Problem solved 
. Those 24V. from the amplifier power supply are poorly filtered or their characteristics have been lost. I have connected the phono preamp to an external 12V power supply and it sounds wonderful, but with the main cord of the amplifier grounded. Now I only have to connect a power supply inside the amp only for the phono preamp. I don't even bother looking for the cause of the bad filtering of those 24 V.
. Those 24V. from the amplifier power supply are poorly filtered or their characteristics have been lost. I have connected the phono preamp to an external 12V power supply and it sounds wonderful, but with the main cord of the amplifier grounded. Now I only have to connect a power supply inside the amp only for the phono preamp. I don't even bother looking for the cause of the bad filtering of those 24 V.
