This type of protection is used in the "1Wamp Electric Guitar Amplifier"
http://www.electrosmash.com/1wamp
"D3 and D4 are 5.1V 1-Watt zener diodes. They are surge protection against static discharges, they clamp the input signal if the levels exceed ±5.1V. They are optional but JFETs have a sensitive gate and they can blow just with spark at the jack in a dry day. The 1N4733A part is used here due to its easy availability but any 5.1V 1W zener like the BZX85 5.1V could substitute it."
