Help with Adcom GFA 565 DC offset?

craiggoff · 11th February 2006, 05:14 PM

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

Tube_Dude · 11th February 2006, 05:19 PM

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?

anatech · 11th February 2006, 10:39 PM

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

11th February 2006, 05:14 PM	#1
craiggoff diyAudio Member Join Date: Jul 2004 Location: camarillo, ca	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

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11th February 2006, 10:39 PM	#3
anatech diyAudio Moderator 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