Induction Loop problem?

I'm working in a school this after developing SE courses and doing some staff training on ProTools Mpowered.

Their setup is pretty typical of schools in the area with LE, iMac, separate 'control room' between two classrooms, junction boxes in each etc, rode condenser mics, the usual shure dynamics.

They're using an Maudio project mix but have a problem.  When hooked up through the junction to the Project Mix pres, with gains set to 0, all tracks will clip, particularly condenser mics on loudish sources.  Any time we record kit all the inputs clip with zero gain applied.

There is an induction loop in the whole school which is the only thing that I can think of that may be causing this.  They have asked that it be turned off locally in the music department but have met with resistance.

Any ideas?  Would an induction loop amplifiy mic signals in this way?

Would an induction loop amplifiy mic signals in this way?

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I doubt it... I don't know the M-Box well enough but chances are that it simply doesn't offer enough max. input level for hot condenser mics.

Any ideas?

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Get a bunch of those XLR barrel type pads.

Samuel

Hi Samuel

Thanks for your reply. 

The condensers aren't particularly hot (Rode NT5 and NT1a)

I'll clarify if I may.  With ANY mic there is too much gain present with the gain on the channel turned completely off. 

There is definitely some odd gain boost happening somewhere.

The pad idea is a good one, I'll suggest it. 

Jim50hertz said:

The pad idea is a good one, I'll suggest it. 

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It's indeed an excellent idea for troubleshooting, but completely unacceptable as a permanent solution.

What exactly do you mean by "with gains set to 0"? The microphone input gain is set with the potentiometers at the top of the unit.

I work in a venue that has a induction loop and I do not experience any boosts or problems what so ever.

The only issue I've ever had is when feeding electric guitar and bass to the loop. This would cause feedback at extreme gain because of the magnetic pickups on the guitars.

Other than that, nothing at all.

I'll second what Abbey Road is saying and ask what do you mean by "0". If you are referring to the labeling on the front panel then the label usually means nothing at all.

If by "0" you mean that the pot set to the complete counter clockwise position, then you got a problem...

The best way to set your input gain is by using the meters in your DAW.

JD

The M Audio stuff is weird because there is a software control panel that adjusts things and you can sometimes lose your settings and things get all wacky.

Open the control panel and make sure the Input Bus isn't cranked.

Matthew Jacobs said:

If by "0" you mean that the pot set to the complete counter clockwise position, then you got a problem...
JD

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This is precisely what I mean.  There is a problem.

mushy said:

The M Audio stuff is weird because there is a software control panel that adjusts things and you can sometimes lose your settings and things get all wacky.

Open the control panel and make sure the Input Bus isn't cranked.

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I used a firewire 1814 for years without any weirdness whatever so I know the Maudio stuff well.  Can't figure this one at all.

They're going to order a bunch of 20db attenuators as a work around till we get to the root of the problem.

Thanks for all your responses, I think I can rule out the loop at least.  I'll post back when/if I can find out what 's going on...

As far as I recall these units don't feature any calibrated gain setting so "0" simply refers to minimum gain. Note that for mic preamps there's nothing like the -inf position of faders; even at minimum gain there will some signal get through, and for many preamps there will still be more than unity gain.

Samuel

I haven't heard of an induction loop causing this, but I'm constantly amazed at the number of mic preamps on the market that can't take a close mic on a drum kit without clipping even at minimum gain. IMO this renders them almost useless for recording bands, but there are a lot of them out there, some with +20dB minimum gain.

A possible workaround (for non-phantom powered mics) if you have balanced inputs on your outboard gear is just to bypass the pre-amp entirely and plug straight into the line input of the next device in the chain. If there's a -10/+4dBu switch, you even have a choice of level. The other solution is to buy a lot of inline mic pads (eg. the Shure ones).

Is there a mic-splitter involved? Any other transformers or inductors in the signal path.

I once commissioned a system where the building power entered the building and ran underneath the studio and control room. Large building lot's of current. There were some series inductors in the pre-amps for RFI suppression and they saturated and caused significant distortion in the mic signals.

Solution was to move the preamps to a different location.

Best,
j