Lots of products are hitting the market to improve the performance of audio streamers and DACs and I wanted to post some information on what's real and not.
Digital Ground Loops
A digital ground loop is exactly like an analog ground loop. Two different devices have two different paths to ground, and the current flow over imperfect conductors causes a differential voltage. We call it digital because the ground loop occurs over a digital connector, such as:
- Coaxial S/PDIF
- USB
- Ethernet (more below)
The symptoms are different however. With an analog ground loop you can have hum. With a digital ground loop you can experience:
- Increased jitter (inaudible to audible harshness)
- Incorrect behavior/clicks (very audible)
- Analog hum
The truth is in 2023 A.D. we should never experience digital jitter but it happens when streamer manufacturers fail to isolate their connectors. Sadly it's not always true that they do.
Lets take these issues one at a time.
Ethernet
Ethernet has almost no chance of creating a ground loop EXCEPT when shielded. The twisted pairs are connected to miniature transformer isolators at both ends. If you use shielded cable you might want to have an isolator at the streamer end to make sure you aren't introducing a new ground path near your gear, but otherwise there's just no way to do a ground loop with Ethernet. Ethernet was exactly designed not to have them so you could run 300' of Cat 5 or better without worrying about it.
Those built-in transformers block 100% of DC and ground loops but there is what's called parasitic capacitance across those isolation transformers, so it's possible, but never measured, that some noise can jump across so (purely in theory) adding an Ethernet isolator could reduce incoming noise to a DAC but sadly no one has taken the time to show this actually happens in a streamer or that you could even hear a benefit if it were blocked.
There's a second reason for considering Ethernet isolators though: Surge protection on long runs or from your cable modem or fiber connector. Any copper that goes from outside your home to inside can be a surge path, so isolating it is important. Use an isolator like this one that meets IEC-60601 only. Stay away from any boutique brand that does not. This standard ensures the device is tested to withstand 4,000 Volts of breakthrough voltage. Also, avoid shunt style surge protectors INSIDE your home for Ethernet. If it has a ground wire, it's the wrong kind. Stick to isolation.
This recommendation is relatively different than what has been suggested in the past, so I encourage you to read this article for the full explanation. The short explanation is that if a lightning surge voltage occurs we want it to stay upstream, and damage as little as possible downstream. Isolators do this while surge protectors encourage surge current to flow downstream. That's bad.
S/PDIF
Optical S/PDIF cables are practically air-gapped. That is, there's no metal connection so no chance of a ground loop. They are ideal in this sense and a great way to troubleshoot. If switching to optical fixes your sound problems it's probably a ground loop. Coaxial S/PDIF however can create a ground loop when the DAC manufacturer fails to use an isolated transformer as the receiver.
USB
Honestly I'm so disappointed and sad that USB receivers for DAC's are not always isolated, but they are not. My Mytek Brooklyn for instance is not isolated this way so when I run a signal from a PC 50' away a hum immediately appears. You don't have to get expensive. Any number of medical grade isolators are available for $50 or under. The bad news here is not all DAC's will work with isolators. Mytek does not for instance.
In my particular case I don't use a PC as the source normally so it's not often an issue. My source is Ethernet.
There have also been published reviews of DACs without isolators in which severe jitter artifacts occur when testing via the USB interface, so this effect is real even if you don't always hear it.
Coaxial TV/Internet
Kind of on a related note is that one major source of ground loops in home entertainment systems is the coaxial cable that feeds the set-top box INCLUDING satellite receivers. They make ground loop isolators for set top boxes and some even have a pass-through for your dish power supply.
