Coax cable for dac to ?

Discussion in 'Digital Sources' started by CT Jim, May 7, 2018.

  1. CT Jim

    CT Jim AK Subscriber Subscriber

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    As I investigate cables to go with the plethora of new dacs and converters and headphone amps I'm acquiring the Coax plug has been confusing. It appears that the coax hook up is typically a decent shielded RCA ended cable for a so-called balanced hook up? Instead of 3 conductors, L+, R+, and ground -, only two are provided, carrying the L+ and the R+.
    So technically, any RCA cable can be used for a coax hook-up, but the poorly shielded cheap ones may cause a problem.
    Now I can see all 3 connectors on a 3.5mm plug, but this 2 conductor versus 3 is??? What about a Coax to a 3.5mm sound card jack? 2 connectors to a 3 connector jack??
    Do I have that right, or do I need to put a piece of Duct tape over the coax jacks and just not go there?
     

     

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  2. judsonw

    judsonw Well-Known Member

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    First, not knowing what you are looking at does make it a little difficult to conjecture where your questions are coming from, but I will try.

    The Coax in DACs is generally a digital input channel, not an analog output. When used for digital, the digital signal can carry more than one audio channel, so you only need a signal line and a ground line.

    There are a number of ways a digital signal can be transmitted: USB, Optical (TOSLink), and Coax. There are others as well, and there can be a lot of arguments about which is good, which is bad. I generally prefer coax myself.

    For a Balanced DAC, the balanced aspect is generally the analog outputs, where each channel has three connectors, (which to my understanding are) a signal, an inverted signal, and a ground. The point of the inverted signal is that when it reaches the other end, the inverted signal is inverted again and added the to the regular signal. The reason for this is because in theory any noise picked up along the way would be the same for both regular and inverted, so when you invert the inverted, the signal is doubled and the noise cancels itself out.

    The 3.5 mm plug is versatile and can be used for balanced. I have seen that 2.5 mm balanced are becoming a thing too, possibly to help differentiate between the two and possibly just so they can market more cables (depending on how pragmatic or cynical you are feeling). That being said, using a coax to 3.5 mm would not work if you are using the coax out; that signal is digital and the 3.5 mm input is probably wanting an analog input.
     
  3. CT Jim

    CT Jim AK Subscriber Subscriber

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    OK, so coax out should go to coax in, not analog in. Basically, coax out to dac coax input, then continue with the analog connections as necessary. OK, we can leave the 3.5mm and 2.5mm connections out, because they are analog.

    so, in simplistic terms any RCA to RCA cable can be used as a digital coax connector cable? Now some have better shielding, but rather than beat the cables to death, I want to understand the simple part.
     
  4. botrytis

    botrytis Trying not to be a Small Speaker Hoarder Subscriber

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    No - COAX is a 75 ohm cable.
     
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  5. CT Jim

    CT Jim AK Subscriber Subscriber

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    Ok by that, so a coax audio cable is a 75 ohm cable with RCA each end
     
  6. House de Kris

    House de Kris Loud-n-Deep

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    Often times it is good to start with some working definitions of terms to help keep things straight. Coax, or coaxial, simply means a cable constructed with a single conductor completely surrounded by another conductor. The center conductor is typically the signal carrying conductor, with the surrounding conductor being the return path (usually referred to as 'ground' by many). Coax cables have been used in audio for many decades. Even the cheap pack-in cables with the 70s Pioneer cassette decks were coax. A huge majority of audio interconnect cables are coax, with the typical connector being the RCA connector. In the analog world, it is one cable per channel carried. The digital world may also use coaxial cables for signal transmission. These may be terminated in the same connectors as the analog coax cables, RCA. The cables carrying digital signals may carry multiple channels on one cable. Since the frequency carried on digital interfaces is typically way way higher than analog interfaces, the characteristic impedance of the cable becomes important. For digital audio, this impedance has been selected to be 75ohms. Coax cables come in other impedances as well. Often times, a coaxial cable intended for analog audio with RCA connectors on both ends will work for the task of being a digital interconnect. It may not be ideal, may not be the pinnacle of performance, but it will work 'good enough' for many.

    Hope this helps.
     

     

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  7. CT Jim

    CT Jim AK Subscriber Subscriber

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    so would it make sense to use the video cable included in one of those 3-way or 5-way cable sets used for home theater, at least as a test cable? Now that I know what to get, they're not very expensive.
     
  8. House de Kris

    House de Kris Loud-n-Deep

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    Video cables work well for audio. Both composite video (yellow connectors) and component video (red, green, blue connectors) are designated as 75ohm cables. These will work fine for both analog and digital audio. There will always be the possibility of some manufacturer putting together a bogus cable, that just happens. I once tested a pack in cable that came with a VCR that had yellow ends, and it turned out to be a 50ohm cable. It is still not clear to me what you are trying to accomplish, but I don't think that really matters.
     
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