Help with Adcom GFA 565 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 11th February 2006, 05:14 PM   #1
diyAudio Member
 
Join Date: Jul 2004
Location: camarillo, ca
Default Help with Adcom GFA 565 DC offset?

I know that these amps have a problem with DC offset as they age. I have changed out the opamp which nulls the offset and have gotten down to 500 ma from a high of 900 ma, but that is still too high!

I remember that Muscial Concepts used to extensively modify these amps, including disconnecting the DC servo altogether and substituting a 470uf cap and shunt resistor into the feedback loop so that DC would not be amplified any longer.

Does anyone have a detailed analysis or set of instructions on how to implement this change? At this point, I'd rather just ditch the servo and go with the capacitive coupling in the feedback loop.

Or maybe someone has had direct experience with this issue in the 565 and has other ideas?

I greatly appreciate any and all suggestions, as the frustration at this end is definitely rising!

Craig Goff
Camarillo, CA
  Reply With Quote
Old 11th February 2006, 05:19 PM   #2
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Default Re: Help with Adcom GFA 565 DC offset?

Quote:
Originally posted by craiggoff
I know that these amps have a problem with DC offset as they age. I have changed out the opamp which nulls the offset and have gotten down to 500 ma from a high of 900 ma, but that is still too high!

Offset is measured in mV not in mA...have you 500mV offset?
__________________
Jorge
  Reply With Quote
Old 11th February 2006, 10:39 PM   #3
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi Craig,
I've repaired these under warranty and out. Just finished one and am in the middle of another.

Fix the problem. Period. Do not remove the DC servo. Do not remove the input coupling capacitor. These amps normally give a half volt DC offset at turn on which will drop below 10 mV unless there is something wrong.

Once your amplifier is repaired properly, you can enjoy it for many more years.

-Chris
  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
Adcom GFP-750 Ossi Pass Labs 13 12th May 2009 03:17 PM
Adcom GFA-565 again, fluctuating DC offset haash Solid State 5 4th May 2009 03:08 PM
Adcom GFA-555II with DC Offset sjdemark Solid State 19 28th February 2009 01:33 PM
Adcom pre-amp pot? rj45 Solid State 11 3rd June 2006 07:19 AM
Adcom GFA-585 went bad - calling Adcom experts hangguy Solid State 14 24th December 2004 08:09 PM


New To Site? Need Help?

All times are GMT. The time now is 01:10 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