DC buildup in Linkwitz Transform - Page 2 - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, 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 15th April 2012, 08:53 PM   #11
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Yes, either a dry joint or a crack in a PCB trace. If you are using modern lead-free solder then I understand that dry joints and good joints can look identical (I always use the old 'poisonous' solder). If you showed us the circuit we could probably tell you where to look for the problem.
  Reply With Quote
Old 15th April 2012, 10:30 PM   #12
Simon B is offline Simon B  United Kingdom
diyAudio Member
 
Join Date: Dec 2011
Location: North-East England
I'd look for an op amp input fed by a capacitor, that also has a resistor to ground, or to some other circuit node that direct current can flow out of.

If that resistor is not properly connected, the small bias current flowing out of the op-amp input charges up the capacitor and hey presto, the very fault you describe.

Probably, learn to solder properly, you've got a dry joint at the end of a resistor. Before you fix it, look at it, see how it looks different from good joints. As DF96 says (and I paraphrase a little) "use proper solder".

As DF96 also says, possibly a crack in a pcb trace. They can be very fine indeed and still be an open circuit. But my money's on a dry joint.

Last edited by Simon B; 15th April 2012 at 10:33 PM.
  Reply With Quote
Old 15th April 2012, 10:47 PM   #13
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

Cracks in PCB traces can be obviated by tinning all of the PCB tracks
with solder, but doesn't work that well for high current tracks.

rgds, sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
  Reply With Quote
Old 16th April 2012, 05:22 PM   #14
petarkb is offline petarkb  Bulgaria
diyAudio Member
 
Join Date: Jan 2008
I couldn't find any obvious bad joints, but I "untangled" some blobs where lots of leads met. On the third one the buildup disappeared, and the output is DC-free!

Thanks a lot to everyone, and especially to Simon B. I went untangling the capacitor junctions first. The troubled capacitor was closer to the output, but was a part of the feedback loop. It's the rightmost C2 in the schematic attached.

There are 'before' and 'after' pictures, if anyone's interested.

DF96, sorry for misleading, but I didn't realize at the time. As you can see, the before picture doesn't have obvious 'bad' joints, just lots of evil blobby ones. Guess I underestimated their evilness.

My next project will be adding a high pass filter before the Linkwitz Transform. My speakers have Visaton W170S woofers, and the LT extends them to Q=0.7 and F3=30Hz. They don't do 30Hz like I expected. Distortion is bad, and cone movement is scary with anything except music. My room needs less deep bass anyway.

Going to use a 1st order high pass at 50Hz. If I mess that up, I'm gonna end up with a flat forehead, from a wall.
Attached Images
File Type: png lt_schematic.png (4.2 KB, 25 views)
File Type: jpg before.JPG (531.9 KB, 24 views)
File Type: jpg after.JPG (547.2 KB, 23 views)
  Reply With Quote
Old 16th April 2012, 05:57 PM   #15
petarkb is offline petarkb  Bulgaria
diyAudio Member
 
Join Date: Jan 2008
My bad, the affected capacitor is C3, not C2.
  Reply With Quote
Old 16th April 2012, 06:05 PM   #16
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
As I said:
Quote:
Originally Posted by DF96
A wiring error?
A short caused by too much solder in the wrong place is a common fault, often introduced during a 'repair'.
  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
Will a Linkwitz transform...transform the driver? AllenB Subwoofers 20 17th May 2010 10:24 PM
Greater than 20dB gain at DC with Linkwitz Transform - bad? Jim85IROC Subwoofers 14 25th November 2006 03:49 PM
linkwitz transform peace brainerd Subwoofers 4 31st January 2006 01:13 AM
Linkwitz Transform ding Multi-Way 10 11th July 2004 09:55 PM
Linkwitz Transform? Aunkst Chip Amps 6 28th March 2004 11:59 PM


New To Site? Need Help?

All times are GMT. The time now is 02:35 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