ADCOM GFA-555 DC Offset?

gimpod
gimpod Posts: 1,793
edited December 2011 in Electronics
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.
“The two most important days in your life are the day you are born and the day you find out why.” ~ Mark Twain
Post edited by gimpod on
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    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.
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    gimpod:

    Read this:

    http://www.diyaudio.com/forums/solid-state/172779-adcom-gfa-555-output-dc-offset-voltage-measurements-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.
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • gimpod
    gimpod Posts: 1,793
    edited December 2011
    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 :eek:
    “The two most important days in your life are the day you are born and the day you find out why.” ~ Mark Twain
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  • SDA1C
    SDA1C Posts: 2,072
    edited December 2011
    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
    Too much **** to list....
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    gimpod wrote: »
    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 :eek:

    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?
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • gimpod
    gimpod Posts: 1,793
    edited December 2011
    PreCd wrote: »
    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.
    “The two most important days in your life are the day you are born and the day you find out why.” ~ Mark Twain
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    gimpod wrote: »
    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?
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • gimpod
    gimpod Posts: 1,793
    edited December 2011
    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.
    “The two most important days in your life are the day you are born and the day you find out why.” ~ Mark Twain
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  • FTGV
    FTGV Posts: 3,650
    edited December 2011
    gimpod wrote: »
    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.
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  • PreCd
    PreCd Posts: 786
    edited December 2011
    gimpod wrote: »
    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.
    SDA2BTL
    Marantz CD5004
    Adcom GFA-545
    Bottlehead Quickie Tube Preamp
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  • lanchile
    lanchile Posts: 560
    edited December 2011
    gimpod wrote: »
    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-state/172779-adcom-gfa-555-output-dc-offset-voltage-measurements.html?highlight=ADCOM+GFA-555+DC+Offset
    Make it simple...Make it better!
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  • gimpod
    gimpod Posts: 1,793
    edited December 2011
    FTGV wrote: »
    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.
    PreCd wrote: »
    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.
    PreCd wrote: »
    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.
    “The two most important days in your life are the day you are born and the day you find out why.” ~ Mark Twain
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