A  simple hardware circuit in a small ABS box, including a rattle free internal linear power supply.
RF filtered keyline and audio connections.
Connects to a TRX, PC audio, headphone and an active speaker.
Settable high/low key line active input.
Switch able audio output and mute action.
The PCB is available the second half of april 2021.

Click-right on a picture to copy it.

         Purpose.
More and more amateurs suffer from strong QRM, preventing them to receive weaker signals.
A solution for comfortably listening to other radio stations, is the use of a to the internet connected PC, and a low noise "WebSDR" (like "Maasbree" http://websdrmaasbree.hamshack.info:8901/ ). The drawback is that 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 with appropriate data connections, "CATSinc" software can do that for you.

But 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.

        Connections.
Four 3,5mm (or 1/4") stereo busses are used for connecting two inputs and two outputs. An RF filtered "Keyline" bus must be connected by a screened cable to a transmitter "Power Amplifier" relay output. Preferably not connected to the cabinet (ground) of the transmitter.

        Actions.
When transmitting, the transmitter shortens or opens its "Power Amplifier" relay output. The response of this muter (do mute or do not mute when keyline is shortened) can then be set by two jumper blocks jmp1 and jmp2. The two LEDs indicate open or closed "Key line" input.

        Audio.
The stereo input L and R lines are individual loaded by 33 Ohm to simulate a headphone load, ensuring a neutral frequency response and mono output. The headphone and speaker outputs are mono and capacitive RF filtered. The speaker output is designed to be connected to a high impedance active speaker or audio amplifier. It is low-impedance-terminated, to prevent hum during muting.

        Switches.
Two toggle switches enable :
S1 : continues PC sound output,
S2 : continues speaker mute. 
The muter interconnects the "ground" of the muter PCB, and that of the "Speaker" output.

        Power.
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, making fusing unnecessary.

An external 9-16 Vdc supply could be used instead. In that case, all supply "minus" of muter, PC, and "Speaker" will become interconnected.

        Housing.
The double sided PCB has two ground surfaces, and is designed for mounting into a Hammond 1554G ABS box. 
The box stands with its drilled bottom up. Its lid will  rest on the table, no holes will be drilled in it. 

First drill all holes at the bottom and back of the box. 
Only a few Watts are dissipated, so ventilation holes are not needed.
The empty PCB can be used as a drilling template for the bottom of the box. 
The four stereo busses (and mains cable strain relief) can be installed into the back-side of the box.

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.

        Finishing.
The finished PCB can by means of the switch shaft nuts be fastened into the "bottom side" of the box. No other screws are needed. 
Adjust the nuts for nice flush protruding switch shafts.

After installing the PCB, install the four stereo busses into the back-side of the box. Short connecting wires to the stereo busses can be directly soldered into the PCB, or screwed into PCB terminals.