Large offset and large differential input to input pairs... but amp works..? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

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 4th July 2015, 07:11 AM   #1
diyAudio Member
 
Join Date: Feb 2012
Default Large offset and large differential input to input pairs... but amp works..?

Have a strange problem I have no encountered before. Here are the symptoms.

Amp works fine, I get a clean signal output, under load, is stable (clean square wave no overshoot). But there is something going on with the LTP offset and amp offset.

Speaker out (vout) offset is ~90mV.
Feedback input is ~90mV
Signal Input is +50mV (???????)

Closed loop gain is fine (25V/V vs 31V/V from feedback ratio), so I don't think the open loop gain is as bad as the differenital input to the LTP suggests. Somehow it seems there is some kind of offset going. I took a look at some the bias voltages and everything seems to look ok EXCEPT each input to the LTP.

Where to debug next?
  Reply With Quote
Old 4th July 2015, 07:26 AM   #2
Banned
 
Join Date: Apr 2014
Location: US
90mV at the output is not the end of the world!!! If you have complementary LTP IPS, it's not that surprising.
  Reply With Quote
Old 4th July 2015, 07:29 AM   #3
diyAudio Member
 
Join Date: Feb 2012
Quote:
Originally Posted by Alan0354 View Post
90mV at the output is not the end of the world!!! If you have complementary LTP IPS, it's not that surprising.
Yes, it doesn't kill any audio quality or anything, but I still want to understand it. I've built many amps with LTP input stage and I usually get an offset of 1-10mV. Ive never seen 90mV and I have never seen a LTP offset this large before, which leads me to believe there is some misconnection, broken part, or broken trace on the PCB, but I can't find it so far.
  Reply With Quote
Old 4th July 2015, 09:32 AM   #4
diyAudio Member
 
JonSnell Electronic's Avatar
 
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
Send a message via Skype™ to JonSnell Electronic
A schematic would help but anything up to 150mV is OK. This is usually caused by a misbalance on the differetial matched pair input transistors hfe. If you are worried about this minor issue, swop them over and read the difference.
No idea what "LTP" or "IPS" stands for. Don't do text speak.
__________________
Support for Flying Mole Class D, PMC DS-001 (Hypex),Valve Equipment and designs both new and old. www.jonsnell.co.uk
  Reply With Quote
Old 4th July 2015, 10:01 AM   #5
diyAudio Member
 
Join Date: Feb 2012
I was just dicking around in spice playing with mismatched emitter resistor values and yes mismatching values does create the exact effect I am seeing, which would translate into mismatched bjt re values...

Maybe ive just lucked out and never built and amp with LTP transistors that never mismatched this much.
  Reply With Quote
Old 4th July 2015, 10:05 AM   #6
diyAudio Member
 
Join Date: Apr 2008
LTP = Long-tailed pair and refers to a differential amplifier.
See Figure 2 here: https://en.wikipedia.org/wiki/Differential_amplifier
IPS = InPut Stage.
__________________
Regards,
currentflow
  Reply With Quote
Old 4th July 2015, 11:00 AM   #7
diyAudio Member
 
JonSnell Electronic's Avatar
 
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
Send a message via Skype™ to JonSnell Electronic
Quote:
Originally Posted by currentflow View Post
LTP = Long-tailed pair and refers to a differential amplifier.
See Figure 2 here: https://en.wikipedia.org/wiki/Differential_amplifier
IPS = InPut Stage.
Thanks for the deciphering.
(I still send texts in English!)
__________________
Support for Flying Mole Class D, PMC DS-001 (Hypex),Valve Equipment and designs both new and old. www.jonsnell.co.uk
  Reply With Quote
Old 4th July 2015, 10:02 PM   #8
bwaslo is offline bwaslo  United States
diyAudio Member
 
bwaslo's Avatar
 
Join Date: May 2006
Location: Portland, Oregon!
Blog Entries: 1
Is it by chance a late 70s vintage Hafler DH200? I ran into and fixed a number of those, with those same symptoms, back in the day. The high voltage diff pair transistors went leaky. Fixes were to replace those (all 4 in each channel) or (more difficult) change the stage to a cascode.
__________________
DIY horn spreadsheet SynergyCalc/
XSim crossover design Xsim; Depot diffusor super-easy diffusors
  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 such a large input series resistor? ammel68 Headphone Systems 8 26th September 2014 03:23 AM
Adding a third input to a differential input stage? maudio Solid State 15 11th October 2006 03:16 AM
Input Differential Pairs and Zetex Transistors EchoWars Solid State 6 19th June 2003 02:41 PM


New To Site? Need Help?

All times are GMT. The time now is 06:41 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki