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Thread: ADCOM GFA 555 II

  1. #1
    Senior Member Guido's Avatar
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    ADCOM GFA 555 II

    I managed to get a GFA 555-II. This is a rally beefy amp. Amazing headroom.

    BUT I have a slight hum when there is no music signal. Is this one of this ground probs?
    What can I do?

    Thanks for help.

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    Do you get hum with nothing at all, (no cables or anything), connected to the inputs? Have you tried changing the location to see if it's interacting with something else?

    John

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    Senior Member Guido's Avatar
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    Quote Originally Posted by johnaec
    Do you get hum with nothing at all, (no cables or anything), connected to the inputs? Have you tried changing the location to see if it's interacting with something else?

    John
    Nope! With disconnected input cables no hum.
    BUT When I connect my preamp to my harman amp I do not have any hum.

    For clearifiction I'm talking about a slight slight hum.

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    What about with just cables and no pre-amp? Bad cables? Does changing the polarity, (power plug), of one unit or the other make a difference?

    John

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    Senior Member Guido's Avatar
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    Thanks for help John.

    It's 2AM here and I'll check this tmorrow. I'll come back to you with this.

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    Quote Originally Posted by Guido
    It's 2AM here and I'll check this tmorrow.
    Aw, c'mon Guido - it Saturday!

    John

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    Senior Member LE15-Thumper's Avatar
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    re - Triple nickle II

    From my previous work as an ADCOM warranty guy here in Winnipeg, I remember some flameproof ground float resistors opening on a few occasions. If you feel handy and have a multimeter. I would check R104 and R154 (one in each channel) They are 100 ohms-1/4 watt and don't show any signs of heat when they open. They also cause a DC offset which your woofers will not like !! They are located on the small input board at the rear.

    Make sure those fat main filters are at zero volts as well. :shock: Use a 22 or 33 ohm-10 watt resistor to discharge them after the amp is shut off. Although they already have a 3.9k across them according to my schematic, so your fingers should be safe. Always have the amp unplugged and your speakers disconnected as well when doing work like this. I know that may sound obvious, but it is not obvious to all. Good luck, let me know if you need more help
    LE15-Thumper
    "Give me JBL, or give me death"

  8. #8
    Senior Member Guido's Avatar
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    Interesting, seems to be some kind of ground loop. The adcom amp continues playing music for some 20 seconds after switching of. When I pull the main plug the hum stops even though the music still plays.

    LE 15 thumper. With your help I'm sure I can fix it. A schematic will help I PM'd you.

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    RIP 2011 Zilch's Avatar
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    Quote Originally Posted by Guido
    When I pull the main plug the hum stops even though the music still plays.
    Guido has discovered cold fusion again?

    [It's the residual charge in the main power supply capacitors, actually....]

  10. #10
    Senior Member Guido's Avatar
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    Quote Originally Posted by Zilch
    Guido has discovered cold fusion again?
    Unfortunately it's only the old capacitor principle

    But the hum stops so it's an ground issue as LE15 thumper supposed.

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    Junior Member P.Warner's Avatar
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    I'm grabbing at straws here by assuming you are using this in Germany (230v-50 hz.) If you know that you are working from the same power source and a common (neutral) and ground for all equipment then it's time to get curious about the Adcom's transformer primary. If this 555II was originally 120v then cross check any conversion work with an Adcom schematic.

  12. #12
    Senior Member LE15-Thumper's Avatar
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    Quote Originally Posted by P.Warner
    I'm grabbing at straws here by assuming you are using this in Germany (230v-50 hz.) If you know that you are working from the same power source and a common (neutral) and ground for all equipment then it's time to get curious about the Adcom's transformer primary. If this 555II was originally 120v then cross check any conversion work with an Adcom schematic.

    That's a good idea too !!
    LE15-Thumper
    "Give me JBL, or give me death"

  13. #13
    Senior Member Guido's Avatar
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    Quote Originally Posted by P.Warner
    I'm grabbing at straws here by assuming you are using this in Germany (230v-50 hz.) If you know that you are working from the same power source and a common (neutral) and ground for all equipment then it's time to get curious about the Adcom's transformer primary. If this 555II was originally 120v then cross check any conversion work with an Adcom schematic.
    I think it is an original 230V model but will check this. Waiting for the schematic

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    Senior Seņor boputnam's Avatar
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    Quote Originally Posted by Guido
    BUT When I connect my preamp to my harman amp I do not have any hum.
    Since you're going through all of it, tell us just what are these, and how were they interconnected?

    Oh, and do you still get the GL hum from the preamp Adcom 555-II combo when there is nothing plugged into the preamp?
    bo

    "Indeed, not!!"

  15. #15
    Senior Member Guido's Avatar
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    Quote Originally Posted by boputnam
    Since you're going through all of it, tell us just what are these, and how were they interconnected?

    Oh, and do you still get the GL hum from the preamp Adcom 555-II combo when there is nothing plugged into the preamp?
    Bo you where right!

    It was the russound A-Bus Module connected to my Rotel RC-972 that caused the hum. With disconnected A-Bus Module everything is fine.

    But now I have another Problem. What to do with the A-Bus? Anyone here with experience in this Field?

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