Audio isolation transformer

  
If you want to build one yourself, you have to get two audio transformers which have 1:1 transformation ratio and greater than 1 kohm impedance. There are high quality audio transformers in the markes that meet those specs, but those can be quite expensive. Another option it to use 600:600 ohm isolation transformers widely available for telecommunications and other uses. Those are not that high quality as good audio transformers, but can be well adequate for many not so demanding multimedia applications like computer audio if suitable transformer is selected.
Audio isolation transformer - schematic

I built some of my audio line isolators usign two high quality telephone line coupling transformers which have 600 ohm impedance (I built later some new ones using some high quality audio transformers). This is the most commonly transformer type used in high-speed modems. Best of those are quite wideband devices (far more bandwidth than usual 300-3400 Hz as used in telephone). Using two of those transformers and few RCA connectors made quite satisfactory (but not really hifi) audio isolator. The connection is easy: connect primary side of the transformer to one audio connector and secondary to other. The frequency measurements were made with Nacamichi T-100 Audio Analyzer and the isolator circuit was connected between it's 600 ohm output and 50 kohm input. I don't know if EOP Z1612 transformers are still available at Farnell, but you can try. If you are looking for other high quality transformers which could be a used, I would try ETAL P2001. I haven't been able to test their performance in this application, but they have proven to good transformers in other laboratory test and applications. Avoid cheapest telephone and audio transformers, because their performance is very poor at frequencies over about 5 kHz (for example Radio Shack (273-1374)). Notice one thing in the transformer wiring: Many transformers have the coil starting marked with dot. If you put the audio signal to the transformer in such way that the center...



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