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RE: question is:

I've actually yet to find a FW interface where the first order effects are very low. I'm sure they exist somewhere, but I haven't found one yet. The Metric Halo units are probably the best candidates at this point, but I don't have one (they only work with Macs and I don't have Macs).

I have several ethernet interfaces with very low first order effects, but the second order effects due to proceesors etc are pretty high.

S/PDIF is an interesting case. Almost all implementations have significant first order effects (the PLL used to recover the clock). There IS a way to get around this, send a clock or S/PDIF stream from the dac to the source. In this case the data is synced to the DAC clock so it can be reclocked with the local clock, so you don't need to use the recovered clock. This does a good job of getting rid of first order effects, BUT they still use a PLL based receiver. The jitter from that receiver is still creating ground noise which can get into the rest of the system. Note that a pulse transformer on the input to the S/PDIF receiver makes no difference to this.

There is a way around this: don't use a PLL based receiver when you are syncing the source to the local clock, its unnecessary. I've actually done this using an FPGA to do the S/PDIF decoding, it works very well.

I've not done work on whats going on with cables and why they make the differences in modern DACs so I can't really answer that question. So far I've just been looking at how changes in the computer affect ground plane noise in the DAC.

In reading the posts I see I have not actually described my testing setup. I built a very low noise differential probe which I plug into a spectrum analyzer. The probe "tip" is a short length of shielded twisted pair. I directly solder the two wires to different locations on the ground plane under test. The probe electronics are battery powered and heavily shielded.

I then plug the DAC into different computers and try various different settings on the computers. I do this for several different locations on the ground plane and with different DACs.

John S.

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