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Help with Adcom GFA 565 DC offset?
Help with Adcom GFA 565 DC offset?
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Old 11th February 2006, 06:14 PM   #1
craiggoff is offline craiggoff  United States
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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
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Old 11th February 2006, 06:19 PM   #2
Tube_Dude is offline Tube_Dude  Portugal
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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?
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Old 11th February 2006, 11:39 PM   #3
anatech is offline anatech  Canada
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Help with Adcom GFA 565 DC offset?
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
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