Forum Rule: Always post complete source code & details to reproduce any issue!
Results 1 to 5 of 5

Thread: Filtering VGND DC

  1. #1
    Junior Member
    Join Date
    Mar 2014
    Posts
    19

    Filtering VGND DC

    I installed my working prototype in an aluminum box before I realized the metal barrel 3.5 jack is shorting GND to VGND.

    As I understand it, the problem with VGN is that it has DC present.

    Is it possible to effectively filter out the DC with a capacitor, without degrading audio quality, allowing GND and VGND to abide?

    If so, what would the circuit look like?

  2. #2
    Senior Member+ Theremingenieur's Avatar
    Join Date
    Feb 2014
    Location
    Colmar, France
    Posts
    2,459
    You might cut the connection between the 3.5 jack and the SGTL's HPVGND pin and insert a 220uF electrolytic or tantalum capacitor (+side towards the SGTL pin 4). Will reduce the channel separation for lowest frequencies a little bit, but that's the price to pay.

    If not, why not replace the jack by a bigger insulated variant where the plastic is long enough to go fully through the enclosure, so that no conducting contact can happen?

  3. #3
    Junior Member
    Join Date
    Mar 2014
    Posts
    19
    Thanks for your response.

    I think it's best to isolate the 3.5 jack as a quick workaround.

    I have reservations about the VDC, however.

    On any given device, many end users employ the headphone jack as a line out when necessary. I know I do.

    Not sure how many devices are expecting VDC on input. And this could cause VGND to be shorted at some point beyond our control.

    This seems an unnecessary danger for a feature I can't find a use for.

    Why does the SGTL design allow VDC here at all?

    In any case, in the manual, https://www.pjrc.com/teensy/SGTL5000.pdf, p 60, fig 20, shows a cap-coupled design where VGND is not even connected.

    Click image for larger version. 

Name:	Screen Shot 2018-03-13 at 12.57.59 PM.png 
Views:	42 
Size:	82.6 KB 
ID:	13282

    I'll give this a try.

  4. #4
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Posts
    20,344
    The main reason is to avoid those huge 220 uF capacitors. They're not that expensive, but they require a lot of PCB space and stand up much taller than everything else.

  5. #5
    Junior Member
    Join Date
    Mar 2014
    Posts
    19
    Quote Originally Posted by PaulStoffregen View Post
    The main reason is to avoid those huge 220 uF capacitors. They're not that expensive, but they require a lot of PCB space and stand up much taller than everything else.
    I see. Fortunately I have room if I choose to go that way. thanks Paul

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •