I'm an occasional member of a worship group and last night we were running through some hymns for a forthcoming song service.

Our guitarist and flautist are both miked up and going straight into the wall box that feeds to the mizer and hence the hall's PA system, which isn't set to a very high volume anyway.

I'm running my PSR3000 via a small Laney amp, to get some local foldback of the keyboard. The amp has a "DI" output which I'm feeding into my Behringer UB1002 mixer purely to use the balanced main output as a feed into the PA wall box. I do not have anything else plugged into the PSR3000.

If I now turn up my mixer I get audio feedback!

However if I feed my mixer from the PSR alone, I don't get feedback. I can in fact completely overload the entire system to chronic distortion levels without any feedback.

But if I now send the amp a signal out of the PHONO output of the PSR, I get feedback... and I can here the flute coming out of the little amp!

All the wires are checked and wired correctly.

Anyone got any idea whats going on here? Could it be some strange earthing behaviour?

I'm completely baffled!