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YouAgain 28th August 2012 12:03 AM

Adding a transformer to a pc's S/PDIF output
I'm looking for some discussion on the correct way to implement a S/PDIF
transformer to a computer motherboard that presently has no transformer isolation on it's digital output. My outboard DAC has a 75 ohm input transformer
which has a 1:1 winding ratio and is loaded by a 75 ohm resistor across the secondary. The transformer which I intend to add to the PC is also 1:1 with a charicteristic impedance of 75 ohms. My thought is to just add it in series with the motherboards digital output. The jack will be isolated from the PC chassis.
What confuses me is that I have looked at the service manuals for my Pioneer, HHB, Denon and NAD CD players and they all have the S/PDIF output transformers connected to the jacks by a 75 ohm resistor in series. This seems to me to be wrong. My thoughts are: since the DAC's input transformer is loaded with the correct impedance on it's secondary. And I'd be using a cable with a charicteristic impedance of 75 ohms. A 1:1 transformer at a CD player output or my PC board output would be happy with the 75 ohm load presented by the DAC WITHOUT any series resistance. I'm hoping that somebody with a deeper understanding of the engineering can tell me why the CD players outputs use a series resistor and if it is just a compromise. I don't have any information on the chariceristics of the transformers used in any of the CD player outputs. I do understand that I need to be sure that the transformer is protected from DC on the primary with a blocking capacitor.

nigelwright7557 28th August 2012 01:28 AM

The 75R is series will be to limit DC current when the transformer is a virtual short.
It also matches the source impednace to the other ends 75R impedance termination.

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