(C) G. Forrest Cook 2004
This circuit is useful for removing ground loop hum on a remote line-level audio signal line. It can be used to to connect a computer sound card to a stereo amplifier's line input. Your author uses several of these isolators to drive multiple amplifiers from one computer sound card for a whole-house sound system.
The circuit can be useful for removing common mode ground noise in 12 Volt automotive/marine audio systems. This can be an issue if you connect a music player to a car stereo aux input and power the music player from the car's cigar lighter plug. The circuit can be used in mono applications by using only one channel.
Connections: L and R RCA Jacks, stereo 3.5mm jacks Operating voltage: 1V P-P nominal (line level audio) Input Impedance: 10K ohms Output Impedance: 10K ohms
The Left and Right audio signals are passed through magnetic fields in the two transformers from the primary winding to the secondary winding. Ground loop noise that would otherwise travel from the ground on one side of the isolator to the other side is blocked.
A custom PC board was made for this circuit, it could easily be built with perforated circuit board, or free-wired with the connectors mounted in a small metal box. Multiple connector types as shown in the photo are optional. If the circuit is put inside of a metal box, ground the box to the input side ground that connects to the transformer cases.
The specified Mouser transformers have a center-tap pin on both the input and output sides, these should be left disconnected.
Here is the printed circuit artwork in PostScript format, it can be printed onto Press-n-peel blue transfer material for making a PC board.
Connect this circuit between an audio source and an and audio destination. Turn on the signal source, listen for the audio signal on the destination. The circuit is reversible, either side can be used as the input or output.
For the whole-house audio system, the sound card output feeds the computer speakers directly and that signal is tapped and fed to a long shielded stereo cable. The isolator is connected to the far end of the shielded cable and the remote amplifier is plugged into the output of the isolator with a short RCA cable jumper.
4x PC mount RCA jacks, Jameco 112475 2x PC mount 3.5mm stereo jacks, Mouser 161-3501 2x 10K - 10K audio transformer, Mouser 42TM018