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Thread: DC offset question.

  1. #1
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,885
    I'm Adrian.

    Default DC offset question.

    So to cut a long story short my ARC ref 75 has just been returned after 3 months at the service department. Diagnosis of fault DC on the input of one channel - busted FETs.

    I've hooked it up but not switched on yet. I measured for DC at the outputs of the Truth pre. With the multimeter set at 200mv I get 0.02 on both channels. Please can someone tell me if this is ok? It seems very very little but I'm so paranoid and dont want another 3 month absence and 900€ bill.
    Technics SP10 mk2
    Jan Allaerts MC 1 Boron mk1 cart
    Miyajima Shilabe cart
    Hashimoto HM-X SUT
    Siggwan (gimballed not unipivot) Cocobola 12"
    Aurorasound Vida LCR Phonostage
    The Truth linestage
    Dave Slagle Autoformer Volume Controller
    Cary 805c SET amps
    Audio Note ANe-SPX speakers
    Townshend Isolda speaker cables
    Cardas Golden Presence interconnects

  2. #2
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 6,089
    I'm Alan.

    Default

    I think a little more clarity is need on the reading.
    If the multimeter is set on the 200mV range then isn't the 0.02 a reading of 20uV?

    Adrian can you repeat the measurement on the 2V range and report back?

    Good reply Andrew but I'm a bit unsure of the reported reading.

  3. #3
    Join Date: Aug 2012

    Location: Hartlepool UK

    Posts: 1,640
    I'm Alan.

    Default

    The offset seems very low TBH
    Could the damage have occurred by careless hot phono plugging between pre and power where the signal makes a connection but no ground so you get the classic oscillation screech ?

    Alan
    Turntable - Garrard 401/Jelco 750L/Ortofon Kontrapunkt B, Pioneer PLC 590, Micro Sieki MA505 , Denon DL103R - DIY Paradise Phono stage - Reel 2 Reel Studer A810, Otari MX55,Tascam BR20, Revox A77, B77, PR99, TEAC X1000 & 3440, Digital HTPC / Young Dac - Preamp - DIY B4, 821, Power Amp's DIY Avondale NCC300 Mono Block, Speakers Wilmslow Kit Volt BM220.8 / Scanspeak D2905/9500

  4. #4
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 6,089
    I'm Alan.

    Default

    Could be Alan, I have to admit to damaging things myself doing that

    What a dork I am. Sorry cancel that, was.

  5. #5
    Join Date: Apr 2012

    Location: London

    Posts: 685
    I'm James.

    Default

    Another possibility is a knackered output cap on the preamp (or input cap on the amp...) allowing dc into the amp. Wouldn't it need a fair level of dc to destroy the fets? Would 20mV be enough?

    Edit: 20mV dc coming out of a pre sounds like abysmal design if it's 'normal' for the pre... I'd be angry if I got that out of my power amps, I'd be straight inside and checking/adjusting the bias.
    Sent from my GT-I9195 using Tapatalk

  6. #6
    Join Date: Mar 2010

    Location: Sheffield

    Posts: 2,898
    I'm Simon.

    Default

    Yup. Should be 20uv at that range.

    If no show on 2v he's good.


    20mv on a preamp is attrocious, it should sub 1mv
    Kuzma Stabi/S 12", (LP12-bastard) DC motor and optical tacho psu, Benz LP, Paradise (phonostage). MB-Pro, Brooklyn dac and psu, Bruno Putzeys balanced pre, mod86p dual mono amps, Yamaha NS1000m

  7. #7
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,885
    I'm Adrian.

    Default

    I have to admit I'm confused by the multimeter. I've included a pic of the settings. I did actually adjust the DC offset internally to zero, and perhaps the 0.02 reading is dodgy connection of the probes on the RCA outputs. I could use alligator clips when adjusting internally. (The maker guided me through the process and both channels read 0.00). I don't want any DC offset - nada. But I find it hard to interpret the meter.

    Could I connect a coupling cap across live/earth at the outputs to get rid of any? If so what kind of value?

    Here's the meter...

    Technics SP10 mk2
    Jan Allaerts MC 1 Boron mk1 cart
    Miyajima Shilabe cart
    Hashimoto HM-X SUT
    Siggwan (gimballed not unipivot) Cocobola 12"
    Aurorasound Vida LCR Phonostage
    The Truth linestage
    Dave Slagle Autoformer Volume Controller
    Cary 805c SET amps
    Audio Note ANe-SPX speakers
    Townshend Isolda speaker cables
    Cardas Golden Presence interconnects

  8. #8
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 6,089
    I'm Alan.

    Default

    Put it on 2000m and see what it reads.

    What did you use to adjust internally to zero?
    If it is showing 0.02 on the 200m range that is more than likely the expected error of the multimeter and is nothing to worry about.

    Though to put your mind fully to rest (we are all behind you in this) report what the 2000m range reads.

  9. #9
    Join Date: Apr 2012

    Location: London

    Posts: 685
    I'm James.

    Default

    No! The cap goes in series with the output.

    Edit: not aimed at Firebottle
    Sent from my GT-I9195 using Tapatalk

  10. #10
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,885
    I'm Adrian.

    Default

    Quote Originally Posted by Firebottle View Post
    Put it on 2000m and see what it reads.

    What did you use to adjust internally to zero?
    If it is showing 0.02 on the 200m range that is more than likely the expected error of the multimeter and is nothing to worry about.

    Though to put your mind fully to rest (we are all behind you in this) report what the 2000m range reads.
    Hang on....
    Technics SP10 mk2
    Jan Allaerts MC 1 Boron mk1 cart
    Miyajima Shilabe cart
    Hashimoto HM-X SUT
    Siggwan (gimballed not unipivot) Cocobola 12"
    Aurorasound Vida LCR Phonostage
    The Truth linestage
    Dave Slagle Autoformer Volume Controller
    Cary 805c SET amps
    Audio Note ANe-SPX speakers
    Townshend Isolda speaker cables
    Cardas Golden Presence interconnects

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