Checking DC offset

BKphoto
BKphoto Posts: 409
edited June 2014 in 2 Channel Audio
So,

first off not sure why I'm getting a negative reading but I'm hoping someone will tell me...

readings are -60 mV and -15 mV

Amp running for about 5 mins and nothing at all plugged into it....




Talk to me Goose...
Marantz 1152 DC- Denon DP 1200, Soundsmith Carmen MKII- ADS L980 - Blue Jeans IC's
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  • DaveHo
    DaveHo Posts: 3,526
    edited June 2014
    Positive or negative makes no difference. 15mV offset is fine. 60mV is not. Your not at the going to damage things stage yet, but that amount of offset is probably producing audible distortion. There's a lengthy thread over ay AudioKarma that can explain things better than I can.
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  • BKphoto
    BKphoto Posts: 409
    edited June 2014
    yeah.....at first i was chalking it up to many new changes in my system, giving things time to settle in but its sound worse now....low level sounds great, then I start adding volume it starts sounding odd, bad is also a word you can use...

    ugh...

    I wanted to play everything for a while and let everything settle before I rebuilt my amp....but doesn't seem like I'm gonna get that chance...
    Marantz 1152 DC- Denon DP 1200, Soundsmith Carmen MKII- ADS L980 - Blue Jeans IC's
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  • deafbykhorns
    deafbykhorns Posts: 150
    edited June 2014
    You can have positive or negative offset.
    The signal is split in two halves and amplified with a positive and negative voltage. It's combined at some point in the circuit before your speaker terminals. Some circuits have an offset adjustment and some simply use matched pairs to achieve 0 offset.
    This is just a simple explanation which can get far more complicated....
    With that amount of offset you might have a transistor or opamp failing which is in the early stages of amp
    Main System- Scratch built 2A3 , Lightspeed preamp, Technics SP15/Graham/XV1s,Klipschorns w/ALK xovers/Trachorns, Speakercraft MT8(4), Sonos ZP90, Yamaha Aventage for surround
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  • deafbykhorns
    deafbykhorns Posts: 150
    edited June 2014
    If it's your Hafler, check Q1-Q4, those are FETs and one is more than likely damaged.
    Main System- Scratch built 2A3 , Lightspeed preamp, Technics SP15/Graham/XV1s,Klipschorns w/ALK xovers/Trachorns, Speakercraft MT8(4), Sonos ZP90, Yamaha Aventage for surround
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  • BKphoto
    BKphoto Posts: 409
    edited June 2014
    it is my 280...

    wil do,

    thanks guys...Gonna take tomorrow and start checking everything out...
    Marantz 1152 DC- Denon DP 1200, Soundsmith Carmen MKII- ADS L980 - Blue Jeans IC's
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  • BKphoto
    BKphoto Posts: 409
    edited June 2014
    By the way, what is the process of testing a Fet...?
    Marantz 1152 DC- Denon DP 1200, Soundsmith Carmen MKII- ADS L980 - Blue Jeans IC's
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  • deafbykhorns
    deafbykhorns Posts: 150
    edited June 2014
    You probably don't have the equipment to test....its a leakage issue and a meter/diode test wont work.
    A scope and a sine wave will tell you whats going on. If its an offset issue, I'm about 90% sure its the input FETs.
    You can sometimes adjust the offset to 0 but if your still getting some distortion, its a leakage issue.
    They are easy enough to change so I would just do all 4. I have made dozens of these repairs in the past.
    Main System- Scratch built 2A3 , Lightspeed preamp, Technics SP15/Graham/XV1s,Klipschorns w/ALK xovers/Trachorns, Speakercraft MT8(4), Sonos ZP90, Yamaha Aventage for surround
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  • unc2701
    unc2701 Posts: 3,587
    edited June 2014
    IIRC, those older Halfers are considered "in spec" up to 100mv and "keep an eye on it" up to 1v... I only remember this because it sounded so ridiculous. See if you can get a hold of a service manual to confirm. If it were me, I'd want to see it a lot lower, but it might not be realistic for the gear/vintage.
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  • deafbykhorns
    deafbykhorns Posts: 150
    edited June 2014
    unc2701 wrote: »
    IIRC, those older Halfers are considered "in spec" up to 100mv and "keep an eye on it" up to 1v... I only remember this because it sounded so ridiculous. See if you can get a hold of a service manual to confirm. If it were me, I'd want to see it a lot lower, but it might not be realistic for the gear/vintage.
    He said in post #3 that it sounded odd when the volume was turned up. This is a sure sign of a defective component, usually in the differential stages, a scope would tell him for sure.
    Main System- Scratch built 2A3 , Lightspeed preamp, Technics SP15/Graham/XV1s,Klipschorns w/ALK xovers/Trachorns, Speakercraft MT8(4), Sonos ZP90, Yamaha Aventage for surround
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  • BKphoto
    BKphoto Posts: 409
    edited June 2014
    i could see if the values were the same...but one is 15 and the other is 60....which to me says something is wrong...

    pulled the Marantz from mancave duty to living room to make up for the fallen soldier...
    Marantz 1152 DC- Denon DP 1200, Soundsmith Carmen MKII- ADS L980 - Blue Jeans IC's
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  • unc2701
    unc2701 Posts: 3,587
    edited June 2014
    deafbykhorns wrote: »
    He said in post #3 that it sounded odd when the volume was turned up. This is a sure sign of a defective component, usually in the differential stages, a scope would tell him for sure.

    Oops, missed that. Yeah, that needs to be looked at.
    Gallo Ref 3.1 : Bryston 4b SST : Musical fidelity CD Pre : VPI HW-19
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    Backburner:Krell KAV-300i
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  • deafbykhorns
    deafbykhorns Posts: 150
    edited June 2014
    Looked up those FETs, they are obsolete so you will have to cross these over to another FET.
    Shouldnt be too difficult to match up
    Main System- Scratch built 2A3 , Lightspeed preamp, Technics SP15/Graham/XV1s,Klipschorns w/ALK xovers/Trachorns, Speakercraft MT8(4), Sonos ZP90, Yamaha Aventage for surround
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  • BKphoto
    BKphoto Posts: 409
    edited June 2014
    always something....


    Thanks ...
    Marantz 1152 DC- Denon DP 1200, Soundsmith Carmen MKII- ADS L980 - Blue Jeans IC's
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