IC-7300 Interfacing an external RF Speech processor


Why use an external speech processor? Isn’t the one in the ICOM good enough? I have used a DF4ZS PEP RF-Speech clipper for some time with my Kenwood TS-2000 and the effective speech power is vastly improved using this device that connects between the Microphone and the rig. This is not a simple audio speech compressor as is in the IC-7300. The RF Clipper modulates a signal at 455KHz and then detects it back to audio. In so doing the audio is processed to get the maximum possible level of the most important part of the audio spectrum and does not suffer from the “splatter” caused when trying to do this with the simple audio compressors as fitted in most modern rigs like the IC-7300.

I spent sometime trying to get the RF-Clipper to work with the IC-7300 and had major problems with a constant tone (possibly feedback, possibly RF ingress) while the RF-Clipper was connected to the microphone socket. Of course ICOM have their stupid phantom power on the mic lead, which meant I had to add a DC blocking capacitor. Perhaps this was part of the problem, perhaps not. In any case, whatever I did, I could not get the external speech processor to work correctly when plugged into the IC-7300 microphone socket.

I then had an idea – why not try feeding it through the accessory socket? This input is normally used for input from mixer boards an the like and is a 1K 1v input WITHOUT any stupid phantom power. I made up a lead and while the accessory socket can also supply 12v at up to one amp, and the speech clipper only draws 50 mA, the lead became a Y-lead with a phono plug to the power input on the speech processor and an 8 pin mic. plug to take the audio and PTT output from the speech processor. My initial test hit a problem. While trying to plug the leads in, the centre of the phono plug touched earth and the IC-7300 went off and would not power up again. It turned out, luckily, to just be the internal fuse in the IC-7300 which had blown and once it was replaced with the spare that is supplied with the IC-7300, it all worked again. My cable is now modified and a 100 mA fuse added in case this problem occurs again.

End result – the RF Speech clipper runs fine via the accessory socket with no feedback or RF ingress to be found.