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  1. #1

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    Default ADCOM GFA-555 DC Offset?

    Does anybody know what the DC offset voltage is suppose to be for this amp?

    I Just checked it and I don't think it's right or there's something wong with my DMM.

    Right Chanel = -65mv DC
    Left Chanel = -42mv DC

    Also anybody know of a good shop in the Seattle area to take this to for a check up and a good going through.

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    gimpod:

    Mine reads 0.030 vdc on both channels. I have run that number by 2 former Adcom technicians and they say that is a fine number. The number I have been told to avoid is 0.045 vdc. According the them if you begin to see that number something is wrong. Go over to DYI Audio and search GFA-555 DC Offset. There is a ton of info on it there. Good luck and tell me what you find out will you? If those who own these GFA-555s can keep the other informed of technical info it could save someone a lot of grief.

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    gimpod:

    Read this:

    http://www.diyaudio.com/forums/solid...rements-4.html

    You can backtrack the thread but the post by ostripper says what a normal offset is. And from his previous posts he sounds like he knows the 555.

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    I've been over to DYI Audio and it's a nightmare trying to find the right info there as most threads end up going all over the place.

    What has me bugged is the readings are NEGATIVE mvdc

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    Try this thread gimpod. At the bottom there is a short/link on why neg mv doesn't mean anything.


    http://audiokarma.org/forums/showthread.php?t=5634

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    Quote Originally Posted by gimpod View Post
    I've been over to DYI Audio and it's a nightmare trying to find the right info there as most threads end up going all over the place.

    What has me bugged is the readings are NEGATIVE mvdc
    I noticed you had put that in there. Are the leads of your meter poled properly and are you placing the meter leads in the proper places so they are poled properly?

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    Quote Originally Posted by PreCd View Post
    I noticed you had put that in there. Are the leads of your meter poled properly and are you placing the meter leads in the proper places so they are poled properly?
    Yes. - to -, + to +

    Oh well guess it's time to put this puppy in the shop, Anybody know of a good one in my area.
    Last edited by gimpod; 12-17-2011 at 08:03 PM.
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    Quote Originally Posted by gimpod View Post
    Yes. - to -, + to +
    Knowing what you do I figured you had that down! Had to ask though. I really can't say. Is it a 555 II or 555?

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    555, I was thinking of sending it over to Musical Concepts for the LX mod. But when you add shipping that's a lot of money.

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    Quote Originally Posted by gimpod View Post
    Does anybody know what the DC offset voltage is suppose to be for this amp?

    I Just checked it and I don't think it's right or there's something wong with my DMM.

    Right Chanel = -65mv DC
    Left Chanel = -42mv DC

    Also anybody know of a good shop in the Seattle area to take this to for a check up and a good going through.
    Since the power supply is bi polar offset can be either + or -.I'm not sure what is typical for that amp but those offset numbers are not excessively high.
    Last edited by FTGV; 12-17-2011 at 09:24 PM.

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    Quote Originally Posted by gimpod View Post
    555, I was thinking of sending it over to Musical Concepts for the LX mod. But when you add shipping that's a lot of money.
    When I first got mine I thought of doing the same thing before I knew that my offset was in an acceptable level. I have a friend in Canada who is an expert on Adcom 555s. He was an authorized service tech on that amp. Let me run this by him. He will probably have a few more questions to ask.

    By the way I really like what you have done with the boards. The website is very informative and the circuit boards speak for themselves.
    Last edited by PreCd; 12-17-2011 at 09:34 PM.

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    Quote Originally Posted by gimpod View Post
    Does anybody know what the DC offset voltage is suppose to be for this amp?

    I Just checked it and I don't think it's right or there's something wong with my DMM.

    Right Chanel = -65mv DC
    Left Chanel = -42mv DC

    Also anybody know of a good shop in the Seattle area to take this to for a check up and a good going through.

    http://www.diyaudio.com/forums/solid...-555+DC+Offset
    Make it simple...Make it better!

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    Quote Originally Posted by FTGV View Post
    Since the power supply is bi polar offset can be either + or -.I'm not sure what is typical for that amp but those offset numbers are not excessively high.
    Thanks, I figured that out after going over the schematics (I actually found one you can read). As far as I can find out the typical DC offset should be around 30 - 40mv DC. So I'm running a tad bit on the high side not much but enough to bug my analistc nature so I think it's going off to a shop for a complete check up. Apparently according to the service manual (that I have) there is no way to adjust the DC offset on this amp, the only adjustment is the bias.

    Quote Originally Posted by PreCd View Post
    I have a friend in Canada who is an expert on Adcom 555s. He was an authorized service tech on that amp. Let me run this by him. He will probably have a few more questions to ask.
    That would be great and would love to hear what he has to say. As concise info on these amps are hard to come by without having to wade through pages and pages on some forum that's constantly being derailed.

    Quote Originally Posted by PreCd View Post
    By the way I really like what you have done with the boards. The website is very informative and the circuit boards speak for themselves.
    Thank you very much it is very appreciated.

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