Counterfeit Opamps? Adcom GFA-555II Weirdness

EchoWars · 18th January 2011, 09:36 PM

A good friend (who lives nearly a continent away) was recently working on a pair of Adcom 555II amps. In the course of things, it was decided that the original opamps would be removed, a good machined socket installed, and new opamps put in. He asked what he should use for the opamps, and I stifled my usual reply of the AD820 and parroted what I had heard here so many times: Go with the OP97.

So he did. One amp ended up with offsets of about 5V, and the other about 7V. We assumed some error in the work done, but nothing could be found. Eventually, I asked him to straighten and clean up the leads on the original Adcom chips and reinstall in the new sockets to see what happened. When we did that, offset dropped to less than 1mV.

I asked him to send me the OP97's (which were purchased from Digikey) so I might see what the deal is. I have them here, but now that push has come to shove, I can't think of a good little bench test for them to determine if they are good devices & original... That, and I was under the impression that Digikey buys directly from manufacturers, so before I contact them, I want to be able to tell them something relevant and definitive.

Any thoughts on a good bench test with +/-15V supplies? What should I look for?

singa · 19th January 2011, 01:29 AM

Hi, Download OP97 datasheet,in it there are test schematics that the manufacturer show that you can verify the device characteristics of a typical device.Results are not guaranteed as your layout may defer
but it will be close. Good luck.

jaycee · 19th January 2011, 01:41 AM

What were the original opamps?

beun · 19th January 2011, 01:46 AM

The OP97 has offset null pins, if the original opamp has those as well in that spot you may have to look at that circuit, the two circuits may be incompatible.

Andrew Eckhardt · 19th January 2011, 02:04 AM

IIRC the opamps in that power amp are only the offset servo. Unless you had a problem with the performance of that you may as well put the originals back in. BTW, it's pretty unlikely that Digikey gets Analog Devices parts from anyone but Analog Devices.

jackinnj · 19th January 2011, 02:06 AM

Were these the "house-branded" opamps which Linear Tech made for Adcom?

gaetan8888 · 19th January 2011, 04:23 AM

Hello

Linear Technology and Analog Device made renumbered ic's for Adcom, here is the list of the real numbers of those Adcom IC.

Bye

Gaetan

18th January 2011, 09:36 PM	#1
EchoWars diyAudio Member Join Date: Dec 2002 Location: Left of the Dial	Counterfeit Opamps? Adcom GFA-555II Weirdness A good friend (who lives nearly a continent away) was recently working on a pair of Adcom 555II amps. In the course of things, it was decided that the original opamps would be removed, a good machined socket installed, and new opamps put in. He asked what he should use for the opamps, and I stifled my usual reply of the AD820 and parroted what I had heard here so many times: Go with the OP97. So he did. One amp ended up with offsets of about 5V, and the other about 7V. We assumed some error in the work done, but nothing could be found. Eventually, I asked him to straighten and clean up the leads on the original Adcom chips and reinstall in the new sockets to see what happened. When we did that, offset dropped to less than 1mV. I asked him to send me the OP97's (which were purchased from Digikey) so I might see what the deal is. I have them here, but now that push has come to shove, I can't think of a good little bench test for them to determine if they are good devices & original... That, and I was under the impression that Digikey buys directly from manufacturers, so before I contact them, I want to be able to tell them something relevant and definitive. Any thoughts on a good bench test with +/-15V supplies? What should I look for?

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19th January 2011, 01:29 AM	#2
singa diyAudio Member Join Date: Apr 2004 Location: near	Hi, Download OP97 datasheet,in it there are test schematics that the manufacturer show that you can verify the device characteristics of a typical device.Results are not guaranteed as your layout may defer but it will be close. Good luck.

19th January 2011, 01:41 AM	#3
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	What were the original opamps?

19th January 2011, 01:46 AM	#4
beun diyAudio Member Join Date: Feb 2010	The OP97 has offset null pins, if the original opamp has those as well in that spot you may have to look at that circuit, the two circuits may be incompatible.

19th January 2011, 02:04 AM	#5
Andrew Eckhardt diyAudio Member Join Date: Nov 2005 Location: PA	IIRC the opamps in that power amp are only the offset servo. Unless you had a problem with the performance of that you may as well put the originals back in. BTW, it's pretty unlikely that Digikey gets Analog Devices parts from anyone but Analog Devices.

19th January 2011, 02:06 AM	#6
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	Were these the "house-branded" opamps which Linear Tech made for Adcom?

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