Adcom 2A IC for GFA 565 - Page 2 - 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 15th August 2006, 01:40 AM   #11
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi James,
Get more than one IC then. I hate ordering from multiple suppliers and I usually end up with Newark, Digikey (great shipping!) and Mouser (horrible shipping!). Since I'm in Canada, things are a little tougher for me. Even Canadian suppliers charge me in USD, I lose on the conversion.

While you are at it, grab some other useful IC's and bits. I can't afford to order without looking at what I need and what I might need.

As far as Adcom goes, remember they are dealing with old skews. The originals where from LT. Adcom was bought and moved, stock included. I have seen much worse markups than that in my time. The markup probably isn't that bad when you look at their original pricing and the fact that these had to go into the system. On top of custom marking and a likely reorder.

I'm not trying to make excuses. This is the way the industry works.

-Chris
  Reply With Quote
Old 15th August 2006, 02:54 AM   #12
CompNut is offline CompNut  United States
diyAudio Member
 
Join Date: Aug 2006
I always order more than one when I need something. Also I always end up adding a bunch of other stuff to the order. I like to have a good stock of stuff on hand for doing these side jobs so I don't have to haul things into work. I have been doing TV/VCR/Audio and various other electronic repairs for so long I am starting to get tired of it. Oh well, it pays the bills!!
  Reply With Quote
Old 26th January 2007, 09:47 AM   #13
diyAudio Member
 
mjraudio's Avatar
 
Join Date: Jan 2007
Location: USA
@CompNut: How did that IC work for you? Were you able to fix the GFA-565?
__________________
--MJR
MJR AUDIO My System
  Reply With Quote
Old 26th January 2007, 03:16 PM   #14
woodman is offline woodman  United States
diyAudio Member
 
Join Date: Dec 2003
Location: New Jersey
The 820 is not the right chip for the 565. The 820 is best for a monopolar supply like the 555 mkII. Use a good quality OP 97 as per Walt Jungs suggestion.

There is a lot of confusion here between the 555 II and the 565/585 - they are not the same amplifier! The 565/585 have dual differential darlington inputs that are biased with the same +/- 13v supply as the dc servo - it is a very complex circuit to troubleshoot. The 555 II uses a + 13V supply for the dc servo and is a standard differential input it is a complex circuit as well, but fairly straightforward for an experienced tech.
__________________
-JamesW-
www.thevoorhees.com - on hold temporarily.
  Reply With Quote
Old 26th January 2007, 11:19 PM   #15
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi James,
The 555 II has a built in DC offset that takes time for the servo to correct as the amp comes up. Not a very nice feature. That's why the could use a single supply. They knew where the DC offset was going!

-Chris
  Reply With Quote
Old 27th January 2007, 04:08 AM   #16
CompNut is offline CompNut  United States
diyAudio Member
 
Join Date: Aug 2006
Yep, it worked great, thanks for the help.

James
  Reply With Quote
Old 2nd February 2007, 03:46 PM   #17
diyAudio Member
 
mjraudio's Avatar
 
Join Date: Jan 2007
Location: USA
A very helpful gentleman who responded to my post over at AudioAsylum stated that:

Quote:
If needed, you can order a replacement 2A IC directly from Adcom -- or get a Linear Technology LT1012 from Digikey for a lot less money.
You can read his entire post either at AudioAsylum or here at THIS thread (near the bottom of the page).

@woodman: Yes I read Walt Jung's post where he made that suggestion. I don't doubt Walt or you, however, CompNut was able to use the 820 and Charlie S (the gentleman from the AudioAsylum post) was able to use a LT1012. Could you clarify what type of supply the 565 has (monopolar, bi-polar, etc...) and explain why a certain type of op-amp is better for the 565 versus others? This would help cement the correct choice in all of our minds. Thanks!

@anatech: How is the DC servo circuit in the 565 different from the 555 II? There is not much information on the workings of this circuit in the 565, could you shed some light on how it works exactly? The input stage PCB is VERY complicated. Do you agree with Charlie's description (from AudioAsylum) of the 565 DC servo nulling circuit? Thank you!
__________________
--MJR
MJR AUDIO My System
  Reply With Quote
Old 2nd February 2007, 06:15 PM   #18
woodman is offline woodman  United States
diyAudio Member
 
Join Date: Dec 2003
Location: New Jersey
Quote:
Originally posted by mjraudio
[B]A very helpful gentleman who responded to my post over at AudioAsylum stated that:



You can read his entire post either at AudioAsylum or here at THIS thread (near the bottom of the page).

@woodman: Yes I read Walt Jung's post where he made that suggestion. I don't doubt Walt or you, however, CompNut was able to use the 820 and Charlie S (the gentleman from the AudioAsylum post) was able to use a LT1012. Could you clarify what type of supply the 565 has (monopolar, bi-polar, etc...) and explain why a certain type of op-amp is better for the 565 versus others? This would help cement the correct choice in all of our minds. Thanks!
The LT1012 is the Adcom 2A (though, as I recall, they were selected for minumum offset from the manufacturer) - it will work. If you look at the 565 schematic, you will notice the network hanging off J101 & 102. This is a shunt regulator that drops your rails from +/- 80v to +/- 13.8V to power your DC servo & bias the current mirrors.

Let me re-quote what Walt said regarding the opamp choice:
"1) The servo op amp must be a low power part, as the current it draws is supplied from a low power zener string (the series LM329 diodes, +/-13.8V). This fact alone rules out most std parts like 741s, 711s, etc. The LT1012 was the original Adcom 2A part, but has suffered excessive failures (not due to any circuit stress, for sure). A quality OP97 will substitute directly, and would be preferred from a no-other-changes-required point of view. Drop it in, and if the PCB is clean and nothing else is broken, it will work. "

Why is a certain type preferrable? Because Walt (the designer) said so. He has been the opamp master for many years, so I will defer to his explaination. Some parts will draw too much current, some are too noisy, some may have too much drift, and yet others may work better with a single supply as in the 555II.

Charlie S is exactly correct in his reply.

Good luck,
__________________
-JamesW-
www.thevoorhees.com - on hold temporarily.
  Reply With Quote
Old 2nd February 2007, 07:16 PM   #19
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi mjraudio,
James is right on here. Many op amps draw too much current for this particular circuit. There is also the DC offset and noise to consider.

Walt Jung helped design the input stages and is the master of op amps as far as I am concerned. Whatever anyone says, listen to Walt. End of story. To the best of my knowledge, LT1012 is the original device.

The DC servos are similar in operation in these amps. I can't remember exact details on each and would have to look them up to tell you for sure. They don't usually present a problem for me to troubleshoot, but you never know .... Seems there is always an amplifier that has a fault caused by an odd component failure mode. They pop up every now and again.

-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 GFA-565 sourceoneaudio Solid State 11 13th July 2011 01:43 PM
Adcom GFA-555 eelekim Pass Labs 2 14th May 2009 04:42 PM
Adcom GFA-555 claiborne Solid State 8 27th September 2007 11:20 AM
Adcom GFA-585 went bad - calling Adcom experts hangguy Solid State 14 24th December 2004 09:09 PM
Adcom GFA-555 Pro jbanneck Solid State 2 15th February 2004 10:33 PM


New To Site? Need Help?

All times are GMT. The time now is 05:41 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