No 8.33 problems when I switched from KX155 to GNC 255 with Garmin audio. The same with GTN650 / GNC225 combo on another aircraft. The user configurable sidetone took a bit of work but we got there in the end.
As Peter says it’s I’d most likely that skimping on the wire quality and not paying attention to correct grounding of the shields is likely to be the cause of most of the problems.
I’m using a 430 with KMA20 (!) and Sigtronics intercom. No noticeable difference in 8.33 vs 25Khz audio quality – maybe i need new headsets (or hearing!).
Peter wrote:
It isn’t so much the loss (yes RG400 is a GHz cable) but the leakage through the shield of a crappy coax which feeds back all over the place.
But the leakage (especially at the low power levels used by a VHF COM radio) is absolutely trivial even with RG58 (the stuff I have has a foil screen under the braid).
If the coax shield is radiating, this is usually because of a poor antenna installation, and will happen even with the most expensive, thickly shielded coax. RF in the cockpit is PROB99 caused by the shield radiating, and not by leakage.
