Mystery challenge

[Phosphene]

Here's a mystery one to ponder. All suggestions gratefully received. We've been called to a problem with the sound in a remote located, small, Medieval parish church. The system (not one we originally supplied and installed) has a few radio mics, lead mics and a CD player fed via a 1U rack mounted simple knob operated mixer to one of the Aux ins of a 100V line mixer amp, the slave of which also feeds an AVX loop amp. So far so mundane.Reportedly, at 28 minutes after the start of a service, (the clergy and church-wardens claim you can almost set your watch by it) a loud, and by lay descriptions fairly high pitched, noise appears on the system, and is only removed by fading down the mic in use at the time and then bringing it back up slowly. On the last occasion (last Sunday) they were working with a lectern mic when it happened, rather than a lavalier radio.On the occasion I made a site visit during one of their services to hear this phenomena sod's law worked and there was no problem. However it is reportedly almost invariably in evidence otherwise.

We don't think that this is feedback, though the description sounds like it, as it has, apparently, happened with different officiating clergy, and different mics and service running order locations. My thought that it might be some totally external, timed, transmission has been scotched by finding it has also happened on a wedding and a funeral, both of which happened outside the normal church service times.I have trained the 'operator' to fade down all mics not in use at any given moment, without this having any effect on the problem. The system has very high gain before feedback in any case, and when you do force it into feeback it is initially, at least, fairly low frequency.

I am having them run next Sunday's service with the loop amp turned off to eliminate loop feedback and await reports.

Comments anyone? Component breakdown after such an accurate time period seems odd given the vaguaries of ambient temerature.The customer is so frustrated by the situation that they are discussing having us replace the entire system. (Perhaps I just shoud.)

 

 

[Stuart91]

If they record their services, do you have the option of listening to the sound on a recording? There might be some clues if you can actually hear it yourself rather than going by their descriptions.

 

I'd be thinking about external sources to be honest. If it happens that regularly, it may be something else going on in the building that causes it. One church I dealt with had a very "noisy" central heating pump that would cause interference on their projection feed when it kicked into action.

[Alec]

Obviously hard to diagnose without witnessing.

You could provide them with a recorder, so that you can at least hear the nature of the "interference".

Even then, it may take hands-on time to diagnose.

Can it be reproduced out of working hours? If so, and if caused by a component of the system, then it could be easier to look at.

And, either way, best to work on eliminating each component, if possible. So try swapping out mixer/amp separately, as it *sounds* like more than one mic does it. Also try swapping out cables and disconnecting/turning off the loop system. Though hearing the sound may give you an instant clue as to what's happening.

 

Nothing more than standard fault finding techniques, really...

[TomHoward]

What are are the radio mics and what freq are they on? We had a problem recently where a lapel mic was still on 858MHz and 4G phones in the congregation caused interference, but only when the mic was a certain distance from the receiver.

[Owain]

Is there a church heating system which is set to come on before each service or other event such as a wedding or funeral? That might be turning off when the church is warm enough ... about half an hour into the start of every service. Or the tea urn in the kitchen which someone puts on at the start of each service, etc.

 

If it's electric heaters a heating contactor switching off might cause a spike on the supply, or direct RF interference, or a change in earth potential somewhere which might be sending the audio system into parasitic oscillation which needs the system gain turned down to kill it. If it's wet central heating, a pump turning on or off might cause a similar spike / earth potential etc.

 

Other random thought - after about half an hour into the service a (filament) lamp somewhere has got hot enough that metalwork has expanded and made a contact between 'lamp earth' and 'anything else earth'. I've heard filament striplamps 'singing' sometimes, and they're often used in lectern lights. Such things may not have been rewired at the same time as the main building wiring as they're 'heritage' or 'portable appliances' (on a 2 pin 5 amp socket in a lot of old churches!)

[MarkBarl]

These mysterious sounds always make me think induction loop. You mention a loop amp. Has this been investigated? Some kind of build up within the system that after 28 minutes of their normal use breaks out?

[timsabre]

These mysterious sounds always make me think induction loop. You mention a loop amp. Has this been investigated? Some kind of build up within the system that after 28 minutes of their normal use breaks out?

 

My money would be on this answer having been there a few times (we used to get ping ping ping at a high frequency as the loop amp compressor kept activating, but sounds the same sort of thing). Get them to unplug the loop amp for a couple of services. I bet if you megger the loop cable it'll be short to earth somewhere.