearity (or lack thereof), steepness of cutoff curves etc etc... Link to comment Share on other sites More sharing options...
Rob_Beech Posted February 26, 2009 Share Posted February 26, 2009 Agreed. I am sure that for most applications the thomann crossovers work fine but there is alot more to it. To get them to act in a linear fashion, with true slopes and cutoff points, to get them to effect and colour the sound as little as possible, and to get them to handle lots of power without altering the sound takes a very good design. There is a reason an Ohm one will be £70 and a thomann one less than half that. There's also a reason a D&B one is several hundred pounds. 2.5khz is probably too high aswell for a 15. not to mention 600 watts MAX maybe not being enough. Link to comment Share on other sites More sharing options...
aidso Posted February 26, 2009 Share Posted February 26, 2009 The coils that are in the crossover are just impedance matching for the two drivers. They are simply copper wire with enamel coating. When you say melted plastic. Is it black? What plastic? You will find most of the board is covered in something like epoxy resin to help prevent fall damage. Easy way to check these is not to take them off the board but simply meter across the input and see what the impedance of the box is. Probably 8ohm. if it is 7 - 9 ohm then its fine if it’s less or more then you have a fault. As for reliability as long as you don’t thrash the life out of it, it should be fine. Link to comment Share on other sites More sharing options...
d_korman Posted February 26, 2009 Share Posted February 26, 2009 The coils that are in the crossover are just impedance matching for the two drivers. They are simply copper wire with enamel coating. When you say melted plastic. Is it black? What plastic? You will find most of the board is covered in something like epoxy resin to help prevent fall damage. Easy way to check these is not to take them off the board but simply meter across the input and see what the impedance of the box is. Probably 8ohm. if it is 7 - 9 ohm then its fine if it’s less or more then you have a fault. As for reliability as long as you don’t thrash the life out of it, it should be fine. You can't measure the impedance of a speaker with a mutimeter - all that gives you is the DC resistance which is not the same thing at all. You would expect the DC resistance to be significantly lower than the nominal impedance but how much depends on the design. A shorted turn in a crossover inductor will make little difference to the DC resistance but could make a huge difference to how the speaker performs. Link to comment Share on other sites More sharing options...
KevinE Posted February 26, 2009 Share Posted February 26, 2009 If you're left in any doubt, you could always post a snapshot of the burnt offering on here! High power crossover coils will measure nearly short anyway so I wouldnt bother metering one. Link to comment Share on other sites More sharing options...
Chris Adam Posted February 27, 2009 Author Share Posted February 27, 2009 cheers everyone. I'll pop a link to a pic next time I'm in the office.I'd think that it will be fine going with people's recommendations. On this particular crossover the melted (white) plastic has minimal damage. I knew that there was no real simple way of working out if it was ok just with a multimeter which is why I posted to see if anyone had any experience of this. I'm sure people's speakers get humped all the time to varying degrees. It was more a second opinion I needed. much appreciated. Link to comment Share on other sites More sharing options...
Ken Hughes Posted February 27, 2009 Share Posted February 27, 2009 In answer to the original question the answer is "possibly".If a coil has become hot enough to melt there could be at least one or more shorted turn however, it will have become weaker due to its becoming very hot. Random, off the shelf passive crossovers don't work - properly.They make a noise but so do thunderstorms and they're not particularly musical either. A passive crossover is as important to a decent box as the bass, midrange or HF drivers.Using something off the shelf when there is a correct crossover available is just plain wrong. I would suggest removing the crossover from the box, send it off to OHM with an explanation of the possible fault to see what they say.They may, like ourselves, wind their own inductors in which case it would be a fairly cheap repair but you do need to replace like with like. Link to comment Share on other sites More sharing options...
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