Digital Twilight: Analog audio via UTP twisted pair

Hi all! Every time I want to listen music from my collection in another room (ex. kitchen), I have to take my tablet, connect it to audio system and setup streaming over WiFi. It's boring.

But what about direct audio from sound card to another room without network transcoding? I know there are devices starting from low costs to high-cost multiroom systems. Not my choice. It is interesting for me to make some kind of "multiroom audio" by myself =)

What about requirements?

I don't pretend to "crystal clear" HiEnd audio quality, but sound quality must be sufficent for most mid-range audio equipment (like AV receiver, etc).
Stereo is sufficent. No need in more channels (but it must be easy to add them).
Audio cables may be laid near 220V power in some places (oops)
True galvanic isolation of power supply, low power consumption from single 12V power supply (i have 12V over all my home).

I decided to use balanced lines. Balanced audio signal must be translated via twisted pair. Twisted pair... hmm... UTP Cat5? Why not? =) 2 pairs for signal and 2 pairs for power supply. As opamps have low power consumption there will be no significant noise on audio signal.

So I need two types of modules: transmitter and receiver. One transmitter and several receivers.

Click on any picture for full size.

Transmitter

Here is a circuit schematic for stereo converter from unbalanced input (from sound card in my case) to balanced output. L1 and L2 are 1210 470uH SMD inductors, C3,C4 - 47uF/16V, C6,C7 - nonpolar electrolytic 2.2-10uF (I've used 10uF). All other capacitors - SMD ceramic.

Op-amp: TL074, one IC per channel.

Balanced signal transmitter

Converter is simple: inverting and non-inverting amplifier pair. With parts shown transmitter amplifies input signal twice. Output resistors may be 75-510 Ohm (I've used 100). They protect opamp output from short-circuit in line.

Unbalanced to balanced converter

Receiver

Parts selection is the same as for transmitter.

Balanced signal receiver

Input buffers for differential amplifier were added to allow use of 5% resistors in differential amplifier section and also reduce influence of input voltage dividers resistance.

Balanced to unbalanced converter

Suppressors are used to protect opamp input from possible voltage spikes.

Power supply

TL opamps need true dual supply (+/- 12V shown on schematics). But I have only single +12V.

One way is to use small 50Hz AC transformer. Simple, but I need some kind of remote control. Of course relay can be used on AC side.

The other way is DC-DC converter. I have some IR2153 chips, so there is no problem to make it =) And also it's interesting: will SMPS affect sound quality al low signal levels.

DC-DC converter

This is a simple converter. IR2153 is used in push-pull mode. As IC has no feedback pin, I've added L7812 and L7912 at output. Transformer parameters shown at schematic (designed for 10-15V input). As there no voltage feedback take care of output voltage value at maximum input.

After all units were made, time for test. For my opinion sound quality is fine (tested on come FLAC's). Power converter does not make noise to audio at all. Tested with 20m (don't have more) unshielded twisted pair.