Doepfer

Hi!

I have a problem with my eurorack system. 
when I connect A138 mixer to general mixer of my studio, the system make an high noise (zuuuuuuuuuuuuuuuuummmmm) though other modules aren't connect to A138.  
the same problem it is made with all three A138 mixers that I have, and with A106-1. 
if I connect the mixer to vca the problem disappared. 
what is the problem?
I use an home made wood case and doepfer A-100 DIY Kit #1 (I connect all component of kit be very careful). 

Thanks to all!
AS