Triple compound output amp stability - 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 6th March 2012, 03:30 PM   #1
rss388 is offline rss388  United States
diyAudio Member
 
Join Date: Dec 2007
Default Triple compound output amp stability

Hi all,

I am working on a Harman Kardon integrated amp HK6950r. The pre-amp works fine and the power section would on power up either work for a few minutes or just directly trigger the protection circuit.

I have isolated it to one channel where the output stage current jumps up enough to trigger the protection circuit via Q477 or Q478 (not sure why there are two labels). I noticed that when I take the power amp section of of the chassis, that symptom goes away and the amp continues to operate. Soon as the heatsink touches the chassis, the protection circuit trips from the one channel.

I have tried a number of things. I replace most of the caps on the amp and adding bypassing via surface mount caps, installing base resistors between 0.5 to 4.7 ohms per device and even doubling the Miller cap on the driver stage. Nothing helped and with all those in place, even the good channel does not operate.

Any suggestions? Unfortunately I do not have a functional scope so DVM is what I have to work with (and a lot of patience...) Many thanks!

Here are the two pages for the power stage. I apologize for the page break but that is how the manual comes.
Attached Images
File Type: jpg power amp driver stage.jpg (580.4 KB, 424 views)
File Type: jpg power amp output stage.jpg (716.0 KB, 404 views)
  Reply With Quote
Old 6th March 2012, 03:33 PM   #2
rss388 is offline rss388  United States
diyAudio Member
 
Join Date: Dec 2007
Maybe I can add a couple of things. The protection circuit disables the input biasing via Q405 (prot signal line). I tried disconnecting that line to see if it was some kind of spurious operation but no, the main fuse (8A) blows every time I tried that so current was indeed being pulled.
  Reply With Quote
Old 6th March 2012, 07:10 PM   #3
diyAudio Member
 
Join Date: Nov 2010
I know this sounds too simple, but check for a bad insulator on one of the outputs.
  Reply With Quote
Old 7th March 2012, 01:22 AM   #4
rss388 is offline rss388  United States
diyAudio Member
 
Join Date: Dec 2007
Quote:
Originally Posted by alhaynes View Post
I know this sounds too simple, but check for a bad insulator on one of the outputs.
I ran home from work to check this out. Wouldn't this be great! Unfortunately (for me), I changed out the pads and same thing. Now I notice that even if the amp would power up to work, soon as I touch the heatsink, the protection kicks in. Could it be some kind of oscillation?
  Reply With Quote
Old 7th March 2012, 01:38 AM   #5
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Blog Entries: 2
I have read that people have experienced issues with oscillations through poorly grounded heatsinks.

Normally the amplifier ground is isolated from the chasis via a 'protected earth' circuit - essentially a pair of back to back parallel diodes. This is to reduce the possibility of earth loops which create 'hummmmm'. If the heatsink isn't already inadvertently connected to something it should be then you could try connecting the heatsink to the amplifier ground - not the chassis. Hopefully they are not the same thing in your amp.
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
  Reply With Quote
Old 7th March 2012, 01:44 AM   #6
rss388 is offline rss388  United States
diyAudio Member
 
Join Date: Dec 2007
Quote:
Originally Posted by Bigun View Post
I have read that people have experienced issues with oscillations through poorly grounded heatsinks.

Normally the amplifier ground is isolated from the chasis via a 'protected earth' circuit - essentially a pair of back to back parallel diodes. This is to reduce the possibility of earth loops which create 'hummmmm'. If the heatsink isn't already inadvertently connected to something it should be then you could try connecting the heatsink to the amplifier ground - not the chassis. Hopefully they are not the same thing in your amp.
Thanks for your input. As luck would have it, the way HK did the grounding is that they star grounded at the heatsink between the two channels already.
  Reply With Quote
Old 7th March 2012, 09:44 PM   #7
rss388 is offline rss388  United States
diyAudio Member
 
Join Date: Dec 2007
In my digging, I found some comments about the critical nature of C413 and C469 (cap between the sources of the input FETs and between the emitters of the driver output. Does anyone have any thoughts about how these might affect stability of the amp?
  Reply With Quote
Old 8th March 2012, 10:29 AM   #8
Account disabled at member's request
 
Join Date: May 2006
Is there a cap between the two LTPs ?? Could you icrease the size of the schematic to have a closer look at it ??

I can suggest for now the use of a small cap of around 10 to 15pf between the two bases of the ccs s feeding the LTPs just to be sure the problem doesnt start here.
Btw I dont like this outputstage much, the use of those inductors together with miller caps on the drivers and a nonlinear capacitance with the diode in the cfps is just asking for trouble. I can see why later models used different setups.
  Reply With Quote
Old 8th March 2012, 02:46 PM   #9
rss388 is offline rss388  United States
diyAudio Member
 
Join Date: Dec 2007
@homemodder, I took the liberty of emailing the pdf file to you. The schematics is toward the end of the file. Otherwise I am not sure I can improve on the readibility.

If you would not mind, can you let me know the exact nodes where I should attach the 15p cap you mentioned?

You mentioned HK's later modelling having a different set up. Is that easy enough to "upgrade" to?

Many thanks!
  Reply With Quote
Old 8th March 2012, 03:23 PM   #10
Account disabled at member's request
 
Join Date: May 2006
No problem, I have found a way to get a clear view.
The cap I suggest is between the bases of Q407 and Q419 and also one between Q409 and Q415. Try small value but if dont help you could use a value upto 100 pf.

Ill try post a schematic from another HK model tommorrow. The outputstage is the same setup but without some of those passive parts. I never use those inductors to the outputs with cfp drivers and have never had any problems although HK seem to persist with this.
  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
LME49810 using triple CFP output stage Mr Teal Chip Amps 16 18th April 2012 07:28 PM
Darlington or Sziklai (compound) pair output stages? GregH2 Solid State 16 12th September 2011 07:28 PM
LM4702 triple darlington output alexcd Chip Amps 24 16th June 2006 03:10 PM
fet output stage stability lowlevel Solid State 11 17th January 2005 11:48 AM
Switches - Dual Triple Pole, Triple Throw jackinnj Swap Meet 0 27th January 2003 07:18 PM


New To Site? Need Help?

All times are GMT. The time now is 06:59 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