I had used Heil Proset with HC4 mic at least 15 years with my IC-781. At the beginning there was some noise (hum), so i wrapped mic/headphone cable over ferrite rod (4 inches long).(I didnīt checked ferrite material, but the thing helped.) The hum dissapeared, my audio was excellent..
In the May this year I heard one U.S. ham using TEN TEC 715 RF Speech Processor. I was positively surprised with audio, I had heard. I bought the new toy, just inserted it between mic and transceiver. The hum again appeared.. Any ideas? Thanks..