barryman Posted July 17, 2014 Share Posted July 17, 2014 [color=#141414][font=verdana, geneva, lucida,]We have a Laney 8300 PA amp that has twice cut out during a gig. This amp is rated for use from 8 ohm down to 4 ohms. The first time it cut out we subsequently found one of the speakers was open circuit so we thought it was that. We also had the amp looked at, a new thermal fan unit was put in so we thought that would be OK. However, having replaced the dodgy speaker, the amp has tripped out again. Something tells me it's a speaker problem rather than the amp itself. Interestingly, when it cuts out it is the thermal warning light that comes on, not the load protection light!![/font][/color] [color=#141414][font=verdana, geneva, lucida,]I have now been running it at home using a guitar and a mic with amp connected to [b]just one[/b] 8 ohms speaker and it has been 100% fine, no cutting out at all. So, questions are:[/font][/color] [color=#141414][font=verdana, geneva, lucida,]1. If the two PA speakers were mismatched, ie one was 8 ohms and one was 5.5 ohms, could that cause the amp to cut out??[/font][/color] [color=#141414][font=verdana, geneva, lucida,]2. Before buying a new set of speakers, is there any way of testing ie placing a "dummy" load on the amp to see if it cuts out again. How can I test it without forking out yet on new speakers?[/font][/color] [color=#141414][font=verdana, geneva, lucida,]3. Bearing in mind that we play small places so don't push the amp to excessive volumes, can an amp make speakers go open or short circuit, ie can they get blown by something other than just volume??[/font][/color] [color=#141414][font=verdana, geneva, lucida,]If anybody can offer any advice I'd really appreciate it.[/font][/color] Quote Link to comment Share on other sites More sharing options...
LawrenceH Posted July 17, 2014 Share Posted July 17, 2014 Not familiar with that model of Laney and can't see the pic well enough to tell, but a lot of those mixer/amps are mono, if that's the case here then a 8 ohm plus 5.5ohm would drop you below the minimum and overheating would be a real possibility. If the amp is stereo then ignore this irrelevant contribution! Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted July 17, 2014 Share Posted July 17, 2014 (edited) Barryman Without knowing the full spec. of the amp or the speakers you're using it's difficult to be precise. We can say the amp is running too hot because the thermal light comes on and the protection circuit cuts the amp off (hopefully) before any damage. It is common for amps being underloaded to run hot. Most PA speakers are either 4 or 8 ohms (nominal impedance). So if you're running two 4 ohm speakers in parallel (1 out of each socket) you'll be putting a 2 ohm load on the amp - i.e. underloading it, causing it to run hot. Same for a 4 and an 8 (about 2.7 ohms). So your logic is bang on there. IF that's the case. Even with two 8 ohm cabs you'll be running at the minimum 4 ohm load (which will run the amp hotter than a single 8). First thing would be to establish what the speaker impedance is. And if the total load in parallel is less than 4 ohms. Even if it is 4 ohms, the amp may be running a bit hotter than it should for a myriad of other reasons. You can get a basic multimeter from maplins and test the resistance across the two poles of the speaker to get a rough idea. Second thing, if possible, would be to run the speakers in series. You won't do as much damage to the amp running at 16ohms than you will at 2, it will just be quieter. As you don't run it at full whack anyway this might be better. Third thing to consider is this. If your speakers are mismatched for impedance they will not be getting equal power. e.g. if you have a 4ohm and an 8 ohm, the 4 ohm will be getting twice as much out of the amp as the 8. This is the reason it is best to have a matching pair of speakers for a PA. hope that helps... EDIT: There's a link to an appalling looking manual here, but it only goes to confirm what you have said. The mono amp has a total minimum load of 4 ohms (i.e. 2 x 8 ohm speakers) and the speaker sockets are indeed parallel. http://pdf.textfiles.com/manuals/STARINMANUALS/Hohner%20-%20Laney/TH6150,%20TH8300.pdf Edited July 17, 2014 by brensabre79 Quote Link to comment Share on other sites More sharing options...
Dad3353 Posted July 17, 2014 Share Posted July 17, 2014 When measuring speakers (either as chassis or as cabs...), it is not sufficient to measure the resistance with a multi-meter. The amp specs are concerned, not with [i]resistance[/i], but with [i]impedance[/i], which is not directly measurable with a meter. The passive resistance of 5.5 ohms would suggest, to me, that the speaker in question has, in fact, an impedance of 8 ohms. If the amp cuts out after a while, but then works again later, only to cut again, this would suggest an overheating problem. Speaker mis-match could well cause this, so be sure that the [i]total [/i]load on the amp, from whatever combination of cabs, does not fall below 4 ohms. 2 x 8 ohm cabs would be fine; an 8 ohm and a 4 ohm would not. Overheating can occur for other reasons, such as running the amp in a hot environment (some clubs get very sweaty..!), or having insufficient cool air available for the cooling fan to be efficient (too close to a wall..? Partially covered with a jacket or hat..?). Accumulated fluff and dust can equally affect the thermal dispersion; removing the casing (yes, unplugged of course, you fool..!) and blowing out the dust with an airline and soft brush is a good routine way of keeping amps fresh. Do not do this in the kitchen, and wear goggles, don't ask how I know to do that. There are other, more technical, reasons why an amp becomes intermittent; a qualified tech should be able to find the root cause and suggest a remedy; it is best to explain that the fault is, indeed, intermittent, and be patient, as sometimes such anomalies are difficult to provoke. Quote Link to comment Share on other sites More sharing options...
SubsonicSimpleton Posted July 17, 2014 Share Posted July 17, 2014 You could drop the speaker chassis out of the cabs and check the labelling on the drivers - some individuals will slap any old driver that is the same size in to replace a faulty unit and claim that they have made a good repair because the cab now emits sound. Quote Link to comment Share on other sites More sharing options...
JTUK Posted July 17, 2014 Share Posted July 17, 2014 I would take it out with another set of cabs if poss... I think it doesn't like the load of the other cabs and has gone into self protection mode as it is over-heating..?? Quote Link to comment Share on other sites More sharing options...
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