PSK31 Interface for the K2

I have been running PSK 31 for about 2 years now with an extremely simple interface to the K2, namely the received signal taken out before it gets to volume potentiometer in the K2 and fed directly into the microphone imput of my old trusted IBM A20p. The transmit side was equally simple, but there I had at least put in a 10K resistor before feeding the speaker output of the IBM lap-top directly into the KSB2.

I admit that it is crude and primitive, but it worked and worked and .......

Well, eventually I decided that the time has come to build a somewhat better interface, one with a bit of amplification and above all a pair of transformers, so that there is a complete galvanic isolation between the K2 and the PC.

This is the amount of space that the good people from Elecraft has left over when you have a K2 with KPA100 installed. And if you also have the KDSP option installed (which I'm planning on), you have even less space to add something.
What you have is an area 1 cm in heigth, 8 cm in width and 2 cm in depth.

Here is the diagram of the circuit that I have designed. The ideas have been stolen from the best.

The greatest hurdle was to find a 1:1 transformer with a impedance of 600 Ohm and a physical size that made it possible for the transformer to fit inside the K2. There is one, but with a price of around 100 US$ each I decided to look for something which was a bit cheaper. Here is the solition: A Ferrite-bead with 10 mm outer diameter and 6 mm inner-diameter wound with 1 meter of 0.2 mm copper wire as the primary. The secondary is also 1 meter of 0.2 mm copper wire. The transformer is 1:1, but is definitely not 600 Ohm impedance.
But since my old setup worked, why should this setup not work - so I decided to give it a try.

After have fiddled a bit with the PCB layout I finally managed to get it to stay with the boundaries that was specified.

This is what the assembled board looks like viewed from the bottom side.

This is what the assembled looks like view from the component side.

The connection to the PC is done using a mini-Din connector, where I have removed the shield, and also made the hole destined for a BNC connector round.

Here is the fully assembled and installed board. And it works too.