Amplifier Distortion, DC-Offset, and You!

Discussion in 'Solid State' started by EchoWars, Mar 19, 2003.

  1. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    9,267
    Location:
    New Jersey
    Short answer caps shouldn't be what making DC offset off
     
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  2. Alan0354

    Alan0354 Super Member

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    2,735
    Location:
    Bay Area
    If you are talking about the aging power supply filter caps ( the big ones after the rectifiers). No, they have NOTHING to do with the DC offset at the amplifier output. Absolutely nothing......unless it short out the power supply.......then all hell break loose!!!

    In normal operating condition, output DC offset is governed by the offset of the input differential pair. Some amps use a trim pot to trim out the DC offset and it can drift over time.
     
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  3. xschop

    xschop Member

    Messages:
    57
    Technics SA-GX170 Class H+
    Been running for years in a dusty and sometimes hot shop environment....

    Left... 4.8 mV
    Right... 4.0 mV
    20170711_131915.jpg
     
  4. reuada

    reuada New Member

    Messages:
    16
    Location:
    uk
    Very interesting thread

    Radio Shack Realistic STA-820 Receiver...
    L: 4.3 mv
    R: 7.1 mv
     
  5. beeluv

    beeluv New Member

    Messages:
    8
    Location:
    Newport, VT
    Sony STR-6800SD
    Left: 17 mv
    Right 9 mv
     
  6. starcopy

    starcopy AK Subscriber Subscriber

    Messages:
    176
    Location:
    NE Ohio USA
    Hey, I'm sort of late to the "check your DC offset" party but I did bring my multimeter. And I plan to test most of my receivers. Thanks for posting the process. But first I have a few dumb questions if this thread is alive -- and if you have the time to reply:
    1. The receiver must be on during all testing, I'm assuming?
    2. So if you have both A and B speaker jacks, is there any need to test both A and B -- or does one reading apply to both?
    3. Speakers disconnected -- but you can leave speakers connected to "B" while testing "A" with speaker switch set to "A."
    4. But what if the receiver only has one set of speaker jacks -- rare, but found on some low-wattage units? Leave speakers disconnected while testing the speaker jacks? Safe to do?
    5. "Give the amp 10 minutes to settle." So, turn receiver on and wait 10 minutes before taking a reading? Or take two readings: one immediately and another after 10 minutes?

    If you can reply, many thanks. If not, I understand and still appreciate all the info you've already shared.
     

     

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  7. Dylanl

    Dylanl Active Member

    Messages:
    122
    Location:
    Pittsburgh, PA
    So just bought a Kenwood ka-9100 and here are the readings. the left channel -129mV DC that needs adjusted? I was able to adjust the bias both right and left inside. Why woukd that have drifted so far off?
     

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    Last edited: Aug 12, 2017
  8. Alan0354

    Alan0354 Super Member

    Messages:
    2,735
    Location:
    Bay Area
    Yes, I would adjust to below 30mV.
     
  9. Koppa

    Koppa New Member

    Messages:
    3
    Measured my sansui 9090DB last night. Was positively surprised to find "just" 18.5mV (R) and 9.0mV (L). The only thing that's been done to this old (40 yrs!) girl is replacement of bulbs that had gone dark behind the front panel.
     
  10. Zolton

    Zolton Well-Known Member

    Messages:
    817
    Location:
    Southern Ontario Canada
    My old faithful standby "when all else fails" Sansui AU-5900

    Left 10mV
    Right 20mV

    I would normally adjust but don't have the option on this unit.
     
  11. 363watt

    363watt New Member

    Messages:
    42
    Location:
    Cincinnati,OH
    Carver M500t -1.5mvdc right channel 64mvdc left channel.
    No issues with sound quality. Inverted style amplification internal wiring.
     

     

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  12. DrTebi

    DrTebi AK Subscriber Subscriber

    Messages:
    76
    Just checked my "new" Accuphase P-300L. Came to me from HifiDo in Japan, who said that it was checked through by Accuphase last year.
    DC offset is 0.3 mV on the left channel, and -0.4 mV on the right channel. I suppose that's quite good :) Love that pretty little monster :)

    My Sony TA-3200F measured a bit differently... I just did a full recap on it (it had some weird noise in the left channel). I am now measuring:
    9 mV on the right channel
    between 0 and 35 mV on the left.
    The left channel is constantly fluctuating. Does that mean there is something wrong?
     
  13. RJolly87

    RJolly87 AK Subscriber Subscriber

    Messages:
    600
    Location:
    Las Cruces, NM
    Being still new at this hobby, I was beginning to realize my recently acquired Pioneer SX-780 didn't really seem to be living up to its reputation, with one channel fading out at low volume, that seemed to improve as I fiddle with input cabling.

    I checked bias voltage as mentioned, and found -130mV in the left channel, and -150mV in the right channel. I didn't let the multi-meter settle for 10 min admittedly, but the amp had been running for at least that long, and readings were stable when I stopped.

    Needless to say, the amp is now getting sidelined for the time being.
     
  14. RJolly87

    RJolly87 AK Subscriber Subscriber

    Messages:
    600
    Location:
    Las Cruces, NM
    Checked my Marantz 2245.

    Left 14mV, Right 95mV.

    Sounds much better for the time being, but it's obvious I have my work cut out for me.
     
  15. Homer4beer

    Homer4beer AK Subscriber Subscriber

    Messages:
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    Location:
    Brisbane - Australia
    Great post.
    Many thanks.
     
  16. cferry1

    cferry1 Member

    Messages:
    81
    Appears to be a itnteresting thread. Yes, Just noticing this thread.

    Prior to reading through 225 pages. I will be attacking this read later today.

    Question to any member who may dare to answer involved as :
    What Experience or Relationship between Power Amplifier DC Offset / Distortion to Replacement Parts primarily Transistors used - Generic Partnumbers vs Original Manufacturer Partnumbers ???

    IT WOULD BE A WONDERFUL EFFORT FOR SOME MEMBER OR ADMINISTRATOR TO SUMMARY THE HIGHLIGHTED INFO OF THIS THREAD AND POST SOMEWHERE !


    cferry1
     
    Last edited: Sep 26, 2017
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  17. jcm2012

    jcm2012 AK Subscriber Subscriber

    Messages:
    107
    B&K ST2140 - right around 6.7 and 6.5

    Just received it yesterday. So far so good, there is a bit of a pop when I turn it on. Sounds great otherwise!
     
  18. AUDIO1MAN

    AUDIO1MAN New Member

    Messages:
    11
    Location:
    Thousand Oaks, CA
    Hi All
    I have designed hundreds of audio power amplifiers during my career @ many company's like BGW, JBL and others. The DC OFFSET for an audio amplifier should be a max of 10mv. The DC OFFSET caused current to flow into the speaker and this can cause higher speaker distortion and hotter output transistors. If the offset is >10mv fix the problem.
    Duke
     
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  19. cferry1

    cferry1 Member

    Messages:
    81
    Agree. I higher DC Balance Offset Voltage and higher Idle Current Adjustment will result in much higher temperature Output Final Power Transistor's. The Heatsink Fins as well.
     
  20. bikehomero

    bikehomero Active Member

    Messages:
    130
    Location:
    Frankfurt am Main - Germany

    Solved the problem today. It was the transistor Q601. Changed it from the left channel an DC was at 30mV. At the left side i replaced the konw missing Q501 with a BC547 resulting in 17mV DC.
     

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