At 23:10 2/10/99 +1000, you wrote:
>Michael Rigby-Jones wrote:
>
>>>                      | < Connect to negative terminal of PS/Battery
>>> ---------+           |     +-------------+
>>> modem A  |Red -------+-----| modem B     |
>>>          |Green -------+---|             |
>>> ---------+             |   +-------------+
>>>                        | < Connect to positive terminal of PS/Battery
>
>> This surely can not be correct, the presence of a nice low impedance
>> battery or PSU would effectively short the lines for any signal,
>> making comms impossible.  I thought the the supply was effectively in
>> SERIES with the line?
>
>  Well PICked!  I let that one pass when I saw it, but now that you
>mention it, the answer is that it should have an inductor or "virtual
>inductor" in series with the power supply.  "Real" transmission bridges
>use a balanced inductor.
>
>
>  The virtual inductor circuit was referenced a couple of weeks back at
>http://209.1.238.246/1st_pages/A1208.htm (it's Q1 in the circuit) and
>has some desirable characteristics over a real inductor including - no
>inductive "kick".
>--
>  Cheers,
>        Paul B.
>
>


Yeh, Paul I seen this too, and that stuff on TIP and RING.

As for that Q1 thingo....  . The 12V zen will clamp the line voltage and
shunt the audio a bit, as for Q1, it is confuigured in a slow start
configuration that also provides a bit of dynamic current limit control the
intention is not to seperate the audio, but is a side advantage (51 ohms is
not enough to ensure that levels of +3dBr are acheived with QI fully on,
but should be OK for most users)


Dennis