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Thread: DC offset question.

  1. #1
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,683
    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 2016

    Location: Bolton, England

    Posts: 1,142
    I'm Andrew.

    Default

    The "normal" way to build preamps and power amps is to have no DC on the outputs of the preamp and no way for DC at the inputs of the power amp to do any harm. Eliminating DC at the output of the preamp is done by having a DC blocking capacitor. Protection from DC at the inputs of the power amp is achieved by having a DC blocking capacitor there too. Maybe it's a belt-and-braces approach but it's generally safe. Removing that protection is inviting trouble in my opinion.

    20mV DC on the output of the preamp may be amplified about 30 times by the power amp, so you would end up with 600mV DC offset at the speaker terminals. I can't remember what amp you have but maybe it amplifies more than 30 times. Anyway, about 1V DC offset at the speaker terminals is in the danger zone in my opinion.

    BTW, the pre-amp might have 20mV DC on its outputs during normal use but at switch-on there may be a huge DC swing before it settles down.

  3. #3
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 4,123
    I'm Alan.

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    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.

  4. #4
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 1,142
    I'm Andrew.

    Default

    Quote Originally Posted by Firebottle View Post
    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?
    Yep, you're absolutely right. What a numpty

    Still, I'd prefer DC blocking caps for safety.

  5. #5
    Join Date: Aug 2012

    Location: Hartlepool UK

    Posts: 1,335
    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, SME 3009/2 , Denon DL103R - DIY Paradise Phono stage - Reel 2 Reel Studer A810, Otari MX55,Tascam BR20 - Digital HTPC / Young Dac - Preamp - DIY B4, 821, Power Amp's DIY Voyager Mono Block, Speakers Tannoy precision 6.4 / Wilmslow Kit Volt BM220.8 / Scanspeak D2905/9500

  6. #6
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 4,123
    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.

  7. #7
    Join Date: Apr 2012

    Location: London

    Posts: 322
    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

  8. #8
    Join Date: Mar 2010

    Location: Sheffield

    Posts: 2,575
    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

  9. #9
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,683
    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

  10. #10
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 4,123
    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.

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