Humming mic - noise decreasing if I touch the grille

[Hugh Robjohns]

Unless CS70 was also connected to signal ground at the time of touching the (let's assume ungrounded) grille, the noise would have increased, not decreased, for his body would now have been acting as an aerial.

I can't conceive of how such an idea could become practical.

The grille is not part of the mic's audio circuit, it's just supposed to act as a Faraday shield. There is no practical circuit mechanism for the grille (if acting as an aerial or aerial interface) to inject any received signal into the high-impedance part of the audio path (unlike the situation with an ungrounded electric guitar). The grille in a mic is only connected to the mic's earth point (typically via spot welds on the body case). In this case the mic's earth connection via the XLR cable is obviously good as it was working quite happily on phantom power.

In my view, given the mic circuitry is clearly earthed, it is therefore far more likely that the problem is a lack of Faraday shielding around the high impedance capsule. By touching the grille one of two things happened. Either the extra pressure caused a better physical grounding connection, restoring the Faraday shield effect, or an improved earth path formed via the finger touching the grille and (presumably) through the other hand holding the mic body or some other earthed device.

I suspected the grille was connected to signal ground and by connecting himself to that signal ground via the grille his body was now acting as shield for some other poorly shielded components.

That just makes no sense at all to me. Just don't comprehend the mechanics, electrics or physics that would allow that to be the case.

Hopefully CS70 will report back on his findings or those of his mic doctor in time.

H

[Tim Gillett]

Unless CS70 was also connected to signal ground at the time of touching the (let's assume ungrounded) grille, the noise would have increased, not decreased, for his body would now have been acting as an aerial.

I can't conceive of how such an idea could become practical.

By touching the grille one of two things happened. Either

1.the extra pressure caused a better physical grounding connection, restoring the Faraday shield effect,

2.or an improved earth path formed via the finger touching the grille and (presumably) through the other hand holding the mic body or some other earthed device.

No third possibility?

I suspected the grille was connected to signal ground and by connecting himself to that signal ground via the grille his body was now acting as shield for some other poorly shielded components.

That just makes no sense at all to me. Just don't comprehend the mechanics, electrics or physics that would allow that to be the case. A lump of wet meat is always going to be poor shield, and completely irrelevant if there's a properly grounded metal mesh around the high-impedance circuitry.

Sure, irrelevent to the part of the mic which the mesh protects, and is doing its job, and unlike you I suspected it WAS doing its job.

Except that now that "lump of wet meat" imperfect as it is as a shield, could still act as a shield for another part of the mic or system whose own shielding has lost its ground connection. My focus was therefore not on the mesh and what it protected, but the other part of the mic. Can you at least acknowledge that? You dont have to agree with it, and I may be wrong re this particular case, as we only have very imperfect information to go on, but please at least admit that that was my point, thanks.

The electric guitar I regularly play at gigs has a slight buzz when at high gain and just sitting on its guitar stand. When I pick it up and put it into the playing position against my abdomen, the buzz increases in volume. Why? I believe because my body (abdomen) is acting as an aerial for the noise in the air. When I now touch the (connected to signal ground) strings, the noise goes down in volume. Not only does it go down, it actually is now lower than when the guitar sits by itself on its guitar stand. I believe my abdomen now acts as a shield to reduce the noise to even lower than it is when the guitar sits freely on its stand.

Why do those three levels of noise occur? At least with my understanding of it, I think I can explain it. It makes sense to me.

Do you have a better explanation?

Tim

[Hugh Robjohns]

Except that now that "lump of wet meat" imperfect as it is as a shield, could still act as a shield for another part of the mic or system whose own shielding has lost its ground connection. My focus was therefore not on the mesh and what it protected, but the other part of the mic. Can you at least acknowledge that?

What's to acknowledge? I understand your suggestions; I simply don't agree with them or with your suggested fault scenario for all the reasons and I have already explained and my practical experience of capacitor mic faults.

Until the mic is properly fixed we won't know what the actual problem is, and further speculation is pointless and only likely to confuse others.

H

[CS70]

Gentlemen, I much appreciate your help. Hope my question won't be reason for too many heated arguments.

Gave it a try yesterday, and found that I don't have a screwdriver of the right size (doh!). So I'll procure one over the weekend and will attempt the operation and report.

One a more fun note - echoing ManFromGlass - yesterday my rhythm guitarist brought a distortion pedal he hadnt used for years, and didn't remember why. We found out soon enough: when he engaged it, his Twin Reverb started playing loud and clear... the part of another band's guitarist in a nearby room!

After being puzzled for a few seconds, we had a big laugh. The quality was pretty good! Think of the applications - I could ask Joe Satriani to come nearby on scene, engage the pedal, and play air guitar.. and everybody would think I can actually play like Joe!

[Hugh Robjohns]

[CS70]

Really it was impressive. Spooky, even. With the guitar at the right angle, it was like the other guitarist was there with us, playing thru the amp. The pedal transformed my friend's 70's single coils strat in a perfect receiver for someone else's instrument signal!

[Tim Gillett]

I have a mic which is noisy - have tried with a couple of preamps and it's the same. I noticed that if I touch the grille, the noise decreases, suggesting perhaps a grounding issue. So far I've tried with just one XLR cable, so can't exclude it's the cable.

Is it worth to open it and check if there's some sort of grounding on the grille?

I was going to mention this earlier but my contibution was challenged, hence the ensuing long discussion.

Yes noisy but (as Roberta Flack sang) "compared to what?"

I know you're a sensible person CS70, but from the point of view of those of us on the forum, without actually hearing the noise in its context, for all we know the mic could be - at one end of the scale - hugely noisier than it should be. But at the other end, it could be within manufacturers tolerances for that make and model. Or it could be anywhere in between those two markers.

From here we dont really know what we are dealing with. Too many unknowns IMO. Then there are the skills needed to correctly diagnose and work on especially more expensive microphones. Hence my early suggestion you consult an expert in microphone service.

Tim

[CS70]

Yes noisy but (as Roberta Flack sang) "compared to what?"

A good point Tim. Since I'm really not used to DIY on electronics, I was just looking for a generic opinion of whether or not it was worth investing the time to open up the mic and check for.. something. I got that, and again thanks to all who answered. I will open (carefully) the mic and see if there's something obviously odd. If not... well, I am a pretty fast learner, but do not intend to practice any newfound electronics skills on a fairly nice mic; so the next step will be a chat with someone who knows what they're doing when it comes to mics (a name with good reputation was mentioned earlier) and see if it's worth shipping back and forth.

Since people are so kind to actually read and answer, I'm more than happy to satisfy any lingering curiosity on the set up and what I mean by noise. So let me explain a little bit better (even if obviously without any of you being here in the room with me, it may be it cannot be done to total satisfaction.. remote debugging is hard).

What happens is very simple. I tried two mics (a Neumann TL102 and the Dolly) with the same recording chain (two, actually, just to see if the issue was in the pres):

mic -> cable -> Groove Tubes The Brick -> Steinberg UR28M line in -> USB cable -> laptop
mic -> cable -> 11r mic preamp (with appropriate patch) -> Steinberg UR28M SPDIF in -> laptop

With both mics, I set up the gain structure to reach the same, reasonable, average and peaks. Phantom power is provided by the respective preamp sections.

Both mics were shockmounted, and the environment was identical - including the electrical sockets powering the gear. DAW is Sonar X3. Both mics are cardioids and angled to have the laptop fan in the null.

The same happens with both recording chains:

With the Neumann, once things are settled (a few seconds), the DAW meters show no sign of activity at all. Complete silence - or as good as.

With the Dolly, the DAW meters never settle, but keep on showing some green, faintly fluctuating.

Looking at the recorded waveform, it becomes clear that there's a noise peaking at about -42dbFS. If I raise the volume, I can hear it (as I mentioned, in the mix is not really noticeable - I just don't need to place a tape emulation on the vocals.. but this was just a demo, so mixing has been minimal)

I've dropboxed two pics of the waveform at https://dl.dropboxusercontent.com/u/91812191/Dolly_noise.jpg and https://dl.dropboxusercontent.com/u/91812191/Dolly_noise_zoo....

Zooming in, the noise appear to be sinusoidal in nature, which makes me think it is related to the electrical current.

The same mic was fairly silent (in the same sense as above, no activity on the DAW meter) just a week ago when I used to record a backing vocalist.

In handling the mic I happened to take the "head" in my hand; I was wearing the headphones, andthe volume was relatively high, and that's how I noticed that the noise was lowering in volume. Not disappearing, but noticeably lower. Tried a few times to confirm I wasn't imagining it, and I wasn't.

My pc is not connected an interface right now so I cannot bounce a sample of the noise, but I'm gonna download Asio4all, restart the pc and dropbox the result.

[CS70]

Here it is. There's some ambient noise of course. The WAV is 32bit, 44.1

https://dl.dropboxusercontent.com/u/91812191/Dolly%20noise.w...

The noise actually seem to reach -39 or -40dBFS.

[Hugh Robjohns]

Zooming in, the noise appear to be sinusoidal in nature, which makes me think it is related to the electrical current.

If you count the number of cycles in a one second period it pretty much confirms it is picking up radiated mains hum.

In handling the mic I happened to take the "head" in my hand; I was wearing the headphones, and the volume was relatively high, and that's how I noticed that the noise was lowering in volume. Not disappearing, but noticeably lower. Tried a few times to confirm I wasn't imagining it, and I wasn't.

Again, that ties in with the idea that the grille's earthing has degraded for some reason. Earthing through your skin will present a higher impedance path than a direct metal bond, so the screening will improve but not be as good as it should be. Added to which the grille itself and the mic body will probably have some paint or lacquer treatments which will also increase the touch impedance.

So, examining the mechanical construction and grounding design of the grille assembly is still where I'd start looking if it was my mic...

H

[CS70]

So, examining the mechanical construction and grounding design of the grille assembly is still where I'd start looking if it was my mic

Cant wait to get the darn screwdriver!

[ef37a]

Zooming in, the noise appear to be sinusoidal in nature, which makes me think it is related to the electrical current.

If you count the number of cycles in a one second period it pretty much confirms it is picking up radiated mains hum.

In handling the mic I happened to take the "head" in my hand; I was wearing the headphones, and the volume was relatively high, and that's how I noticed that the noise was lowering in volume. Not disappearing, but noticeably lower. Tried a few times to confirm I wasn't imagining it, and I wasn't.

Again, that ties in with the idea that the grille's earthing has degraded for some reason. Earthing through your skin will present a higher impedance path than a direct metal bond, so the screening will improve but not be as good as it should be. Added to which the grille itself and the mic body will probably have some paint or lacquer treatments which will also increase the touch impedance.

So, examining the mechanical construction and grounding design of the grille assembly is still where I'd start looking if it was my mic...

H

Just FYI all. Right Mark Analyser is an invaluable free download for diagnosing noises of all kinds. (do the Display Colour Inversion unless you like buying toner!)

Dave

[ef37a]

Here it is. There's some ambient noise of course. The WAV is 32bit, 44.1

https://dl.dropboxusercontent.com/u/91812191/Dolly%20noise.w...

The noise actually seem to reach -39 or -40dBFS.

Oh yes. The 50Hz is more than 60dB above the midband noise. 100Hz 20dB or so below 50 then a nice series of harmonics.

Classic "wet finger" hum!

Dave.

[Tim Gillett]

Thanks for the extra info CS. Without even dismantling it you could do more precise tests of the mic. This is what I'd do if I had your gear and was seeking diagnostic help from afar:

First we still dont have an absolute reference but from your recording sample (can I hear a radio station in the background?) it sounds like a serious hum level, well above what it should be.

Mount the mic on the stand as you mentioned. Connect it to the Groove tubes Brick. Listen through headphones.

Why not record what you are doing, talking through what you are doing at any given point? So "I'm now touching the grille with the tip of my index finger" etc. Posting the recording of your voice and the resultant sounds would be really helpful I think. Since the fault seems so gross, no need to record to anything hi res. 16 bit would be fine. Compressing to a sensible mp3 would also be fine, I suspect. From your sample recording it seems we're not dealing with subtleties here!

Now to some more precise tests:

With just a fingerpoint, and your other hand not touching anything else, touch various parts of the mic, the grille, the metal body below it, the XLR plug surround etc. Note what you hear.(It's being recorded anyway so we can hear it and your commentary too).

Then this time, with the preamp close by, touch the chrome "instrument" socket. To make it even easier for you, insert a bare 1/4" metal type plug so you have better contact with your other hand. Now repeat the fingertip tests you just did.

You could also group two tests together for each part of the mic you touch, so "Now touching the mic grille. Now touching the mic grill AND touching the instrument metal plug at the same time." "Now touching the lower mic body. Now touching the lower mic body AND touching the instrument metal plug." etc.

If as Hugh suggests the mic grille has lost its ground connection you could try gently squeezing or pushing parts of the grille to see if temporarily the connection is restored. To make this a fairer test, try wearing a rubber kitchen glove so that your hand is insulated from the grille.

This for you might be a long winded process. I'm a service technician specialising in service and repair of audio recording equipment for the past 30 years. For an experienced technician, a fault such as this might at least be narrowed down (not actually fixed necessarily) very quickly. These are the first steps I would take in diagnosis, at least from what you've told us so far.

Cheers

Tim

[Hugh Robjohns]

[ManFromGlass]

So I would surmise from Tim's thorough methodology that there is very little difference, aside from bodily fluids, between a doctor and a service technician!
- except the doctor could always say - "so it hurts when you twist it that way? Well then, don't twist it that way . . . . . .

[Tim Gillett]

Ah yes but you forget: microphones have feelings too!

[CS70]

Haha I guarantee that I make most microphones purr with delight!

Cool idea Tim, my voice is plastered all over the internet already (not to mention to be occasionally heard on the odd radio station) so I'm definitely fine with the idea (even without suitable EQ and reverb.. frightening! ). I'd already tried the XLR wiggling, with no success. But as I said, I have no delusions on my skills in mic fixing: a man must know his limits. So the idea was really not to have a remote debug, but just to check if my idea of a possible issue with grille grounding was even remotely sensible - and that goal was accomplished thanks to you guys. If a quick check doesn't fix it, I'll contact a tech.

I'll definitely gave a try to the "pushing the head down" (even if seems to be fixed quite sturdily) and the rest of the easy stuff. If I manage to speak thru the process, being the weekend, it's more likely you'll had a toddler and "wheels on the bus" as ambient noise

[Tim Gillett]

OK, we'll brace ourselves for the cacophony.

[SimonZ]

Lack of (sufficient) earth continuity XLR-mic body-grille.
No more no less