A Tech Tip from Shure Applications Engineering:  

The musician was bewildered…more than usual. "When my guitar is connected to my amp with a cable, and I sing into a wired mic, I get an electrical shock through my lips. If my guitar is wireless, it does not happen. If my mic is wireless, it does not happen. Why do I get shocked when the guitar and the mic both use a cable?"

Of course, the mic gets blamed because it touches the lips. But the culprit is not the mic, nor the cables. The culprit is the guitar amp.

Because of the electric design of many vintage guitar amps, it is not uncommon for a small amount of current (120 VAC) to "leak" onto the amp chassis.

Here is the path from amp to lips: The guitar amp chassis connects to the guitar cable. The metal guitar strings are grounded/connected to the guitar cable. When hands are on the metal guitar strings, the musician's body is connected to the leaking AC current via the strings, the cable, and the guitar amp chassis. When lips touch the metal grill of the microphone (that is connected to AC ground via its own cable) the circuit is complete. The leaking AC current flows through the wet lips onto the mic grill and then to ground via the mic cable and mixing console. Ouch!

A foam windscreen on the mic grill will prevent the shock to the lips, but a shock could still occur if the metal mic handle was touched by one hand while the other hand touched the guitar strings.

If a wireless mic is used, there is no ground path to the console. If a wireless guitar is used, there is no ground path to the guitar amp chassis. In either case, there will be no electrical shock.

Electrical shock is a serious matter and should not be ignored. The proper solution is to have a competent guitar amp technician examine the guitar amp for the source of the AC current leaking onto the amp chassis and repair the fault. If the AC current leakage gets worse, the musician could burn his lips or suffer a severe electrical shock, perhaps even a life-threatening shock if the AC power source is 240 volts like in Europe.

Editor's note:  This is a real warning.  An older gentleman that I know led a band in the 1990's and his guitar player (also older) was killed on stage from this exact scenario.  Improper grounding and equipment malfunction is not just "perhaps" deadly, but actually so.  Don't take chances.

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