Spin Semiconductor

A question about FV-1 in mono operation : I've noted that the volume of the unit in programs that contain original clean signal (e.g. chorus internal program or delay program) is lower than the bypassed signal.
Is a Fv-1 problem due to ADC - DAC conversion ? Is it necessary an op-amp gain increase? (I've noted that this problem there isn't in effects unit like Z-CAT HOLD DELAY PEDAL that use FV-1 without op-amps)

It's not an FV-1 problem it is most likely a s/w thing. On some of the internal program it expects both left and right inputs to be driven, it adds half of each to get the total signal. If you are using one of those programs and only driving one input then it is only using half the signal. Try driving both inputs and see if that helps.