Mac Mini picking up induction loop

[Stuart91]

Earlier this week we installed a temporary induction loop in a nearby venue. It's a simple perimeter loop driven by the venerable Ampetronic ILD9.

 

Our guys tested it when it was put in and all seemed to be working fine. It is fed from an aux send from the in-house mixing desk. The client has apparently brought in a Mac Mini to use for playback, which needs to be present on the loop signal. What seems to be happening is a classic feedback cycle, as soon as the signal from the Mac is brought up on the aux feed, a high pitched whining starts.

 

Unfortunately I haven't been able to check it out in person, just going on what I was told over the phone. I don't have any exact information on the model of Mac Mini or how things are laid out. The Mac is using the internal headphone output, and the feedback still occurs when the output is routed into a DI box rather than direct into the desk. There is a chance that they could have a long minijack cable sitting coiled up before the DI, but I can't be sure. Apparently it isn't possible to move the Mac outwith the perimeter of the loop cable.

 

I can think of dozens of venues we've worked in where there have been induction loops and computers in operation at the same time, and this is the first time any problems have been flagged up. I'm curious if there's a known issue or vulnerability here, or if anyone has had similar problems in the past?

[dmxlights]

Hi Stuart,

It sounds to me like the the DI box is picking up the loop system and not the mac. I would almost put on a bet that the DI box is a passive box and its picking up the loop system.

I do wear a hearing aid and have done a lot of looking into loop systems and I also use the loop setting on my hearing aid for checking power supplies of laptops and other devices.

The loop system creates a magnetic field and the passve DI box has a tranformer in it which will pick it up. The old power supplies with a wire wound transformer in it also had a bad effect on passive DI boxes.

 

The only anwer is a active DI box or direct in to the desk. It also can be that the loop systems output as in the impedance is set to high for the lenght of the cable.

 

[Stuart91]

It sounds to me like the the DI box is picking up the loop system and not the mac. I would almost put on a bet that the DI box is a passive box and its picking up the loop system.

 

Thanks, Tony - it never occurred to me to suspect the DI. I think it was a passive Radial model that they were using, but not 100% sure. If so, it'll be a good quality transformer but that won't necessarily avoid the problem.

 

99% of the DI boxes in our hire stock are active models, so it's not a problem we've come across before.

 

Apparently they were getting the feedback before the DI was added, but that probably involved a long unbalanced cable run.

 

It also can be that the loop systems output as in the impedance is set to high for the lenght of the cable.

 

Definitely a possibility. We set the loop amp to a sensible level when it was put in, but it may have been cranked since. Plus there's no way of knowing how hot the send from the desk is.

[dmxlights]

I wounder did they plug a jack lead in to the mac line and in to the desk as well which could also casue this. I did this before when I was converting a tape to cd and forgot about the audio out of the laptop to the desk.

 

Another thing to look out for is a jack lead raped aroung the leads of the loops output could posable to casue this.

 

It would be worth while trying a active DI with the mac and the loop system

 

[Charlotte_R]

You could look at using an external sound card too. This would mean that the outputs come out of the mac as a balanced signal without the need of a DI box.

[Dave m]

We had issues once with a pa and loop where an outside organisation connected and used a different earth.Are you all on the same mains feed?As said, an external sound card might help

[Simon Lewis]

As far as I'm aware, most decent DI boxes have a metal can which effectively screens against stray magnetic fields. In this particular case I would try and determine whether the pickup is from the laptop to desk lead, an earth loop problem such as Davem mentions or whether the ILD9 itself is exhibiting a fault. They're a great amp, but can fail, and I have had a few that have shown this symptom. Ampetronic have a very reasonable fixed price repair for their amps.

 

It can be worth tryg a USB pre to see if this solves the issue, but I would also try plugging the loop amp in at the same point as the desk and Mac, if it isn't already powered from there.

It can also be worth trying lifting the signal earth to the ILD9.

 

 

Finally, you can call the Ampetronic guys. They stand by their products, support them and will give good help over the phone...

[Stuart91]

One of us was back on site today, to remove the equipment. It turns out that the computer, and various other bits of audio kit, had been set up directly on top of the loop wire (on a table straddling it). So we're pretty confident that this explains the problems.

 

Thanks for the suggestions nonetheless, it's all useful stuff to keep in mind for the next time something like this comes up.

[sandall]

Reminds me of the tremendous guitar feedback I once got at a gig in a recently-refurbished venue. It turned out that the feeder to the loop in the hall ran just under the stage floor, & the lead guitarist had neatly lined up his pedals directly on top of it :(

[Stuart91]

Similarly, I once found myself in a school assembly hall, which had a standalone loop system, clearly fitted as a box-ticking exercise. It was fed by a microphone built into a wall plate, which was in a position where it would only pick up ambient noise during normal activities.

 

Unfortunately, one of the PA stacks was directly in front of this plate, and the loop amp appeared to be left on 24/7. It produced some spectacular problems before we discovered it and turned it off.

[Simon Lewis]

Reminds me of the tremendous guitar feedback I once got at a gig in a recently-refurbished venue. It turned out that the feeder to the loop in the hall ran just under the stage floor, & the lead guitarist had neatly lined up his pedals directly on top of it :(

 

 

That's a poor install, then. The feeder should be twisted to stop radiation!

 

 

 

Similarly, I once found myself in a school assembly hall, which had a standalone loop system, clearly fitted as a box-ticking exercise. It was fed by a microphone built into a wall plate, which was in a position where it would only pick up ambient noise during normal activities.

 

Invariably, these wretched installs have the mic turned up so high, that a flea breaking wind in Nebraska would send it full scale. I'm guessing the installer thinks the customer will appreciate the flashing red lights....