TOPIC:

I have measured with th oscilloscope the input-output of a sine wave in the balanced Flex
with all parameters on 0 db  (input gain, output gain, main volume gain) the output sine wave is lower than input wave. With gain  put on +6db output matches input, is this normal ?

I suspect you're losing 6db in your measurement scheme.
Maybe your input is unbalanced (via an adaptor) and half the voltage you think it is????

Dave.

i inject  in the input an unbalanced sinewave between ground and hot pin and i measure at output between hot pin and ground so o inject unbalanced but i also measure unbalanced, do you think the scheme is wrong ? unfortunately i don't have a balanced sine wave generator, but i think you are right, perhaps in this way i inject half and measure  half so output is 25%

Yes definitely you were right, i changed measurement scheme, i managed to inject a balanced sine wave connecting another balanced preamp with usb connection with the laptop connected so to let the preamp  generate a balanced sine wave with his dac, now minidsp flex  input equals output with all parameters at 0 db, thank you very much, i'ts the first time i'm handling with balaced connections, up to now i have to remember that hot and cold inputs are always summed.

Good deal.

Whenever testing and you find yourself losing 6db (or gaining 6db) unexpectedly, it's almost always related to a unbalanced/balanced conversion not accounted for.

Dave.