Stuart91 Posted March 9, 2017 Share Posted March 9, 2017 I've made a quick service call this evening to check on a customer who was reporting trouble with their induction loop system. Users were saying that the speech was barely audible and there was a loud buzz. My initial suspicion was that there might be some kind of ground loop between the loop amp and their mixer, or some similar and hopefully easy to fix. I've got an Ampetronic FSM calibrated field strength meter, which can also double as a listener. With the headphones in, there's a loud buzz very obviously noticeable. My guess is that it would be around the 100Hz region but it's hard to tell through tinny headphones. I turned off all the kit, and the buzz is still there, seemingly unaffected. So I'm certain that it is not the induction loop amp that is generating it. What is interesting is that when I go outside, the interference peaks in the street. So all the evidence points to some sort of external source. The problem has apparently only started within the last month or so. There's nothing obvious (like a high voltage power line overhead) and I'm genuinely baffled. I'm planning on giving Ampetronic's technical support folks a phone in the morning, but in the meantime does anyone have any suggestions as to what it could be? And what possible solution there might be for it? Link to comment Share on other sites More sharing options...
Jivemaster Posted March 9, 2017 Share Posted March 9, 2017 Did an electric car recharge point arrive at the same time? They would be kerb side in the road. Link to comment Share on other sites More sharing options...
Brian Posted March 9, 2017 Share Posted March 9, 2017 Neutral fault in the buildings wiring. Link to comment Share on other sites More sharing options...
adam2 Posted March 9, 2017 Share Posted March 9, 2017 Would it be possible to power the induction loop from a vehicle battery and a SINE WAVE inverter, for test purposes so as to eliminate any mains borne interference. Have you tried turning off all lighting in the vicinity, for test purposes, not just stage lighting but normal house lighting etc ? Lighting circuits are sometimes wired in such a way as to produce large external magnetic fields. If this is the case then a process of elimination will show which circuit or part of a circuit the cause. The cause is current related, not voltage related, so simply turning the lights out rather than isolating the lighting circuit will work. A defective power circuit can also produce a stray magnetic field, locate this either by turning all loads off, OR by adding a large load to each socket outlet in turn and observing if the buzz worsens. A 2.5KW fan heater is a useful test load. Link to comment Share on other sites More sharing options...
Stuart91 Posted March 9, 2017 Author Share Posted March 9, 2017 Did an electric car recharge point arrive at the same time? They would be kerb side in the road. That's an interesting thought. I've taken a wander up and down the street and there's no sign of anything. The customer says they're unaware of anything like either. Would it be possible to power the induction loop from a vehicle battery and a SINE WAVE inverter, for test purposes so as to eliminate any mains borne interference. I could probably do something along those lines when I'm back out. However, we're getting the same interference whether the loop amp is on or off, it seems to make no difference at all. Have you tried turning off all lighting in the vicinity, for test purposes, not just stage lighting but normal house lighting etc ? Lighting circuits are sometimes wired in such a way as to produce large external magnetic fields. If this is the case then a process of elimination will show which circuit or part of a circuit the cause. The cause is current related, not voltage related, so simply turning the lights out rather than isolating the lighting circuit will work. I turned off the house lighting in the room. (There's no stage lighting). It didn't seem to make any difference. If I wave my meter towards some of the iffy looking domest1c dimmers, I get some difference noise, but that goes away within a couple of feet. A defective power circuit can also produce a stray magnetic field, locate this either by turning all loads off, OR by adding a large load to each socket outlet in turn and observing if the buzz worsens. A 2.5KW fan heater is a useful test load. That makes a lot of sense. The thing that's puzzling me,though, is that the interference seems strongest in the middle of the road outside. It's actually peaking on the meter. Interestingly, it tails off gradually inside the venue, but drops off quite quickly on the other side of the road. If there's a faulty power circuit, could it be a mains feeder that's buried under the road? I'd expect that however many feet of earth around it would block it though? Link to comment Share on other sites More sharing options...
Dave m Posted March 9, 2017 Share Posted March 9, 2017 Is it practical to turn off the supply to the building? Link to comment Share on other sites More sharing options...
Stuart91 Posted March 9, 2017 Author Share Posted March 9, 2017 Is it practical to turn off the supply to the building? I'd need to ask, but I don't see it being an insurmountable problem. Link to comment Share on other sites More sharing options...
bruce Posted March 9, 2017 Share Posted March 9, 2017 It’s not by any chance built above a railway tunnel, or the glasgow subway? Link to comment Share on other sites More sharing options...
Brian Posted March 9, 2017 Share Posted March 9, 2017 ...the interference seems strongest in the middle of the road outside. Not that unusual. It could be that whatever is causing it is cancelling out inside the building due to relative phase differences and peaking outside the building. What happens if you walk around the outside of the building at the same distance? Link to comment Share on other sites More sharing options...
Stuart91 Posted March 9, 2017 Author Share Posted March 9, 2017 It’s not by any chance built above a railway tunnel, or the glasgow subway? About 60 miles out... No tunnels around (that I know of anyway). The nearest railway is about half a mile away. There is a rumour of an electrical substation somewhere nearby but haven't found it yet. Link to comment Share on other sites More sharing options...
Jivemaster Posted March 9, 2017 Share Posted March 9, 2017 ...The problem has apparently only started within the last month or so... What changed at the same time as the buzz started? New appliances? Link to comment Share on other sites More sharing options...
OddSocks Posted March 10, 2017 Share Posted March 10, 2017 What about a switch to LED street lighting, could this cause magnetic noise? Link to comment Share on other sites More sharing options...
paulears Posted March 10, 2017 Share Posted March 10, 2017 Does your monitor have a null in it's response, so turning it produces a signal drop (or increase). If so you could see if you can get a rough bearing from different points and triangulate for the origin? No idea if that even works on loop systems, but my battery receiver definitely reacts to orientation. Link to comment Share on other sites More sharing options...
pstewart Posted March 10, 2017 Share Posted March 10, 2017 On a perimeter loop system, if you're holding the receiver vertically, you will find a null directly over the loop cable, as the receiver is looking for the vertical magnetic field, and the field round the cable is all horizontal at that point. This doesn't work with phased array type systems as you have contributions from the loops either side. Philip Link to comment Share on other sites More sharing options...
Simon Lewis Posted March 10, 2017 Share Posted March 10, 2017 I turned off all the kit, and the buzz is still there, seemingly unaffected. So I'm certain that it is not the induction loop amp that is generating it. What is interesting is that when I go outside, the interference peaks in the street. If the buzz is there when you have turned off the loop amp, and it is apparent in the street, then it does seem to indicate a change or fault with external mains wiring. It is not uncommon for sections of road to have some significant levels of mains hum. These often far exceed the background level that's acceptable inside a building where a loop is or is to be installed. Link to comment Share on other sites More sharing options...
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