Audio Mute Project

One of the most perplexing and difficult challenges in adding an SDR Receiver to a traditional HF Transceiver is operating CW. There is always blowback in between characters coming through your headphones or speakers - even when using an SDR Switch. My switch of choice is the RX Audite by QRO.cz., which is one of the best commercially available switches on the market. It does its job beautifully. You connect the SEND jack of the transceiver (in my case, the Icom 775DSP) to the PTT jack of the RX Audite. Next, connect the SDR Receiver (in my case, the SDR Play RSP-Duo) to the SMA jack marked SDR on the RX Audite. Finally, connect the transceiver and the antenna PL-259 connectors to the RX Audite, and you're done. The RX Audite also has a separate receive antenna SMA connector. The blowback was especially severe when using a separate antenna. So I opted to use a single antenna and build an audio muting circuit. After a failed attempt with one Amazon relay, I opted to use an HiLetgo 2-channel 12V relay. They were very cheap - a set of two for $8.95. Google AI helped me set up all the connections. I use a Rolls MX51 stereo audio mixer, where I chose to route the audio output of my Mac's SDR Connect through two normally open connections on the relay. The SEND output of the Icom 775DSP connects to the IN side of the relay, and the PTT IN on the SDR switch (in my case, the RX Audite). The relay module is set up to shunt the main to ground to mute the audio. I use headphones with a binaural effect, which works great on CW, courtesy of an audio plugin called SpectralPan, routed through a plug-in host called Insert Basic.  The audio is muted when transmitting whenever the key sends the transmit signal to the relay, thus giving a much better audio experience when operating. Here are photos of the relay, its connections, and the project box I put it in. 

There are virtually no commercial option for this available to the ham community. There was one, but when I inquired, I received no reply. So I decided to build one. The of us who prefer the big, heavy radios operationally, but want the high-quality spectrum scope of software packages such as SDR Console (Windows) or SDR Connect (Mac), this is part of the solution for you. You'll still need an SDR switch of some kind, however, lest you risk "blowing up" your SDR receiver post haste. But this solution gives you a new lease on life for pleasant audio reception, especially on CW. Of course, full QSK still isn't advised, but you can do semi-QSK by setting the delay at the appropriate level. Combining the binaural audio with this mute circuit gives the operator a whole new listening experience with your external SDR.
Another issue to be addressed is delayed sidetone, and the SDR Receiver hearing the leaked CW via the antenna in a delayed fashion. This makes sending near impossible. The circuit described above addresses the audio issue, but what about your own CW sidetone? You will need an external keyer that can feed the sidetone audio into your mixer. I found that attempting to use the built in transceiver sidetone results in a noisy experience due to RFI. Instead, I use an MFJ -495 Memory keyer and feed its real-time sidetone audio into my mixer, which blends seemlessly with my SDR receive audio.
If you have any questions about this mute relay circuit, feel free to email me at kg4o@kg4o.com.

PS: I recently built a second audio mute circuit for our club station KG4F. The rig is an Icom 7600, and instead of a separate keyed for sidetone, I was able to use the built-in keyed. 

73,
Scott, KG4O