Digital audio isolation

Isolator for digital audio is needed in cases where coaxial S/PDIF digital audio link forms a ground loop in your audio system. Typically in coaxial S/PDIF connections the coaxial cable shield is connected to equipment cases. This arrangement easily creates ground loops which can cause noise problems in various places in audio system. A suitable transformer can be used to isolate the coaxial S/PDIF signal and avoid ground loop problems (other option is to use optical connection instead of coax). Here is the circuit of the S/PDIF signal isolator I have used:


ELFA catalogue product 56-550-55 “PE65612 Trafo dig.siirt.” is a tranformer suitable for isolating S/PDIF digital audio signal. The manufacturer for this product is Pulse Engineering and their product code is PE-65612.

Ratio: 1:1
Bandwidth: 100 kHz-55 MHz ±3 dB


Here is a picture of the S/PDIF signal isoltor I built (box open):



More information on S/PDIF and related circuits can be found on my SPDIF document.


  1. Adolfo says:

    Hi, since I found it very interesting and since I have on hand some of these transformers, can I have a more clear picture of the box, or at least an easy schematics of how to connect the 4 pins of the trafo, please? thanks in advance! Adolfo

  2. tomi says:

    I just added a new circuit diagram drawing that shows the exact connection of the transformer I used in my ciruit I built. The RCA connector on the left connects to transformer primary and the RCA connector on the right the secondary. It ideal direction for the signal would be from left to right throught this isolator (although the circuit works also well on other direction).

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    There is an RSS feed button on the right…

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  18. Tomi Engdahl says:

    Use plastic optical fiber for isolation

    Don’t overlook using fiber optics to achieve isolation. The Navy learned to love optical fibers since they are insensitive to EMP pulses from nuclear weapons. Glass fibers are expensive, and you can expect to pay 25 to 100 dollars for the transmitters and receivers. Then again, glass fibers work to GHz frequencies. A much more cost-effective solution is plastic optical fiber (POF). Here data rates are limited to 10 or 100 MHz, but the costs are closer to 5 dollars for transceivers.

  19. Tomi Engdahl says:

    Do It Yourself Musician #4 – Fix USB Audio Noise with HifimeDIY USB Isolator

    use the HifimeDIY USB Isolator to solve a USB ground loop problem in my home recording setup. If you have a high pitched whining noise in your USB audio system, take a look at this video.

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