Ground loop problem in DIY pre-amp - diyAudio
Go Back   Home > Forums > General Interest > Car Audio

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 16th November 2004, 02:55 AM   #1
pfn is offline pfn  United States
diyAudio Member
 
Join Date: Nov 2004
Location: SF BA, CA
Unhappy Ground loop problem in DIY pre-amp

Yay, a forum for audio DIYers :-)

Here's my basic setup:

iPod into volume controller, volume controller into amp in trunk. Currently, I'm using a 100k dual-gang pot to control volume, and, with this, I have no noise. The pot is a cheap piece of junk that I would like to improve on by using something more advanced and robust.

I found this circuit online and would like to use it for performing volume control:

http://www.elecdesign.com/Files/29/2531/Figure_01.gif

Its workings is described by:

http://www.elecdesign.com/Articles/A...2531/2531.html

I've built up the circuit as diagrammed, and tried installing it into my system such that it replaces my current pot. It's powered by a LM2940T regulating car power down to 5V. With the car off, it sounds beautiful, no noise at all. Once the car is turned on, though, all sorts of noises associated with ground loops show through, alternator whine, ignition ticking, etc.

The preamp and my iPod are both grounded by the same connection (factory ground point connected to console sheet metal).

One thing I haven't tried is connecting pin 6 of the digital pots directly to audio signal ground, and not connecting pin 6 into circuit ground. I've tried isolating the audio ground previously by not connecting the shields of the audio lines.

Quote:
The voltage at the digital potentiometer resistor elements must stay within the power-supply voltage rails. Therefore, the 47-kW resistors and associated coupling capacitors maintain a 2.5-V dc bias on these elements. The corresponding audio voltage range is 2.5 V, in which normal audio levels fall.
Given this quote above, and looking at the specs on the digital pot, I don't understand why the voltage drop across the resistor network in the pot must be within rail voltage. Maybe I should bypass the 47K resistors altogether and wire pin 6 directly to the audio shield ground. Would this help, or should I look into filtering the power supply some more?

Any recommendations on how to eliminate this noise problem?

Thanks!
  Reply With Quote
Old 16th November 2004, 05:13 PM   #2
pfn is offline pfn  United States
diyAudio Member
 
Join Date: Nov 2004
Location: SF BA, CA
Hmm, so I looked at the specs on the digital pot and it seems that pin 6 needs the resistors on it to bias the voltage, since the the datasheet on the pot says voltage across the resistive elements needs to stay between Vcc and GND or bad things happen :-(.

What to do now.
  Reply With Quote
Old 19th November 2004, 10:54 PM   #3
pfn is offline pfn  United States
diyAudio Member
 
Join Date: Nov 2004
Location: SF BA, CA
Nobody has any input?

Anyway, it seems I misplaced the coupling capacitor on the Rl to audio GND pin of the pot. I'll try relocating it after I get back from vacation and see if it helps the situation any.
  Reply With Quote
Old 27th November 2004, 09:37 PM   #4
djQUAN is offline djQUAN  Philippines
diyAudio Member
 
Join Date: May 2004
Location: manila
Send a message via MSN to djQUAN Send a message via Yahoo to djQUAN
try adding a 5-10R resistor between the chassis GND and the circuit GND.
  Reply With Quote
Old 28th November 2004, 06:16 PM   #5
diyAudio Member
 
tsmith1315's Avatar
 
Join Date: Jul 2003
Location: Doerun, GA
Have you tried the normal filtering elements on the +12V input, and on the input/output of the +5V regulator?
  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
ground loop buzz problem SCOTT123 Solid State 3 24th March 2008 10:53 AM
Ground Loop problem jleaman Pass Labs 68 9th May 2006 05:32 AM
hum,ground loop,earth loop problem with your answers please frank2395 Pass Labs 2 17th May 2005 07:46 PM
Ground loop problem for sub? AudioIsFun Multi-Way 5 29th February 2004 08:14 AM
Leach Amp: Ground loop problem swede Solid State 7 15th May 2002 07:19 PM


New To Site? Need Help?

All times are GMT. The time now is 06:17 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