Question on DC Offset - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

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 18th July 2009, 04:59 PM   #1
diyAudio Member
 
Join Date: Jul 2009
Send a message via AIM to davodavo Send a message via MSN to davodavo
Default Question on DC Offset

Hi. This isn't really a DIY question, but I'm hoping your collective expertise will instantly know the answer. I searched the forums, didn't see it...

I've got a fairly old quasi complementary symmetry, direct coupled amp that has 140 mV DC offset on one channel. The amp works, but that offset is about 100 mV higher than you'd expect. Naturally, there is no adjustment for either bias nor DC offset.

The transistors all seem OK, no burned components, no obvious cap problems.

For this kind of amp, which transistors most directly effect DC offset? where would I look to tweak a resistor value to adjust?

Circuit diagram is up at www.taberconsultling.com/circuit.jpg
  Reply With Quote
Old 18th July 2009, 06:03 PM   #2
mjf is offline mjf  Austria
diyAudio Member
 
Join Date: Jan 2009
Location: Austria
hello.
slip of pen.
http://www.taberconsulting.com/circuit.jpg

when parts are old they are often out of tolerance,especially caps and res.
you can measure the resistors and compare the values of the two channels,the better one with the bad one.perhaps you find some different values.
and you can recap the little elcos with new ones.
greetings..............
  Reply With Quote
Old 18th July 2009, 08:35 PM   #3
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Personally I wouldnt be getting too upset at 140mV DC offset.

The transistors blowing is another matter.
It could be the bias is far too high.
You need to measure the volts across output resistors to get an idea if the bias is faulty or not.
__________________
PCBCAD50 software. http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 18th July 2009, 11:41 PM   #4
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
One simple try you can do is to let input pair, Q3 and Q4,
change places.

Another better try is to find and buy 10 pieces 2N4036.
Then measure volt B-E, at a current of 1mA, to find a good matching pair.
Or just try different combinations of those exemplars until you get low offset at output.

One resistor that has got some effect on DC-offset is R12, 27k.
But is not optimal to begin to change the value of this resistor
because probably the amplifier should have 1+1 mA in input pair.
Best is if they share exactly 50-50%
And so my guess is that the Supply is around 2x55 Volt DC.
( 54V/27k = 2 mA .. and each input transistor takes 1 mA )

----

I agree with previous poster. 140mV is not too much to worry about.
Will not harm the quality of the output or hurt your speakers.
So, there is not much to gain from fixing this moderate Dc-offset.
__________________
lineup
  Reply With Quote
Old 19th July 2009, 04:58 AM   #5
taj is offline taj
diyAudio Member
 
taj's Avatar
 
Join Date: Feb 2005
Just curious, trying to learn. What is Q1 and Q2 used for before the LTP? It looks like an extra gain stage to my under-educated eye.

..Todd
  Reply With Quote
Old 19th July 2009, 05:16 AM   #6
gni is offline gni  United States
diyAudio Member
 
gni's Avatar
 
Join Date: Sep 2006
Q1 and Q2 look like gain but could be just buffer. . . those caps should
stop DC before LTP. . . Q3 and Q4 are probably not matched. . . .keep an
eye on the 140mV. . . .not really bad. . .but it could be much lower. . . .

I like the idea of swapping them. . .that would be simple and then you would
know. . .

Would love to hear how it works. . .
  Reply With Quote
Old 19th July 2009, 05:39 AM   #7
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
Quote:
Originally posted by gni
Q1 and Q2 look like gain but could be just buffer. . . those caps should
stop DC before LTP. . . Q3 and Q4 are probably not matched. . . .keep an
eye on the 140mV. . . .not really bad. . .but it could be much lower. . . .

I like the idea of swapping them. . .that would be simple and then you would
know. . .

Would love to hear how it works. . .
hi,
Q1/Q2 forms a non-inverting gain buffer with a gain of around 3 if i am right....

with today's sources, maybe not required...the main amp istelf has a voltage gain of about 12 sp overall gain is around 36 which just about good for a PA amp....
__________________
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
  Reply With Quote
Old 19th July 2009, 06:15 AM   #8
mjf is offline mjf  Austria
diyAudio Member
 
Join Date: Jan 2009
Location: Austria
hello.
at the base of q3 there is a res without designation........may i guess..............that is the one for trimming the offset?
greetings.................
  Reply With Quote
Old 19th July 2009, 06:22 AM   #9
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
that resitor should be 10k like R17.....
__________________
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
  Reply With Quote
Old 19th July 2009, 07:13 AM   #10
mjf is offline mjf  Austria
diyAudio Member
 
Join Date: Jan 2009
Location: Austria
yes,that is a point........in a nearly perfect working amp these res values should be equal............;
amazing that r-c network across r17...............seems to give some extra punch in the bass tones...........
  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
DC offset question tschanrm Class D 9 23rd February 2006 02:32 AM
DC Offset Question dsavitsk Chip Amps 14 10th May 2005 11:52 AM
dc offset question? crippledchicken Solid State 4 7th April 2004 10:37 PM
SOZ DC Offset question. moe29 Pass Labs 4 12th August 2003 05:28 PM
DC Offset question wheezer Solid State 2 13th January 2003 10:30 PM


New To Site? Need Help?

All times are GMT. The time now is 02:17 PM.


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