[URC] Phone patch circuit

[David Basden]

Hi everyone,

I'm wondering if I could get feedback on a circuit that I made to patch from
a soundcard to a phone handset. It's REALLY not designed to be plugged into a
PSTN line at all (for a start the battery would have to go ;-)), but just to
look like a normal local-loop to the phone. 

There are plenty of circuits on the net to do this sort of thing, but the
ones that don't use active components are all pretty dodgy looking.

I haven't designned anything quite like this before, so if there is anywhere
I have gone badly (or even slightly) wrong, let me know.

Cheers,

David

            9V battery
               | |                 C1                    R2  9k ohm
        +-----||||------------+----||-------+     +---+-----/\/\/---o Output to soundcard
        |      | |            |   10uF      |     |   |    
  phone o                     /             } |#| {   |        C2
                        600   \ R1          } |#| {   +--------||---o Input from soundcard
handset o               ohm   /             } |#| {           10uF
        |                     |             |     |       
        +---------------------+-------------+     +--------/\/\/----o Signal GND
                                              T1       R3  150 Ohm


T1 is a 600-600ohm line isolation transformer taken from a modem. It can't
take more than a few uV before saturating, hence the caps. I really would
have preferred a centre-tap transformer, but I couldn't easily find.
600 - 300/300 one. This one will give really good isolation anyhow ;-)

C1 and C2 are polyester caps taken from the same modem, and should be
large enough to not lose too much of the low-end voice frequencies.

The output to the soundcard should idealy be a 10k ohm match.
The input from the soundcard should idealy be a 1k ohm match.
I've obviously gone more in favor of the higher impedence one
being matched. Both inputs should see an inductive impedence.

R1, R2 and R3 are all 1/2 W resistors.

The phone handset is a current loop, matched at 600 ohm. Even
with it showing as a short, there shouldn't be a large amount of
current flowing across the 9V battery.