"Brent Brown" writes: > A proper optical fibre link should perform better than copper +=20 > optocouplers. Will for > sure be something commercially available... probably at commercial prices= =20 > :-( Thank you to everybody who replied. =20 Dwayne Reid writes: >The standard approach is to use an audio isolation transformer. I=20 >have had very good success with issues such as what you are describing. > >Shameless plug: my company sells little isolation transformer modules=20 >for not much money. These use a semi-custom transformer from=20 >Datatronics and really do work very well. I think that cost is=20 >around Can $25 in singles (soon to increase due to supplier cost=20 >increases) plus shipping. >Let me know if you want to try this out. I'll arrange to send one to=20 >you and you can either pay for it or return it as you see fit. That is quite reasonable and thanks for the information. You got me to thinking, though, and I have a large spool of CAT 3 cable which would provide 4 twisted pairs of telephone wire. I do have some little 1-to-1 isolation audio transformers so the only problem would be if the windings of the transformer capacitively coupled the low-level EMI smog in to the radio. The common mode signal in this case is the low-level RF noise while the differential signal is the desired audio. The noise shouldn't make it across the transformer unless it does so via capacitive coupling. I'll just have to try it and see. The fiber optic approach that Brent Brown describes would be a great solution as there would be nothing conductive between the ends of the cable. I imagine the best fiber optic link would be either a digital carrier made by pulses of light at some high sampling rate such as 10 Mb/S or so with a decoder back to audio at the other end but then you get the possibility of your digital link generating more noise to have to deal with which totally defeats the purpose. Before I retired in 2015, I worked at Oklahoma State University in our Network Operations Group which managed our Ethernet and telephone networks on campus and the links to our auxiliary campuses and the Internet. The thing that impressed me about fiber optics is that the fiber isn't that terribly expensive but terminating it is. You've got to polish the cut end of the fiber until it is optically flat and free of deformations or one must use an electric arc to melt two ends of fiber such that they fuse together creating one continuous fiber. Equipment that will do that reliably is quite expensive. Some of the consumer HiFi gear that uses fiber-optic audio cables may use plastic fibers for all I know but still terminating them is not trivial. Basically, if the fiber optic connection is meant to be easily connected and disconnected, you polish and lose 2 or 3 DB per interface or you fuse if you are permanently joining a fiber to something that you don't plan to remove under normal circumstances. Those fusion splices don't have any loss since there is no boundary. In this case, thanks to all. I'll temporarily connect the audio to a short jumper and the isolation transformer and then the long line from the secondary of the transformer to the audio mixer and see if the smog survives all that. If it does, it's back to the drawing board for something different. Martin McCormick WB5AGZ --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .