DACs with transformer isolated co-axial SPDIF input?

[Eah123]

I'm looking for DACs in the <$300 USD range that have a transformer isolated co-axial SPDIF input. I saw that this was a feature for the JDS Labs EL DAC II+, ("Coaxial S/PDIF input is isolated using a Pulse transformer to minimize the possibility of ground loop noise.") which seems like a good option, but I'd like to audition it vs. other DACs.

The reason I need this is that I'm distributing a SPDIF signal from my streamer (Sonos Connect 44.1khz/16b) to multiple rooms via BNC coax which are the existing house wiring for cable TV . I've tested this using a cheap $10 DAC and found that BNC coax introduces a strong ground loop hum, which I'm able to defeat by using a DDC coax to optical to galvanically isolate the DAC. Although I'm not opposed to keeping the DDC in the stack since it doesn't introduce any noticeable delay, it would great to be able to consolidate 2 devices into 1.

Any recommendations?

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[antcollinet]

I'm looking for DACs in the <$300 USD range that have a transformer isolated co-axial SPDIF input. I saw that this was a feature for the JDS Labs EL DAC II+, ("Coaxial S/PDIF input is isolated using a Pulse transformer to minimize the possibility of ground loop noise.") which seems like a good option, but I'd like to audition it vs. other DACs.

The reason I need this is that I'm distributing a SPDIF signal from my streamer (Sonos Connect 44.1khz/16b) to multiple rooms via BNC coax which are the existing house wiring for cable TV . I've tested this using a cheap $10 DAC and found that BNC coax introduces a strong ground loop hum, which I'm able to defeat by using a DDC coax to optical to galvanically isolate the DAC. Although I'm not opposed to keeping the DDC in the stack since it doesn't introduce any noticeable delay, it would great to be able to consolidate 2 devices into 1.

Any recommendations?

​

Unless you are using an active splitter - don't do it.

Coax digital requires impedance matched (75 ohm) cable runs point to point. If it splits off all over the place to go to different rooms you are going to get all sorts of reflections and signal distortions. I'm surprised you've managed to get it to work at all.

Other than that, I don't know how to find out which DACS isolate the SPDIF - it doesn't seem to be anything that manufacturers specify. You could try putting a coax to toslink converter (Around £20) at the DAC end, and go toslink into the DAC. That would eliminate ground loop issues.

 

[Eah123]

Unless you are using an active splitter - don't do it.

Coax digital requires impedance matched (75 ohm) cable runs point to point. If it splits off all over the place to go to different rooms you are going to get all sorts of reflections and signal distortions. I'm surprised you've managed to get it to work at all.

Other than that, I don't know how to find out which DACS isolate the SPDIF - it doesn't seem to be anything that manufacturers specify. You could try putting a coax to toslink converter (Around £20) at the DAC end, and go toslink into the DAC. That would eliminate ground loop issues.

Your solution is what I am currently using, and it does work.

I understand the issue with impedance mismatch between RCA and BNC coax cables, but if it works and no issues with signal drop out, why not do it? The question is only rhetorical. The beauty of digital signals is that it is low voltage so no danger of harming the equipment and it either transmits or you get garbage.

Interesting thing I read in this article is that a good SPDIF coax input is supposed to be transformer isolated, so possibly my cheap DAC just cut this corner. I’m going to poke around to see if any DACs provide circuit diagrams which would indicate this.

https://www.soundonsound.com/sound-advice/q-can-digital-audio-cables-cause-ground-loops

Thanks for your response.

 

[Eah123]

In case anyone is reading this and looking for a similar solution, my research indicates that the coax to optical conversion and using the optical SPDIF input to the DAC is the only way to completely remove ground loop hum. This is because the SPDIF coax specification requires hard grounding. Using an isolation transformer can reduce ground loop hum, but cannot completely eliminate it.

 

[somebodyelse]

Interesting thing I read in this article is that a good SPDIF coax input is supposed to be transformer isolated, so possibly my cheap DAC just cut this corner. I’m going to poke around to see if any DACs provide circuit diagrams which would indicate this.

https://www.soundonsound.com/sound-advice/q-can-digital-audio-cables-cause-ground-loops

The AES and EBU pro specs require the sending end to have the pulse transformer, as shown in the article. The consumer specs don't require a transformer at all unfortunately. I think there's a Japanese spec that requires the transformer at both ends, but few follow this. That's why it's unusual to find a DAC with a transformer on the input. Your JDS is very much the exception.

 

[somebodyelse]

https://www.sonifex.co.uk/redbox/rbaes4b3_ld.shtml 4 x 3-way passive (transformer) splitter - might be similar to what you're after.

 

[Randyman...]

Assuming your Sonos is a 2-prong AC input - Floating your Sonos' AC input with a mains isolation transformer might help break the loop(s) upstream - but you will still end up double or triple terminating the 75 Ohm output with multiple DAC's (some digital output drivers might handle that - lots probably won't) - and will still have multiple DAC's grounded on the RX end via their shields.

In the past for low-current-draw items, I've used a Camera isolation AC transformer specifically designed to get rid of hum-bars (AC mains ground loops) in old analog cameras. Many varieties of mains isolation transformers are available. If its only a 3rd prong ground loop, you can use a "Hum-X" (Ebtech) diode-based ground-loop isolator opposed to lifting the AC ground (never recommended!).