If used at all, most digital transformers used in the outputs of CD transports or the input of d-to-a converters are cheap jobs with ferrite cores, in order to improve on this we have commissioned a toroidal core made from very thin laminate spun 80% nickel strip, housed in a small plastic “doughnut”. Buy one, compare the quality and be prepared to be shocked, there are good reasons why Audio Note Level One and higher Level 1xoversampling™ d-to-a converters have the reputation they have and the digital interface transformers have a great deal to do with this.
The three-wire version is for AES/EBU inputs with balanced XLR inputs, see Q&A plus diagrams of how to wire the above transformers below.
Some Q & A’s regarding the 2 and 3-wire digital transformers.
Do a continuity check of the 2 (x 3) twisted wires, which gives you 3 electrical pairings.
1. The 2 pairs are arbitrarily selected for the 110R side, and the remaining for the 880R side?
That’s right, all three windings are identical.
2. It does not matter which of the set of 3 twisted wires is selected as the "start" wires?
Right again, as long as the same group of 3 ends is always referred to as the start set. The other set are of course the finishes of those wires.
3. 1/2 watt 880R Audio Note Tantalum resistors do not appear to be available. I intend to use 1200R and 3300R resistors in parallel instead.
Use 910R, the specification for the AES/EBU termination impedance is +-20%.
4. Is the 880R side facing the DAC (chip) side? If so, how are the 3 110R wires hooked up to the AES/EBU e.g. "start" to + signal, "start/finish" pair to ? (DGND ?), finish to - signal? If not, what is the hook up wiring to be done?
Yes, the 880R side faces into the DAC circuitry. The 220R side “start” goes to the +XLR pin, the “start/finish” goes to gnd, “finish” goes to the –XLR pin. The phasing of the secondary is irrelevant as the information is biphase mark encoded. But you may like to keep thins as before by ensuring that the “starts” and “finishes” of the new and original transformer are kept go to the same points.