How do you measure the DC offset of an amp

ben62670
ben62670 Posts: 15,969
edited October 2013 in Electronics
I need to know how to measure the DC offset of an Adcom Amplifier (GFA-585).
Thanks Ben
Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
Thanks
Ben
Post edited by ben62670 on
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  • disneyjoe7
    disneyjoe7 Posts: 11,435
    edited January 2007
    I don't think an end user or consumer needs to be worried about that. An offset is measured in mv at a point between the output transistors, so this point can be before any final filtering circuit which to output terminals are connected to. If you suspect trouble woofer being pushed in or out with amp on, the amp needs service and I would do this ASAP before speaker damage.

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  • Dennis Gardner
    Dennis Gardner Posts: 4,861
    edited January 2007
    Here is the best discussion I have found on the subject................http://audiokarma.org/forums/showthread.php?t=5634
    HT Optoma HD25 LV on 80" DIY Screen, Anthem MRX 300 Receiver, Pioneer Elite BDP 51FD Polk CS350LS, Polk SDA1C, Polk FX300, Polk RT55, Dual EBS Adire Shiva 320watt tuned to 17hz, ICs-DIY Twisted Prs, Speaker-Raymond Cable

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  • ben62670
    ben62670 Posts: 15,969
    edited January 2007
    Thanks for the link I am on it
    Ben
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • scottyboy76
    scottyboy76 Posts: 2,905
    edited October 2013
    Never would have thought you could let an amp sit for ten minutes not hooked up.
    humpty dumpty was pushed
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  • scottyboy76
    scottyboy76 Posts: 2,905
    edited October 2013
    Is this a record? Diggin up bones.
    humpty dumpty was pushed
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  • F1nut
    F1nut Posts: 50,567
    edited October 2013
    You can let a SS amp sit powered up without a load all day and twice on Sunday. Do NOT try it with a tube amp.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."

    President of Club Polk

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  • lanchile
    lanchile Posts: 560
    edited October 2013
    To check dc offset, do this:
    a) Turn on amp and let it warm up (half hour will do it)
    b) disconnect all inputs and speakers.
    c) Connect a multimeter (DC mv range) in the speaker terminals (+ - )
    d) it should read very low (10 mv or lower) the lower the better.
    I would consider a good and healthy reading in the 5 mv range, My diy amplifier is spot on the "0" mv.
    it will be a good idea to check the bias too, but this will need more steady hands and knowledge of what you are doing.

    PS: Every time you fix or upgrade an amplifier (change capacitors, resistors, transistors etc), you should check bias and dc offset.
    Make it simple...Make it better!
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  • scottyboy76
    scottyboy76 Posts: 2,905
    edited October 2013
    Bought a carver tfm35 yesterday, along with a ct-3, think i will check dc offset after first auditioning with my speaks.

    If this blows up, i will haunt you guys in the afterlife.
    humpty dumpty was pushed
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  • scottyboy76
    scottyboy76 Posts: 2,905
    edited October 2013
    just checked the tfm 35, nothin, the needle moves slightly as the probes touch the terminals,.
    humpty dumpty was pushed
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  • scottyboy76
    scottyboy76 Posts: 2,905
    edited October 2013
    after 45 minutes, i get 1mv on one channel, and 2mv on the other.
    humpty dumpty was pushed
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  • Glen B
    Glen B Posts: 269
    edited October 2013
    scottyboy76 wrote: »
    Bought a carver tfm35 yesterday, along with a ct-3, think i will check dc offset after first auditioning with my speaks.

    It should be the other way around. Whenever I acquire a used/vintage amp, I always check the DC offset first before hooking up to speakers, and an expendable pair at that. If the amp has a major problem, I don't want it hooked up to my good speakers.
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  • scottyboy76
    scottyboy76 Posts: 2,905
    edited October 2013
    just hooked it up to some **** speaks, and didnt push them.

    Other question, it was an analog meter.
    humpty dumpty was pushed
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  • zingo
    zingo Posts: 11,258
    edited October 2013
    I also measure DC offset of amps that are new to me, and it's saved me more than once on 80's amps that had +200mv offset!
    Jake
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  • skrol
    skrol Posts: 3,378
    edited October 2013
    The DC offset is the first thing I check on a new to me amp. I looked at a Hafler amp a few years ago that had a way high DC offset in one channel.

    Oh yeah, use a good DMM like a Fluke. Cheap DMM's can be trusted at such low levels.
    Stan

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  • scottyboy76
    scottyboy76 Posts: 2,905
    edited October 2013
    skrol wrote: »
    The DC offset is the first thing I check on a new to me amp. I looked at a Hafler amp a few years ago that had a way high DC offset in one channel.

    Oh yeah, use a good DMM like a Fluke. Cheap DMM's can be trusted at such low levels.

    Did you mean to type "cant" be trusted at such low levels?

    If so i will borrow a better meter, im using a cheap sperry from home depot i have always used to do home wiring projects.
    humpty dumpty was pushed
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  • lanchile
    lanchile Posts: 560
    edited October 2013
    scottyboy76 wrote: »
    Did you mean to type "cant" be trusted at such low levels?

    If so i will borrow a better meter, im using a cheap sperry from home depot i have always used to do home wiring projects.

    You can always trust Fluke multimeters. They are the best multimeters your money can buy!. in my book, under 5mv dc offset is very good and 0mv is ideal!!!!
    Make it simple...Make it better!
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