Looking for a good lavalier mic that is impervious to GSM / cell phone interference

[Paul R Johnson]

The snag with interference of this nature is that there are two main ways it gets into the system to cause problems. The first is that the mic cable acts as an aerial - the screen feeding the interference to the mixer/pre-amp input where it causes problems. Secondly, it gets to the inner conductor(s). Radio microphones don't need mics with balanced connections, as they power the mic by applying the polarising voltage superimposed on the audio line, or my putting the voltage on a separate core. In most cases, even when a lav mic has an XLR connector, it's often one of those extra long ones, with the pre-amp in the plug - so although it has balanced output, the input may well be unbalanced - and therefore more susceptible to the stray RF currents. Solving these issues is always difficult because the RF noise is unpredictable, but my own experience is that some mics, usually the ones commonly used on wireless systems that have unbalanced cabling, are the worst. If you need the RF immunity then you'll have to set up some tests to establish which items you have that are contributing to the problem. Some input devices seem more prone to the problem. An AKG I use from time to time is totally fine when plugged into a Yamaha mixer, but the same mic plugged into a JVC camera is prone to the problem. I'm certain the issue is cable pickup, but a mic input with poorer RF rejection characteristics. Lots of trial and error to find combinations that work better. I'm not sure you'll ever cure it. I had the problem a few days ago when I left my own phone on my macbook, that had the headphone output connected to the usually good Yamaha mixer. I guess the phone on the aluminium chassis of the macbook allowed the RF to travel straight down the cable into the mixer - and the nasty noises emerged! I have an old transformer isolator and using this, which has an output with no direct wired connection to the input socket, does work to be a good isolation device. Difficult to use one of these on every input, though. probably a step too far, but could you use ordinary microphones on desk stands? Properly balanced and more reliable?

[Greg Miller]

As several people have stated, this problem most likely comes from the use of an unbalanced mic and wiring at some point in the chain, as is the case with the small electret lavs. For all we know, in some cases the interference might actually get into the high impedance FET which is part of the electret mic capsule itself.

Seems to me that the most trouble-free solution would be to use a balanced dynamic mic, rather than an electret. The mic element itself is low impedance, it's purely passive (no onboard FET), and you feed a balanced signal down a twisted pair (with a shield, of course) directly to the recorder. If the recorder is properly constructed, so as to be RF-immune itself, the problem should disappear. (A transformer-coupled input would be better than an active balanced input, IMHO.)

Of course a dynamic lav is a bit larger than the smallest electrets. But you're not micing a broadway musical, and you don't need to hide the mic in someone's hairline. A visible mic is perfectly OK for depositions.

If I were in your shoes, I'd try something like this: Shure SM11CN Omnidirectional Dynamic Lavalier Microphone | Full Compass

[Greg Bellotte]

Originally Posted by Luke Arndt

XLR is always balanced... right?

not always...many devices that use XLR connectivity are actually unbalanced circuits. Real balanced circuits cost more so it's usually the cheaper devices that are unbalanced. hard to know for sure without looking at circuit diagrams or plain old testing as manufacturers aren't usually given to divulge the inner workings anymore. if you can read a dead short between pins 1 and 3 of an input it is most likely UNbalanced.

I will say that we use senn mke-2 (def unbalanced mic) and sony ecm-77or88 (not sure about balanced) and rarely have issues, but again the source is only half of the equation. I do know the desks i use (calrec, yamaha, SSL, Neve) all have true balanced inputs (active), and many have transformer coupling to boot.

Easy way for you to test, make a call to your own mobile. while it is in call hold it next to your mic. During the call is about the most active rf signal the handset will produce and it continues as long as the call is in progress. If there is going to be a problem with interference this will decidedly reveal it.

[John Willett]

Originally Posted by Greg Bellotte

I will say that we use senn mke-2 (def unbalanced mic) and sony ecm-77or88 (not sure about balanced) and rarely have issues, but again the source is only half of the equation. I do know the desks i use (calrec, yamaha, SSL, Neve) all have true balanced inputs (active), and many have transformer coupling to boot.

All mini tie mics are unbalanced down the cable - they are balanced at the XLR connector.

[Greg Miller]

Originally Posted by John Willett

All mini tie mics are unbalanced down the cable - they are balanced at the XLR connector.

Not quite.

All electret mini tie mics are unbalanced down the cable.

The Shure which I referenced above (Post #17) is a dynamic, and according to the literature and wiring diagram it is a conventional balanced dynamic mic. Nothing unbalanced about it.

Of course it's not quite as "mini" as some of the electrets but it's still small enough to be used with a tie clip.

[Luke Arndt]

I e-mailed Audio Technica to ask about any possible way to retrofit the AT803B to resist the interference.

The response I got was a suggestion to use an inline RF filter, such as this Shure A15RF: Shure A15RF RF Filter, Problem Solver | Full Compass

I wonder if that would do the trick! Seems very intriguing. Anybody have any experience with one of these?

[Richard Crowley]

Unlikely to work. Reason: The RF is getting in to the INPUT end of the mic and filtering the OUTPUT of the microphone won't get rid of the interference after it is embedded into the audio.

My policy is: If the attorneys aren't concerned enough about the problem to order everyone to turn off their cell phones, then it is NOT YOUR PROBLEM. Leave it up to the customer how serious they are.

It MAY be possible to "RF-harden" that microphone, although I would bet against it. But the A-T people know that if you have to ask, you aren't qualified to perform the modification. Not to mention that there is almost nothing you can do that won't instantly void the warranty and carry a good likely hood of destroying the microphone in the process.

The one thing that has a (very small) chance of success would be to use an external ferrite bead filter around the mic cable. Unfortunately, these are typically much larger than the microphone themselves and would render the microphone horribly clunky, ugly and difficult to use. And they still wouldn't work if the RF path is directly into the microphone head which is the most likely problem.

As others have already suggested. If you really MUST operate in an RF-hostile environment, use old-fashioned fully-balanced dynamic microphones with good shielding. And maybe some of those Shure RF filters as well.

[Luke Arndt]

Do they have balanced dynamic microphones that are lavaliers? Or is there some reason not to be using them?

This video is really interesting:

Cell Phone GSM Interference - YouTube

I wonder if soldering some of those X2Y capacitors onto the mic's circuit board itself would help.

Thanks for all the feedback, guys.

[Richard Crowley]

Originally Posted by Luke Arndt

Do they have balanced dynamic microphones that are lavaliers?

Yes, this was already suggested in previous responses. Did you just blow it off?

Or is there some reason not to be using them?

Dynamic mics are larger and not as sensitive as electret. They are also a dying breed.

This video is really interesting: Cell Phone GSM Interference - YouTube

Yes, it would be interesting if you were Audio-Technica. But there isn't anything you can do retroactively as an end-user.

I wonder if soldering some of those X2Y capacitors onto the mic's circuit board itself would help.

Yes and No.

Yes: If you could get them into the right spot, they would protect the very sensitive FET gate against RFI. But I guarantee that there isn't enough room on the board to put ANY of those components.

No: Adding capacitance across the input will attenuate the very tiny audio signal and make the problem worse because it will tend to DECREASE the signal-to-noise ratio (SNR). An electret microphone is a very special kind of circuit that is very difficult to protect against nearby GSM pulse-mode RFI.

Repeating: If you want to use electret microphones you MUST turn off the phones in the room!
If you can't do that then you have a choice of using an old-school balanced dynamic mic, or else just living with the interference. You aren't the first to encounter this problem. You aren't even the 1000th. You seem to continue to believe there is some magic solution. Rest assured that if there were a magic solution we would have all heard about it by now.

[Paul R Johnson]

I read the bit about dying breed and thought 'rubbish', but it's quite right - I had an old AKG which I quite liked, but a bit of Googling showed that in reality - we have a single Shure and a single EV microphone - neither of which are very small - the EV for example, is 3/4" in diameter, so hardly tiny.

The X2Y products (wasn't that a dreadful video?) seem to be pcb based, and it's a shame they haven't produced one built into an XLR-XLR barrel type connector that could be plugged in line.

I guess we're just going to have to accept that phones DO have to be off.

[John Willett]

Originally Posted by Greg Miller

Not quite.

All electret mini tie mics are unbalanced down the cable.

The Shure which I referenced above (Post #17) is a dynamic, and according to the literature and wiring diagram it is a conventional balanced dynamic mic. Nothing unbalanced about it.

Of course it's not quite as "mini" as some of the electrets but it's still small enough to be used with a tie clip.

I said mini tie mics - the Shure is not a mini tie mic.

[Richard Crowley]

Originally Posted by Paul R Johnson

The X2Y products (wasn't that a dreadful video?) seem to be pcb based, and it's a shame they haven't produced one built into an XLR-XLR barrel type connector that could be plugged in line.

The interference has already been embedded into the audio by the time it reaches the XLR output connector. There is NOTHING you can do by the time it reaches the XLR connector. I you don't prevent the RFI from getting into the very first stage (the impedance converter circuit inside the head) it is TOO LATE.

[Greg Miller]

Originally Posted by John Willett

I said mini tie mics - the Shure is not a mini tie mic.

"What's in a name?"... I guess it depends on your definition of "mini."

The first dictionary I grabbed says:
miniature: "-adj., on a very small scale, reduced"

miniaturize: "v.t., to reduce in size"

While the SM11 is certainly not as small as the newer electret tie mics, it is "reduced in size" compared with a lot of the earlier lavs that we saw in past years. The sizes have gotten progressively smaller, and perhaps we're near the end-point of the curve with the SM11.

Shure SM11: L = 33.5mm, D = 14.7mm.

EV 649B: L = 57.2mm, D = 19.0mm.

Shure SM51: L = 62.7, D = 19.8mm.

EV RE85: L = 67mm, D = 23mm.

Altec 686 (ugh!): L = 88.9mm, D = 27mm (max).

Shure certainly expects the SM11 to be used as a "tie mic" as the spec sheet indicates it ships with a "Tie Clasp Assembly" and a "Tie Tack Assembly." Admittedly, I wouldn't want this thing hanging from a pin shoved through my necktie. Then again, I don't remember when I last saw my necktie.

So I think it's accurate to say that the SM11 is a "tie mic" and, given that it's the smallest dynamic I've found, it might be called "mini" as well. But, ya know, if it makes everyone happy I will call it a "small" dynamic tie mic.

BTW I note that the Shure/Countryman WCB6T (electret) is described as a "Micro-Lavalier Microphone" so perhaps we've now reached a new "micro" dimension which is smaller than "mini."

[Richard Crowley]

And remember that even a balanced dynamic microphone may be susceptible to GSM RFI, depending on how it is designed and constructed, and depending on the connectors, cabling and microphone preamp.

There is no guarantee that there is ANY microphone that is truly "impervious" to GSM RFI.

[Greg Miller]

Absolutely correct!

If the dynamic capsule is insulated from the case, it is pretty much balanced... that's a function of capacitance between the capsule, the case, and the microphone coil. At typical lo-z mic impedance (~ 200 ohms) and audio frequencies, any imbalance should be so small as to be insignificant; at RF it becomes more important.

A shielded twisted pair should be pretty much impervious to the RF levels we're talking about. But some shielding is better than other shielding... you need to check the wiring specs to find the percentage coverage of the shield. (This is especially important for RF coaxial cable.)

Still, a bad input stage (mixer, preamp, recorder, etc.) can cause problems. If the case and pin one connectors (of the XLR) do not connect immediately to the chassis where they enter, but rather follow some long "ground" wire to the circuit board, that "ground" wire will act as an antenna, and will re-radiate any RF (which was picked up on the shield) back inside the chassis, where it can easily bleed into everything.

Still, the OP said he had problems only when the phone was near the unbalanced cable, between the capsule and the connector, and not with the cable from the connector to the preamp. If that is truly the case, then I feel pretty confident that a balanced mic (like the SM11) would solve his particular problem.