If you stream music from the internet, I can't recommend this more highly


I had been using a Roon Nucleus to stream Qobuz, with my Chord Qutest directly connected to the Nucleus. I thought I was getting pretty decent sound quality. And then I got a marketing email from Small Green Computer touting some of their optical gear. The basic idea is that normal cables and connections used to stream from the internet pick up noise of one kind or another (radio frequencies and electromagnetic something or other). But fiber optic cables and their connections/interfaces do not. I don’t know anything about anything, but it made theoretical sense to me, it wasn’t a huge amount of money ($1,400), and with a 30 day return policy I figured I could always return it if I didn’t hear any improvement. Well, I didn’t just hear a slight improvement; it was like turning on the lights in a dark room. Much greater clarity and detail, much better micro and macro dynamics, better timbre to acoustic instruments -- overall just more lifelike. Two quick examples: I’ve listened to some of Steely Dan’s top songs 100s of times over the course of my life, and this is the first time I’d ever noticed a particular and very subtle sound characteristic of Fagen’s keyboard in Babylon Sister. It’s hard to describe, but it’s like there’s a slight sound of air being exhaled by it. The other example: the specific timbre of whatever percussive instrument is used at the beginning of Copeland’s "Fanfare for the Common Man" (a recording by the Minnesota Orchestra). There’s more of a metallic sound than a drum skin sound to it that I didn’t know was there before. The metallic sound starts in the center and then projects out and to the sides, like a wave washing over you. Anyway, I’m just thrilled about having stumbled upon the whole "optical" thing and felt obligated to let others know about it. If you stream music over the internet, I highly recommend giving it a try. (The product I got was the opticalRendu, with the linear power supply option, and the Fiber Ethernet Converter Bundle option.)
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I didn't read all the posts, have a simple question. I use wifi with Node2 and have tried both, coax and optical connection to my DAC. According to all this, optical should sound cleaner  (as a fiber filter), however coax cable sounds better. A contradiction or I am missing something? 
Your DAC likely is not able to deal with the higher jitter of an optical interface. Coax has less jitter.
I’ve been using fiber optic between network switch and DAC for almost two years. For me, it’s such an improvement over straight Ethernet, I’ll never go back. Galvanic isolation/ separation is probably the key here. As with most things hifi, doing a lot of little things right leads to noticeable improvements. I put time, effort, thought and money into every component in my network system. It all matters when it comes to digital streaming, just like it matters when it comes to having a great phono/ turn table setup. 
niodari, my experience is that Toslink is poorer as a link in comparison to digital coax. There may be exceptions, but I have not encountered any yet. The discussion on upgrade here is conversion from ethernet to optical. I would not make the mistake of thinking that because the Toslink is inferior to your DAC, it would not be efficacious to use optical connection in eliminating noise/improving the playback by replacing Ethernet. I wrote the article the OP is referencing in regard to the SMC/SONORE products, and discourage people from the conclusion that these two implementations of fiber optic result in similar outcomes. 

I also encourage people to listen to those who have done demo, owned and/or compared.