On Sun, 9 Mar 2014, Ralph D. wrote:

Hi,

I need a cable (ideally about 20ft), stereo 3.5mm male/male, highly
shielded to connect my audio source to a transmitter across a room. This
cable passes many sources of interference, so the shielding is critical.

Just use coax. Even something small like RG174 likely has better
shielding than many audio cables.

Of course, if there is a real issue here, little stereo plugs end up
leaving an unshielded area for the RF to get in. Changing to something
else would complete the shield. I know I'd use BNC connectors for audio
connectors if that pile of 200 male connectors I found on the sidewalk
had included 200 female BNC connectors as well.

Make sure the output feeding the coax is low impedance, that's not for
feeding the coax, but a high impedance would make it easier for the RF to
be picked up. And terminate at the transmitter end, so that point sees
relatively low impedance.

Make sure the usual low value bypass capacitors are at the input to that
transmitter, so even if there is RF pickup, it is bypassed to ground and
won't be rectified by the input stage. Get some ferrite beads (the tiny
ones, and put them on the lead between the input jack of the trnasmitter
and the first stage, or the traditional method would be an RF choke of
suitable value.

Look in any radio handbook and there should be a section on RF
interference, which would describe how to keep RF out of the first stage
of an audio amplifier.

Michael






I would prefer to get it from Amazon so as to get it quick with Prime
(incredibly narrow window of free-time during a currently very busy
schedule) but would be OK with ordering from one of the Ham sites if
turnaround time is very good. I could not get a good search parameter on
Amazon that didn't turn up thousands of hits that I just don't have time to
sift through right now.

Any good cable with known good isolation would be good. I have ferrite
chokes I can use, but would prefer a cable of suitable quality that did not
need them (current cable is getting interference even with them as this is
not just run-of-the-mill 60cycle stuff).

Any good suggestions would be appreciated.


Thanks!