Dennis Plunkett wrote:

> As for that Q1 thingo....  . The 12V zen will clamp the line voltage
> and shunt the audio a bit,

  Quite a lot, I'd think, just *asking* for distortion, quite
undesirable for a modem.  I find it a trifle strange to put it there.

> as for Q1, it is configured 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

  I would *hope* it's intended to separate the audio, because there's a
22 µF capacitor on the other side of the transistor.  If it was just a
slow switch on (and why on earth would you *want* a slow switch on?),
you wouldn't really need the 51 ohm resistor at all.

> (51 ohms is not enough to ensure that levels of +3dBr are acheived
> with QI fully on, but should be OK for most users)

  The 2k7 ohm resistor prevents saturation, so the transistor is
definitely in constant-current mode as far as AC is concerned.  As such,
the AC component doesn't see the 51 ohm resistor, only the 2k7.  Again,
saturation is *most* undersirable as it generates distortion.
-- 
  Cheers,
        Paul B.