Coaxial SPDIF and transformer, DIR9001

[JanErik]

Is it absolutely necessary to use a transformer, or can I connect the coaxial input directly to the DIR9001 input, perhaps with a 75 ohm load resistor?
Will be using motherboard output.

[abraxalito]

Not essential, no. But if you want the lowest noise (normally means best SQ) then use a transformer to gain isolation from your motherboard.

[JanErik]

Seems some people have used the transformer off an old Ethernet card, would that be OK?

[abraxalito]

Probably it'll work, might not be the lowest jitter though because they're not usually very high primary inductance. Whether you're going to be bothered by jitter depends on the kind of DAC its feeding - certain S-D ones (without on-chip switched cap filters) are the most sensitive.

[JanErik]

It will be feeding a PCM1794A.

[abraxalito]

That's one of the more jitter sensitive ones in my estimation as its using off-board I/V conversion. If you want the best sound then probably best pay at least some attention to how the transformer is distorting the S/PDIF waveform - you need high primary inductance and low leakage inductance. But then again if you use the TI recommended opamps for I/V (NE5534) its attention that's not worth paying because the opamps will probably be the weakest link for SQ, not jitter.

[MoeStevens]

The PE-65612, sold by Mouser (and others) is a transformer made for this application. currently it's only $5.26 (US). Futher, The 9001 data sheet page 8, (last two bullets) indicates that with a coaxial input, a buffer is needed. Pavouk suggests a 75176, which requires a 5 volt supply in addition to the 3.3 volts needed by the CD9001 itself. The 75176 output is TTL, which is acceptable to the 9001's input. Careful attention to the transmission and input signal IMHO, will reduce the chance of jitter.