Maybe I've found: it's the headphones adapter, probably the cable that came with the Simplex.
Inserting slowly the 6.3mm jack into the adapter-cable, jus for few millimeters, I have a loud sound on both channels.
Inserting it more I loose one channel.
Inserting it completely I have poor sound on both channels.
My 3.5 mm to 6.3 adapter works fine with Teknetics T2 and Garret Ace 250, so I suppose Nokta adapter is faulty.