Shocks from mic

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Re: Shocks from mic

Post by ajay_m »

Capacitive leakage from double insulated power supplies is very common. The design of the PSU is intended to ensure that it will remain electrically safe but the resulting leakage current can be quite perceptible.
The main risk is to sensitive mic preamps which can easily be damaged by plugging mics in with power on in this situation.
I have not tried the numerous low cost signal isolation transformers available on Amazon or eBay with mic level signals though they do work quite well at line level. They are extremely cheap and possibly worth trying although you'd need to make or buy XLR adaptor cables to the 3 pin headphone mini jack's they use.
Obviously this isn't a safety mitigation, just removes an annoyance.
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Re: Shocks from mic

Post by ian2 »

James - Unlike the guitar-oriented GI100, the Behringer DI100 has two input sockets; a 1/4 inch jack AND an XLR. You can see them both here: https://mediadl.musictribe.com/media/PL ... er1_XL.png

I got out my multimeter at lunchtime. There was no continuity between the metal surround on my Laney CP12 wedge monitor's input jack socket and the ground pin of its mains power socket. Out of curiosity, I found that there was no continuity between the metal surround of the input jack connector and the similar connector for the wedge's LINK socket. (I figured there ought to be continuity?) I then found that there is continuity between the inner part of the wedge monitor's input jack socket and the ground pin of its mains power socket. Forgive my layman's tentative interpretation of this, but would it seem that the input jack socket's line and ground connections have been transposed?!! Is this the culprit?
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Re: Shocks from mic

Post by James Perrett »

ian2 wrote: Mon Jul 04, 2022 11:21 pm James - Unlike the guitar-oriented GI100, the Behringer DI100 has two input sockets; a 1/4 inch jack AND an XLR. You can see them both here: https://mediadl.musictribe.com/media/PL ... er1_XL.png

That's weird - what is the point of an unbalanced XLR input? Weirder still, Behringer claim that it is balanced on their advertising which is obviously wrong if the markings on the DI box are to be believed.

ian2 wrote: Mon Jul 04, 2022 11:21 pm I got out my multimeter at lunchtime. There was no continuity between the metal surround on my Laney CP12 wedge monitor's input jack socket and the ground pin of its mains power socket. Out of curiosity, I found that there was no continuity between the metal surround of the input jack connector and the similar connector for the wedge's LINK socket. (I figured there ought to be continuity?) I then found that there is continuity between the inner part of the wedge monitor's input jack socket and the ground pin of its mains power socket. Forgive my layman's tentative interpretation of this, but would it seem that the input jack socket's line and ground connections have been transposed?!! Is this the culprit?

I've just checked the CP12 service manual and it certainly looks like the input should be grounded. However, one thought occurs to me - what sort of cables are you using between the mixer and the monitor? I wonder if Laney are using the tip of the input connector for the signal and the ring for ground instead of the sleeve?

If you were to use a standard unbalanced cable between the mixer and monitor with TS jack connectors the mixer would be grounded. If you use a balanced cable between the mixer and monitor the ground connection wouldn't be carried through to the mixer and you would end up with no ground in the system.
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Re: Shocks from mic

Post by Wonks »

On the DI100, the XLR input socket has pin 2 connected in parallel with the tip connections on the TRS input sockets, whilst pins 1 and 3 are connected together, as are the R and S of the TRS inputs, to the common input ground bus.

So you are losing the balanced signal aspect of the mic operation by using the Behringer DI100 between the mic and the DI box. The XLR output on the box does give a balanced output back to (in your case) the mixer. But as the XLR output signal is significantly reduced in level, your mic signal could end up very noisy indeed with the extra gain required and the initial unbalanced cabling picking up more noise than a balanced connection.

The ground lift switch (as expected) either connects or disconnects the ground bus connection of the output side of the DI box to the input side.

As the mixer doesn't provide a solid ground connection itself, you are then reliant on the power amp/active speakers the mixer outputs are connected to, to provide a reference ground. And it all depends on the power amp or active speakers as to whether the ground connection is connected to earth, or left disconnected in order to avoid a ground loop.

And if the ground lift button on the DI100 is operated, then there won't be any ground connection to the mixer side of things anyway.

So you are then reliant on the powered monitor providing a ground.

According to the schematic for the monitor, it should do.

What I think you have measured ground from on the CP12 is the chromed plastic socket on the TS input and link jacks which have no connection to ground, they are just there to look pretty!

But there should be a connection from the S connection of the socket to ground, so I'd stick a TS (or TRS) jack cable in the socket and measure from the S on the unused end of the TS (TRS) cable to the earth pin of the mains plug.

What I can't tell from the CP12 schematic is whether the mains earth connection only goes to the speaker chassis, or whether this is also connected to the signal ground. It does look like there may only be a virtual ground via a central tap on the secondary side of the mains transformer (presumably in an effort to avoid the monitor being the source of a ground loop).

It may well be the Laney that's providing the floating voltage on the mic, rather than the mixer if you don't get the tingling when only using the mixer.
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Re: Shocks from mic

Post by Hugh Robjohns »

James Perrett wrote: Tue Jul 05, 2022 12:54 amThat's weird - what is the point of an unbalanced XLR input?

It's there because they've been 'heavily influenced' in their design by the BSS AR113 (and previous generation DI BSS boxes). BSS provided the XLR input for easy connection to pro line level sources... but the reasons for it being unbalanced seem lost in the musts of time! The XLR is definitely wired unbalanced in both cases, with pin 3 tied to pin 1 (gnd).

ian2 wrote: Mon Jul 04, 2022 11:21 pm I got out my multimeter at lunchtime. There was no continuity between the metal surround on my Laney CP12 wedge monitor's input jack socket and the ground pin of its mains power socket. Out of curiosity, I found that there was no continuity between the metal surround of the input jack connector and the similar connector for the wedge's LINK socket. (I figured there ought to be continuity?) I then found that there is continuity between the inner part of the wedge monitor's input jack socket and the ground pin of its mains power socket.

It all depends what you mean by 'the inner part' of the socket. It's not unusual to have jack sockets where the securing nut is electrically isolated from the grounded sleeve terminal.
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Re: Shocks from mic

Post by Wonks »

I'm not sure if we know just what mics you are using, apart from it's not an SM58.

If they are budget SM58-alike types, I've bought a few to see what they are like and have found some weird and wonderful wiring schemes. A lot seem designed to work with an unbalanced TS jack connection (sometimes supplied with the mic and sometimes not).

I notice in Hugh's quote of yours above that I missed your reference to the S connection of the Laney input jack being connected to earth, so in theory the mic should have a good ground reference. I'd be tempted to try and measure the resistance from the mic grille to the Laney power plug earth pin with the system set up as you first described. There should be a good path to ground. If not, unplug the mic and check pin 1 on the mic-end XLR to the Laney mains ground pin.

If that is fine, then there's a bad ground connection in the mic. Though if it's a painted grille, then you might not register anything.
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Re: Shocks from mic

Post by ian2 »

Last night, I found that with either a TS or TRS cable plugged into the wedge monitor's LINE input (thanks for that great tip, Wonks!), I found that there is continuity between the S (sleeve) and the wedge's mains socket's earth pin. (Please therefore disregard my now apparently obviously incorrect attempt on Monday to establish that.)

There was the question of what type of cables were being used to connect my on-stage Phonic MM1002 mixer to the Behringer DI100 DI boxes. The answer is regular TS guitar cables, i.e. unbalanced.

@Wonks: FYI the mics are Sennheiser dynamics. If Mike Stranks has the memory of an elephant he will know the model number, but it was about 11 or 12 years ago that he helped our church upgrade its PA equipment so, in my case, I'd have to boot up an old PC to stand any chance of finding an email with that info. (My previous PC has been kept because it has lots of unfinished recording projects on it, which I'd imagine may remain unfinished indefinitely...!) The mics have a black grill and I'd imagine that it's metal that's been painted. If the model of mic is one that's still made it looks like it would be e835 or e845. All of the church's PA gear is behind three locked doors 24/7 except for the Sunday morning service, which poses a real challenge to verifying that good earth connections are present where expected.
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Re: Shocks from mic

Post by Mike Stranks »

Dumbo here... :lol:

I expect that the mics are Sennhesier e845s... they would have been my mic of choice/recommendation back then.
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Re: Shocks from mic

Post by Wonks »

No problem with the mic’s own XLR wiring then unless a wire’s come loose inside (which can happen).
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Re: Shocks from mic

Post by DGL. »

Regarding capacitive current leakage my old laptop was terrible for that, the metallised top cover would become uncomfortable for touch if any of the ports were plugged into a grounded device, i.e. audio out to my mixer or connecting an external display.
I also have issues with my newer laptops that do have grounded supplies in that if I connect them without plugging in the PSU to my Panasonic radio at work it messes with the touchscreen and touchpad and makes the metal case feel weird.
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