Cheap radio audio interface

This turned into a surprisingly tricky project… I have a signalink as my main audio interface box to my radios. It’s basically a USB sound card that connects to the data port of my radios and lets me use digital modes. It also has a couple of knobs to control levels. It’s great, I love it, but as it costs about €130 I wasn’t going to bu another one just for the convenience of not having to move cables around when I want to use it on a different radio.

The solution – get a cheap USB sound card and make up a cable for it to the data port. I got a sabrent device from amazon and can make up the cable from spares. Simples.

Not so. The cable work fine on receive, but transmit was a different story. There is just no level of either application or device that will make transmit work properly. It’s either too faint to trigger VOX or too loud so that it’s instantly at full power and kicking the ALC meter to the max. Even using the CAT and PTT didn’t help.

This is where those little dials on the signalink come in handy. I put in a various potentiometers in the way to see if I could bring the signal down, and only had limited success. Even adding in 440k Ω couldn’t make the FT-450D happy. So in the end I’ve gone with just putting a 47kΩ resistor in the cable and opting to use this just with the FT-857 which seems happy enough with that if I choose the sound and application volume. That’s ok as the main use for this will be FT8 on 2m, the FT-450D can do HF & 6m with the signalink.

This seems to be a peculiarity to Yaesu radios, or at least to the 450D and to a lesser extent the 8×7/8×8 series. More modern radios come with a USB sound card included, and even those that dont seem to have better matching of levels to a soundcard or some way to adjust the gain of the data input. But with the 857 & 450D you’re stuck with the signal you give it.

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