Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
The two wires of a telephone line are balanced so that each wire has the signal out-of-phase to cancel hum pickup.
Your circuit grounds one wire of the telephone line which causes the other wire to act like an antenna that picks up all kinds of interference.
Usually an audio transformer is used to keep the telephone line balanced and it also isolates DC.
thanks, so adding 600:600 will make it right? because the problem happens as I supply my circuit 's Vcc with USB !
But when I go on batteries, I have no noise
Actually the whole ground is connected to usb's ground, I've found on Net that connecting to USB directly(tel ground- USB ground) generates noise & in order to fix that, somehow a choke must be used.<looking for circuit & nothing found yet)