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Adcoms with lots of DC offset.  Lots!
Adcoms with lots of DC offset.  Lots!
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Old 19th January 2010, 10:48 PM   #71
K-amps is offline K-amps  United States
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Originally Posted by K-amps View Post
lt1022
I stand corrected, it is not LT1022 but LT1012.

The LT1022 might demand more current than the precision zeners can provide.

Both LT1012 or OP97 should work. They have lower DC offset than the other recommendations and are drop ins for the servos in the 565.
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Old 16th February 2010, 04:01 AM   #72
anatech is offline anatech  Canada
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Adcoms with lots of DC offset.  Lots!
Hi Arif,
The current draw of the replacement op amp is important. It it's too much, the "zeners" will become current starved and drop out. There really isn't a good reason to replace the op amp unless it has failed. They rarely go bad.

Hi BTG,
Those resistors will only open if an external ground fault occurs.

The cause of the DC offsets is not the Adcom 3A. The main cause is electrolyte from defective capacitors.

Quote:
Input transistors will cause DC offset if they are not matched properly but with a servo amp it's a non issue. Even a poorly matched pair wouldn't be more than 120mV and that would have failed Adcomm's QC.
I have to disagree with you on this. It is critical to have a properly matched input pair. The DC servo circuit does not respond quickly, it's response time is set by the integrating capacitor. This can take over 10 sec to null out an offset. In the mean time, you may have a large offset that drops slowly over this time. If you have a DC servo supplied by a bipolar supply, the match of the input pair determines the DC offset until the DC servo has corrected the imbalance. If the op amp runs from a single supply, there may be other reasons for the initial DC offset. These can start around 10 ~ 12 VDC (as reported here earlier) until they are corrected. This is not abnormal behavior, but it's not what you want to see. No danger to tweeters unless you are running an active crossover where the tweeter is driven directly.

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If the transistor went bad over the years, you should be able to measure that plus they usually work or they don't.
I wish that were true. Most often they may have a beta change, so the input pair becomes unbalanced. They may become noisy or even leaky, but they will often continue to function rather than go open or short. Of course, it is possible they can fail open or short. Never discount the ways parts can fail, I've seen too many odd failures in my time.

Hi Ben, Eric,
I'm really happy you have your amplifiers running again. That's great, and better still that you are helping others. Thank you for that.

-Chris
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Old 28th September 2014, 03:02 AM   #73
gootee is offline gootee  United States
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Originally Posted by anatech View Post
Hi John,
There's your problem. 100 ohms to speaker ground from the RCA outer shell. The resistors will appear just fine, but they are open.

Get those caps out of there or don't use the amp until they are replaced. The electrolyte is not normally visible to the naked eye.


Yes.

-Chris
It sure was nice to find this old thread, today. I just received an Adcom GFA-555II from an eBay seller, and found that each channel had about 4.5 Volts of DC offset, on its output. After reading the post above, I checked the resistance from input ground to chassis ground and found between 1 and 2 Megohms, for each channel, instead of 100 Ohms. After I replaced R104 and R154, the two channels' DC offsets were both only about 0.3 millivolts!

I didn't have the replacements needed for all of the small caps, on hand, so I have ordered those.

In the meantime, I took a chance for a few hours, so I could see what she could do, through some Vandersteen 2Ce and some Magnepan 3.6 speakers. The sound is exquisite!

"Thank you" to everyone who contributed to this old thread, and to diyAudio for keeping it around. You saved me some frustration and ill will, today.

- Tom
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Old 8th October 2018, 11:27 PM   #74
void97 is offline void97
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I've 5V+ DC offset on both channels, and it go away after I replace the R104 and R154 resistor. Thanks for the great tips.


Louis
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Old 7th December 2018, 02:27 AM   #75
cogeniac is offline cogeniac  United States
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Hi Void;

If you have not seen it, I'd suggest you check out the other long Adcom 565 thread. We wrung those amp offset problems out pretty thoroughly there.

Yet Another Adcom GFA-565 Thread

Scott
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