On 20 Jun, 13:34, Postipoika
wrote:

Please note that a single trafo only in mic line doesn't isolate
satisfactory.
Otherwise there will still remain humming earth loops.

If your rig has a phone patch or a line input it is preferred to use
these. You need not to play with plugs .
In many rigs the VOX is not controlled via line input. In that case PTT
rules on the air.

Be ready to use a trafo also in headphone output line and a relay or an
opto isolator for PTT.

It is most simple to use COM port RTS or DTR for PTT.
KISS: Connect a high bright led and a 5V dil relay in series directly
into RTS. Led is very nice to show "On the Air".

postipoika

Hi Postipoika,

Unfortunately, my rig doesn't appear to have a line input, so using
the mic connector looks like the only option.

I had already planned to use another transformer to isolate the
headphone socket on the radio from the soundcard, and an optoisolator
on on RTS for PTT, although I may get away with just using the VOX.