Problem with leach amp

[AudioG]

Hello,
I have buid two leach ampli with pcb V4.5.
I have a strange problem only on one of two leach low tim ampli:
if I connect only the central pole of input plug from preampli or BF generator, in the input stage something smokes, and the fuse of V+ blows (4A delayed), there is a very high oscillation, not audible but so strong to break a loudspeaker used for test.
After changing the blowed fuse the ampli works!
This only in a channel!

I need some help.

Thank's

Giuseppe

[AndrewT]

Hi,
the general rule is do not change plugs with the equipment live and/or connected to the speakers.
But almost everyone does it even me.
RCAs are a very bad design simply because they allow live before ground connection, XLR and DIN do not allow this.
Have a look at the impedance of the input stages and connections to ground in case there is an error here.
Someone with RF experience might give some pointers with regard to rectification of ultrasonics in the first semiconductor.