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Digi+ w/Transformer signal Quality

Thomas

January 26, 2016 11:53

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Dear Hifiberry Team,

here my feedback to the newer version of the Digi+ Board with Transformer. This new version is having now the Transformer Shielding connected to the Ground of the Connected device.

My DAC is an T+A DAC8 which is fully Grounded by the power socket. The DAC8 can immediatly signalize signal failures and also has 2 modes for signal locking (via internal masterclock or PLL).

In my testing even with the new version I have short signal errors if I turn on the light in the Room or any other significant Power is switched.

The only way to avoid it is to use the small jumper next to the transformer which actually connects the ground of the DAC with the ground of the Digi+/RPI2. THis fixes the effects but I am also loosing the safety of a galvanic separation.

I guess these interferring Impulses get inducted somewere on the Digi+ Board and find its way into the Transformer were they get Transmitted also to teh output side.

Lets say its a quiet small flaw but maybe the layout or schematic can be optimized to avoid these signal glitches as good as possible.

Maybe the upcomming Metal case if grounded properly with the client device ground can help as well.

with kind regards,

--thomas

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9 comments

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  HiFiBerry team January 26, 2016 14:41

  Hi Thomas,

  unfortunately this is nothing the can be improved by schematics or layout. You really need a metal case and grounding of both side: Raspberry Pi AND the DAC. With a device free-floating like the Raspberry Pi with a standard switching power supply, this kind of issues is quite normal. A metal case will help, but for optimal signal quality the metal case itself should also be grounded.

  Best regards
  Daniel

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  Peter January 26, 2016 17:11

  I am surprised that just switching on a light in the room has that much effect. Most of us can't fabricate our own case, or have DACs which indicate signal errors.I am also wondering about the fluorescent lights in my office now.

  I wonder if there is something that can be done at the power supply, or if a better power supply for the RPi would help?  For $5~$10 for a typical 5V 2A supply, you can't expect much.   I am thinking of something like a smaller version of the analog supply that John Hagensieker shows here.

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  Thomas January 26, 2016 22:53

  Hi Peter,

  I have to say too that I was surprised about the effect as well and at the beginning I was sure that the reason was a unshilded Transformer on the board.

  I belive this is only a small glitch when the magnetic field in the surrounding is changing strongly (like turning on/off the lights ;-). I tried to use different more or less cheap Power Supplies all with the same outcome. I think only a "grounded" Power supply can make a difference, to pull the devices potential down. At the end You need a Concept of grounding your Hifi chain. Either from the DAC side or the Power Supply side ...

  As I indicated the easiest way is to bridge the little Jumper next to the transformer which connects the grounds of the DAC with the ground of the DIGI+. Then those glitches wont occur, but also the galvanic separation is not really there...

  BTW: U can quiet nicely use the TOSLINK which is immune to this effect. In my case the toslink even works ok till 192khz.

  with kind regards,

  --thomas

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  Peter January 27, 2016 02:18

  Hi Thomas,

  I don't actually have any 192KHz material, just 24-96 flac and lower bw.

  So far I am very pleased with both DIGI+, and the DAC+ (which I got first).  I understand about bridging across the jumper, but I am reluctant to do that, because I ordered the transformer version to have galvanic isolation on the link to my AV receiver.  Normally I would prefer the coax over the optical link,  but I can use optical if I switch some of my other inputs. 

  I have not actually connected the DIGI+ to the rest of my system yet.  Just got it days ago, and I am using it with an old Nuforce Icon HDP DAC/headphone amplifier, and trying to compare to the DAC+.

  I am just very surprised that turning the lights on in the room would make enough EMI to create an error on a shielded coax link (!)  Normally you would have to have a wire loop close to the Pi (or another kind of source) to do this.  Perhaps there is a spike on the power line, and it is transmitted to the Pi through the power supply?  Have you tried an ordinary mains-type surge protector/filter?

  Regards,

  Peter

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  Thomas January 27, 2016 10:15

  Hi Peter,

  I think the the Shilded Coax Cable and the shilded Transformer can be excluded from the equation. If such spike on the powerline can go through the Switched Power supply is a difficult question as well... (I doubt it). My current belive is that since the quiet large Ground areas on the PI/Digi+ board are not really grounded they are not protecting the signal path from EMI. And somewere on the analog signal Path of the Digi Board this Glitch get inducted and maybe especially through the nature of the transformer this is interferring the signal badly...

  But without an Ozilloscope we can only speculate... 

  A grounded Power supply would be the solution.... but I did not find any yet. Also a smart placed capacitor could maybe remove those spikes ... but I dont know if this is a best practice in audio design.

  I added a Surge Protector to the Power Supply but it did not help.

  br,

  --thomas

   

   

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  Peter January 27, 2016 19:22

  Hi Tomas,

  I agree that if the Pi with the DIGI+ is only connected to its own un-grounded power supply, and then to a grounded dac, then bridging across the jumper is safe, and probably an improvement.  The DIGI+ will be on the same ground as your DAC, and the coax will be doing its job well.

  The isolated side of the transformer (the square pad of jumper you are referring to in your post) is connected to the shell of the coax cable connector, and from there to the ground of your dac, and if your dac is grounded, to the mains ground at the wall socket, perhaps through the ground of your amplifier also.

  But, if you also have the HDMI cable connected to a TV, the shell of the HDMI cable is connected to the TV ground, and from there to the mains ground, because the TV must have a grounded power supply.  So, the ground plane on the DIGI+ board is connected to the mains ground through the HDMI cable, the TV, and the power cable to the wall socket!

  I measured 0.6 ohms resistance between the DIGI board, and the end of the TV power cable (Fluke 115 meter), so I think this is correct.

  So in this second case if you have both the HDMI cable AND the digital audio coax cable connected to the Pi, then bridging the jumper may not be a good idea.  With a TV and an audio amp in the system, you could be making an opportunity for a ground loop, and some noise could be transmitted either from the TV through the Pi and to your DAC.  Or the opposite, from your audio amp to the DIGI+ and Pi. (I hope that Daniel will correct me if I am wrong, but in this case bridging the jumper is no improvement)

  You can also see a more technical explanation of why the power supply for the Pi is not normally grounded in the first answer from this thread in the RPi forum:

  How to properly ground PSU...

  Respectfully,

  Peter

   

   

  Edited by Peter January 27, 2016 19:33
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  Ned January 31, 2016 20:26

  Hi Hifiberry Team, et. al.,

  If a metal case is one of the recommendations for dealing with this issue, is there one available for the Raspberry 2B + Digi+ combo?  It seems that the metal cases on the Hifiberry web site are only for the older Digi and older Pi boards.

  Any help you can give on this would be greatly appreciated!

  --Ned

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  HiFiBerry team February 01, 2016 06:28

  Hi Ted,

  do you really have a problem with your Digi+? I know that Thomas has some problems, but there are very rare reports like this. In most installations the Digi+ works fine without any problem. A metal case might help, but it will only work well, if it is grounded.

  For most installations, the Digi+ WITHOUT the transformer is the better choice. Galvanic isolation is good if both ends of the connection are grounded, but if one end isn't, it can make things worse. 

  BTW: The metal case for the Digi+ will be available in March.

  Best regards,
  Daniel

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  Gus April 09, 2016 11:12

  Hi all,

  I am planning to get a Digi+ and I have a question regarding the transformer version. Thomas mentioned before that there is a little jumper that can be bridged in case there are issues with the grounding. Is this jumper a switch that can be turned on/off, or is some soldering required?

  Since I am not sure in advance which version will give best results with my hifi, having the transformer version with the option to disable it at will would be great.

  Many thanks is advance for your feedback,

  Gus