Electric shocks from SM-58 connected to ADA8200???

[fladd]

As the title says, I have my Shure SM-58 connected to a Behringer ADA8200, which itself is connected via ADAT to a Focusrite Scarlett 18i8 (2nd gen). When I touch the grill of the SM-58, I get an electric shock. I also get an electric shock when I hold the SM-58 in the hand and then touch the body of my MacBook Pro, which is connected to the Scarlett via USB A.

I am not sure what is going on here to be honest. Phantom power is also switched off on the ADA8200. Any ideas?

[Wonks]

It appears you don't have a solid ground in your system, so the voltages are floating. The system "0v" isn't referenced to anything, so can float as high as half mains voltage. When you touch the mic, you reference the ground to yourself, but you briefly experience a sting as the voltage difference reduces.

This shouldn't happen as the ADA8200 has an IEC power socket. So you may not have an earth connection in the IEC lead. Or the power socket you are using may be mis-wired. The latter may seem more likely as once holding the mic, you should be at the system ground potential.

[Sam Spoons]

Definitely time to check that the mains supply has a good earth, the MBP won't have a low resistance earth despite having a metal mains pin, the Focusrite is powered by a wall wart/line rat which probably doesn't either so it's only the ADA8200 which has an earth connection.

It may not be the case that the earth 0V is floating up to half mains voltage is the culprit, I have experienced shocks which were demonstrable caused by static build up, but it is the most likely, and potentially, the most dangerous cause and needs checking so follow Wonky's advice and check the mains lead and the socket.

[Wonks]

And as the ADA8200 is connected via an ADAT optical link, the ADA8200 isn’t providing a ground for the rest of the system either, so that too can float.

So I can see how holding the SM58 and touching the case could possibly give a slight shock, but the SM58 on its own should be properly grounded via the IEC lead (provided that the lead has a plug with an earth/ground pin and that’s not been disconnected.

With an IEC lead plugged into the ADA8200 but not plugged in to a wall socket, a multimeter should tell you whether the earth/ground connection is good all the way from the mains plug earth pin to the units chassis, or pin 1 on a mic input and probably the outer shell of the word clock BNC connector on the rear.

If you then plug the mic lead in, do you get continuity to pin 1 on the mic end of the XLR lead? You aren’t using a lead with the ground disconnected at one end that was once used to cure a ground loop?

And then with the mic plugged in to that, do you get a low resistance reading to the grille from the ground pin?

If this is all fine, it certainly might just be static as Sam mentioned.

Have you got any other mics other than the SM58? Do you get the same issue?

If not, are you 100% sure the SM58 is real (e.g. bought new from a respected store and not used or new but cheap from eBay) and not a fake? Almost all the fake SM58s are wired for unbalanced use (I’ve taken a couple apart myself) and many have weird wiring on the XLR connections with no guarantee the shell is grounded.

[Wonks]

If you find the ADA8200 is properly grounded, it will be worth getting a ground connection run from that to the Focusrite to give the rest of your system a good ground reference.

[Hugh Robjohns]

It's worth remembering that there are two kinds of 'electric shock' which have completely different causes, risks and resolutions.

A static shock is caused by building up a high static voltage on the body — usually due to wearing synthetic materials or shuffling across synthetic carpets, etc.

If you then touch something grounded that static voltage discharges as an instantaneous and momentary 'zap' — you may even see a small spark.

The way to identify that it really is a static shock is to take your hand off the grounded item and then touch it again a few seconds later. If you have already discharged your static in that first zap they'll be nothing left to zap again! (Not until you've recharged yourself by moving about, anyway).

Static shocks are unpleasant, but present no danger to life (although they can kill sensitive electronics!)

A mains shock feels very different! In the most common form, as described above, it is due to leakage currents either from double-insulated devices, or from class-1 devices that have lost their mains safety ground connection for some reason.

In these cases, metalwork that would normally be grounded can 'float' up to half the mains voltage — around 115V in Europe, or 57V in the US.

If you touch that you will feel a continuous 'pulsing' shock which, in Europe at least, is very unpleasant.

Typically, the source impedance of this leakage is normally quite high and thus the current is too low to cause harm... in most cases — but it is indicative of a serious electrical fault somewhere and must never be ignored! Find and fix the problem immediately!!!!

[twotoedsloth]

My mother has a regular gig at the legion hall, and she was getting shocks from her microphone. The obvious solution was to buy her a wireless system, which solved the problem. If you invest in some rechargeable batteries and a suitable wireless rig, you should be able to avoid shocks.

Best of luck, hope you get this resolved.

Peter

[Arpangel]

If you touch a neon mains test screwdriver to the mic it will probably light up, as you are grounded.
This should be investigated and fixed immediately, although not fatal "in most cases" it's very unpleasant and could be indicative of dangerous faults somewhere in your mains wiring. Or floating mains voltages, make sure your system is grounded.

[Jimmy B]

This should be investigated and fixed immediately, although not fatal "in most cases" it's very unpleasant and could be indicative of dangerous faults somewhere in your mains wiring. Or floating mains voltages, make sure your system is grounded.

+1 to this! Also the basis for any installation, even a temporary one, should be a power outlet that has been properly tested , e.g. with one of these:
https://www.toolstation.com/socket-tester/p82826
Failure to do this can be fatal (not always, but do you want to be one of the 1%?)

[fladd]

Thanks for all the replies!

Okay, so I now tested continuity between the ground pin of the power connector of the ADA8200 and (1) the chassis of the device, (2) pin 1 of an XLR input, and (3) the grill of the SM58. I get continuity for 2 and 3, but not for 1! I also realized that the ADA8200 has a ground screw on the bottom of the chassis. I do get continuity on this screw.

I am not really sure what is going on here...Any advice would be highly appreciated.

[Hugh Robjohns]

Okay, so I now tested continuity between the ground pin of the power connector of the ADA8200 and (1) the chassis of the device, (2) pin 1 of an XLR input, and (3) the grill of the SM58. I get continuity for 2 and 3, but not for 1!

That's not unusual if the chassis has a thick paint coating, for example. Sometimes you have to take a screw out of the lid to reveal bare metal for the meter probe.

I also realized that the ADA8200 has a ground screw on the bottom of the chassis. I do get continuity on this screw.

So the chassis _is_ properly grounded, assuming the wall power outlet is properly grounded.

[fladd]

Now that I plugged it in again, the problem is gone also...at least for now...but I would still like to know what was going on, so it does not come back.

I am starting to wonder whether this could be related to the fact that I am daisy chaining multiple power strips until the ADA8200.

What other causes could this have had?

[Arpangel]

It’s preferable to use the star system, don’t daisy chain, it’s easier to trace faults in individual plug boards, like earth discontinuity, and is less likely to cause earth loop issues.

[ef37a]

I cannot find the right schematic for an ADA8200 but some circuits do not take the internal supply zero volts directly to chassis although chassis is earthed via the mains. Instead a pair of back to back diodes are used, sometimes with a few nF across them and sometimes a resistor. The absence of the resistor will give you the "tingle".
This technique is very common in guitar amplifiers.

Re "star strips"? If you are making up a mains diss' system for a "forever setup" It is better in my view to buy rake of 13A back boxes and sockets and mount them on a piece of 19mm ply. Wire them up with substantial 2.5mm cable and fit a cable gland. You can probably fuse the plug at 5A for most small home studio rigs?

I also have a couple of Ally extrusion 300mm high "power poles".

Dave.

[ajay_m]

It's not clear how severe this 'electric shock' is. With double-insulated equipment it is common for there to be some capacitive leakage through from the mains input to the output, as a result of capacitance in SMPS transformers, primarily. This results in a low-level leakage current that is then perceived as a 'vibrating' sensation when you run your fingers along a metal surface connected on the secondary side.
The severity of this of course depends on the capacitance, though it should never be more than just a sensation that the surface you are touching is vibrating, certainly not a tingling sensation or anything like that. I'm guessing the leakage current is maybe 50 microamps or so in this scenario. (a quick back of the envelope calculation for approximately 1nF of leakage capacitance (1000pF))

Even at lower current levels while not dangerous this voltage can be catastrophic to high impedance inputs if cables are connected or disconnected with power applied, which is why in general having some kind of earth on the secondary side is desirable. Of course to be honest with phantom power floating around, hot plugging cables is always somewhat risky, for example, the Akai Force has a known issue where the device can be severely damaged in that scenario, so regardless of mains earthing issues, you always have to be a little cautious with XLR cables and hot plugging, if you want to avoid all risk. In any case, if there were any concerns about grounding, I'm not sure I'd want to be tightly gripping the shell of an XLR and plugging it into something that might have a grounding issue, with power on.

[Hugh Robjohns]

Now that I plugged it in again, the problem is gone also...at least for now...but I would still like to know what was going on....

We all would!

But first, we need to know what kind of 'shock' it was? A one-off static shock, or a continuous throbbing or vibrating half-mains 'belt'.... or the heart-stopping yaaaarrrgghhh of live mains!

We know the ADA is electrically okay, now. If it was a static shock then everything is working exactly as it should — the mic body is grounded through the ADA back to the mains protective earth.

And if the static problem has gone today it's because the air is more humid or you're not wearing nylon underpants today.... (but seriously, different clothes and shoes will change your static build-up dramatically).

If it was a half-mains shock the ADA wasn't getting the earth it requires through the mains supply, so check for dodgy (broken or corroded) leads or extensions, plug-boards and wall outlets.

I am starting to wonder whether this could be related to the fact that I am daisy chaining multiple power strips until the ADA8200.

Potentially, yes. Certainly more opportunity for the safety earth to pass through a dodgy socket connection and/or for the ground impedance to rise to an unhealthy level. Always best to connect plug-boards in a star arrangement as m'learned colleague Arpangel states.

What other causes could this have had?

Nylon undercrackers....

[Wonks]

Your shoe material may also play a big part if static. The more insulating they are, the more static can build up on you without draining away.

[ajay_m]

Rubber soled shoes. My Clarks shoes build up a spectacular charge even walking over hard floor surfaces, leading to a massive static jolt when I reach for the office exit door button, which is grounded. Sure beats that second cup of coffee for waking me up, I can tell you!

[Hugh Robjohns]

I remember attending a huge trade show in Las Vegas (very dry air) years back. Lots of walking across cheap man-made carpet through vast halls wearing comfy trainers. When I reached for a metal door handle into the Gent's loos I drew a visible spark and a crack that everyone around me heard and saw!

[ManFromGlass]

Cars with cloth interiors during low humidity winter. I would exit the car and get a most unpleasant shock through my big toe!

[fladd]

This was not static though! It was a continuous tingling.I could repeatedly touch (for very short, as it was very unplessant) the chassis of the ADA or the mic grill and get it every time.

Some more info: After I reconnected the ADA (at which point, the problem was seemingly gone) I also attempted to measure the voltage that I felt on the chassis and mic grill (I didn't know at this point that the problem was gone, as I hadn't touched it yet). While doing so, realized something else that I found strange. I did this measure by putting the black pin of the multimeter into the ground of a power strip plug (and the red pin on the chassis/mic obviously). However, as soon as I put the black pin on the ground, and with the red one being just held in the air by me (so not touching anything yet), the multimeter registered a voltage between 4 and 9 V (fluctuating based on the position of the pin in the air) when using a power cord further away from the wall socket for the gorund. When using the ground of one closer to the wall socket, the voltage was around around 0.1 - 0.3 V.

Not sure if this helps to diagnose what is going on, especially since at the point of the measurement I wasn't getting any shocks anymore, but who knows.

[Jimmy B]

In case it might help, I tried the same experiment here using a cheap digital multimeter. I am seeing 0 volts on the AC voltage ranges (which can measure 0.1 volts) and a maximum of 0.1 millivolts (the smallest number that it can measure) on the DC voltage ranges. This is on the second power strip in my chain.
My numbers are what I expected to see.

[ajay_m]

I don't know the provenance of the meter you used for that earth check, and one thing I definitely recommend is NOT using Chinese DVMs of unknown provenance because if nothing else they are often assembled with a somewhat cavalier attitude towards boring stuff like safety standards. Hence in a scenario where you accidentally connect something across 230V (or, far worse, across phases in a 400V three phase installation), and have the meter plugs connected to read current instead of voltage, you want the fuse and circuitry to be designed for exactly that scenario, as opposed to a sudden catastrophic arc flash or other risk. This is why we have CAT certification, which ensures that the meter insulation and fusing etc, meets certain standards.

Hence purchasing from a reliable vendor like RS components or Mouser etc is highly recommended, even for budget meters - not stuff off Amazon and ebay.

Now that said I certainly don't expect any meter, even a dubious Chinese el-cheapo one, to register a voltage with one pin earthed and the other floating around but not directly in contact with anything, including you, except where there are significant RF fields around, such as an FM transmitter or something like that. Otherwise you would expect the probe insulation to be hundreds of megohms and so the probe held in your hand ought not to be able to drive current back through the meter - unless, as I said, there's a significant RF field, because then capacitive coupling becomes a significant factor, at 100MHz a 1pF capacitance has a reactance of approximately 1500 ohms for instance.

Being within a few miles line of site to a powerful FM transmitter will certainly introduce some fairly significant signal into even a few metres of cable, mind you.

It kinda sounds like that there might be some RF background here AND that particular power strip earth has failed. Or that indeed the main power socket earth is faulty and then the earth cable is acting as an antenna for RF, hence, the longer the wire (further away you are from the socket) the more RF the meter picks up.

That's harmless, it's just a symptom of a potentially serious grounding problem, at which point I would seriously look at involving a qualified electrician to check things oout.

[Jimmy B]

I agree with Ajay. It is quite likely that one of the power strips has a safety ground continuity fault, which may be intermittent.
Whatever the fault is, this is not something that can be "lived with", as an important safety feature may be neutralised. It needs to be sorted out quickly.
This issue has previouslybeen covered in great detail in this thread:
https://www.soundonsound.com/forum/view ... tric+shock

[fladd]

Yes, I also initially assumed that there is an issue with the ground in one of the power strips. Yet, I do get continuity between pin 3 of an XLR input on the ADA8200 and the ground of the first power strip. So this should rule out an issue with grounding, right?

Something else: Is it possible that the SM58 got damaged (I mean it still works, but maybe the response was altered) when there was voltage on it?

P.S.: I do indeed have a cheap Chinese multimeter (this one to be precise).