[Elecraft] AF1- active audio filter

[Johnny Siu]

    Hello Elecrafters,
I have just finished the construction of the AF1 for a local radio club.
AF1 works very well.  However, when using my K3 to TX, the AF1 picked up the RF energy and I can hear my voice in my earphone.  I must admit that the antenna is near to my shack but no RF feedback for any other accessories.
In the circumstances, will this problem be solved if I put the AF1 in a metal enclosure?
Have any of you come across the same?  The AF1 does not improve the chance of picking up signals but lessen the listening fatigue.
TNX & 73
Johnny VR2XMC