wrote in message
...
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.
=========
Tom -

That is correct, the low-gain or "zero" models (last digit in model number
being zero of sold-state late 1970s /early 1980s vintage radios) from Icom
(IC-730 being one) ..... did have this problem (pre-amp was in microphone
and inside the radio on the input)
http://www.heilsound.com/amateur/tips/all_icom.htm

You are correct in that you MAY not need the voltage divider -- having an
adjustable input such as that with the Jim Mitrenga, N9ART sound card
interface "A Flexible Digital-Mode Interface". BTW, although that
RadioShack relay is no longer available -- there is an NTE equivalent that
is a "drop-in replacement".
http://www.farcircuits.net/
http://www.qsl.net/wm2u/images/interface_n9art.pdf

w9gb