Uncommon Ground Problem - diyAudio
Go Back   Home > Forums > Source & Line > Digital Source

Digital Source Digital Players and Recorders: CD , SACD , Tape, Memory Card, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th April 2014, 07:28 AM   #1
diyAudio Member
 
Join Date: Oct 2013
Default Uncommon Ground Problem

Hi,

I have a rather "uncommon" ground problem about common grounding of two audio circuits.

Basically what I want to to do is to create an external circuit which amplifies both the line-out and line-in of a PC sound card. The circuit is powered from it's own battery and is floating when unconnected. It's a data acquisition system for some sensors which work in the audio frequency band

Here's the catch:

My sound card has a bias voltage on the sleeve of the line-out jack. I took the sound card apart and figured out that they saved the output capacitor and instead put in a buffered voltage divider so that the jack's sleeve is at the center voltage (signal swing from 0 to 3.3V and the sleeve is at 1.65V with respect to earth ground).

The sleeve of the line-in jack however is directly connected to earth ground (measured with a multimeter).

Now I'm designing my interface board with it's own power supply, usually I'd connect the sleeve of both jack's to the negative terminal of the battery. However in my scenario this would happen:

- The LINE-IN jack's sleeve would pull the ground of my circuit to earth potential.

- The LINE-OUT jack's sleeve which is at 1.65V with respect to earth potential would be shorted to earth -> Problem!

Basically my question is what to do with the LINE-OUT jack's biased sleeve

I've attached a drawing of the situation.

PS: It's an external USB sound card with the VIA VT1620A chipset, but this shouldn't matter. They connected USB GND and USB shield together, that's why the ground on the sound card is at earth potential.
Attached Images
File Type: jpg problem.jpg (35.8 KB, 70 views)
  Reply With Quote
Old 8th April 2014, 09:19 AM   #2
Did it Himself
diyAudio Member
 
richie00boy's Avatar
 
Join Date: Nov 2003
Location: Gloucestershire, England, UK
Use a differential amplifier. Connect signal to non-inverting input and virtual ground to inverting input. Connect real ground to 0V of your device.
__________________
www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more.
  Reply With Quote
Old 8th April 2014, 09:31 AM   #3
Mooly is online now Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Have you measured this "bias" voltage when a load is present. Try a 10k and see if it drops.

Also... a DVM may show unpredictable voltages if there is any high frequency hf present. A scope is a better guide, or make a simple low pass filter (say 100k and 1uf) and measure the voltage across the cap for a true DC reading.

(I find it hard to believe that a true DC voltage exists on the output tbh but I suppose you never know )
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 8th April 2014, 10:06 AM   #4
diyAudio Member
 
Join Date: Oct 2013
Quote:
Originally Posted by Mooly View Post
Have you measured this "bias" voltage when a load is present. Try a 10k and see if it drops.

Also... a DVM may show unpredictable voltages if there is any high frequency hf present. A scope is a better guide, or make a simple low pass filter (say 100k and 1uf) and measure the voltage across the cap for a true DC reading.

(I find it hard to believe that a true DC voltage exists on the output tbh but I suppose you never know )
I'll give it a try and hook up a load and check the voltage on the scope.

The DC voltage is only there because of the missing output capacitor, usually you have 0V with respect to the power ground on the sleeve and the signal swings positive and negative. In my case it's the same, the headphones which are usually connected can't reference the 1.65V to any other potential, so from the point of view of the headphones this 1.65V is the zero line and 3.3V is max. positive swing and 0V is max. negative swing.

I wonder what would happen if I directly connect the line-out to the line-in, theoretically it would blow up my sound card!
  Reply With Quote
Old 8th April 2014, 10:20 AM   #5
diyAudio Member
 
Join Date: Oct 2013
Quote:
Originally Posted by richie00boy View Post
Use a differential amplifier. Connect signal to non-inverting input and virtual ground to inverting input. Connect real ground to 0V of your device.
Thanks for your answer! The differential amplifier is a solution indeed, in my case the amplifier is a MC34119 (http://www.promelec.ru/pdf/mc34119.pdf), a simple headphone amplifier.

As far as I understand from the datasheet I could connect the signal to pin V_in and the virtual ground to pin FC1 and the GND of the amplifier goes to my "real" power supply ground on the board.

Another (rather dumb?) question: If I want to add a simple RC low pass filter before the input to the amplifier, can I connect the filter capacitor of the RC filter to the virtual ground of the input?

Thank you very much
Attached Images
File Type: jpg mc34119_example.jpg (86.2 KB, 55 views)
  Reply With Quote
Old 8th April 2014, 10:34 AM   #6
diyAudio Member
 
Juergen Knoop's Avatar
 
Join Date: Jan 2007
Blog Entries: 3
does your circuit needs a dc connection to sound card ground? You could just insert an electrolytic into that path too.
  Reply With Quote
Old 8th April 2014, 11:16 AM   #7
Mooly is online now Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Quote:
Originally Posted by sled View Post
I wonder what would happen if I directly connect the line-out to the line-in, theoretically it would blow up my sound card!
Thats exactly one of the main reasons I find it hard to believe there is DC present by design.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Why are Switching power supplies so uncommon ? ginetto61 Power Supplies 32 15th March 2013 07:30 PM
HiFi Fanatic - Uncommon ODougbo Everything Else 0 26th November 2012 09:58 AM
Uncommon taste in speaker voicing? Need advice poldus Multi-Way 6 15th August 2007 01:47 AM
no ground problem trancy Tubes / Valves 2 27th March 2004 02:53 AM
does connecting mains ground with circuit ground create a ground loop? jarthel Everything Else 0 25th June 2003 01:55 PM


New To Site? Need Help?

All times are GMT. The time now is 08:56 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2