A  simple hardware circuit, with a noise free internal linear power supply, and filtered connections.
Connects to a TRX, PC audio, headphone and active speaker.

More and more amateurs suffer from strong QRM, preventing to receive weaker signals.
A solution for comfortably listening to other radio stations, is the use of a good "WebSDR". But the listeners own transmitted signal will also be heard, creating annoying echoing. When starting to transmit, the received PC-audio should therefore be muted (silenced). 

When using a modern TRX, "CATSinc" software can do that for you.

In some cases, the use of software is not possible or wanted. Then this hardware circuit can silence the audio from a PC (WebSDR) for you. It is a nice and simple to build project.

Four 3,5mm or 1/4 " stereo busses are used for connecting inputs and outputs. An RF filtered "Keyline" bus connects it to a transmitter "Power Amplifier" relay output. The connecting "Keyline" cable should be screened and electrically be floating from ground. If needed, equipped with good CMCs.

When transmitting, the transmitter shortens or opens its "Power Amplifier" relay output. The response of this muter (mute or not) can then be set by two jumper blocks. The two LEDs indicate open or closed "Key line" input.

The stereo input L and R lines are each loaded by 33 Ohm. The headphone and speaker outputs are mono and capacitive RF filtered. The speaker output is meant to be connected to an active speaker, or audio amplifier. It is low-impedance-terminated, to prevent hum during muting.

Two toggle switches enable :
-  continues PC sound output, and 
-  continues speaker mute. 
The "ground" of the muter PCB, and if the "Speaker" output, will be interconnected.

The muter has its own small linear power supply, which is RF-filtered to be noise free. The little mains transformer is a special "Short Circuit Proof" type. The muter 240V~input needs therefore not to be fused. An external 9-16 Vdc supply could be used instead. In that case, the "minus" of the muter, the PC, and the "Speaker", will become interconnected.

        Housing.
The double sided 92 x 76 mm² PCB has two ground surfaces, and is designed for mounting into a Hammond 1554G ABS box. First drill all holes. The empty PCB can be used as a drilling template for the bottom side of the box. The box stands with its drilled bottom up, and with its lid on the table. 

The switches and the LEDs must be placed at the top surface of the PCB. All other components must be placed at the bottom side of the PCB.

The finished PCB is screwed into the "bottom side" of the box by means of the thee switch shafts. No other screws are needed. 

After installing the PCB, the four stereo busses could be mounted individually, directly under their PCB screw connectors, into the back-side of the box. Short connecting wires to the stereo busses can be screwed into the PCB terminals, or directly soldered into the PCB.