Amplifier Distortion, DC-Offset, and You!

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

  1. 363watt

    363watt New Member

    Messages:
    31
    Location:
    Cincinnati,OH
    Carver M500t -1.5mvdc right channel 64mvdc left channel.
    No issues with sound quality. Inverted style amplification internal wiring.
     
  2. DrTebi

    DrTebi AK Subscriber Subscriber

    Messages:
    67
    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?
     
  3. RJolly87

    RJolly87 AK Subscriber Subscriber

    Messages:
    553
    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.
     
  4. RJolly87

    RJolly87 AK Subscriber Subscriber

    Messages:
    553
    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.
     
  5. Homer4beer

    Homer4beer AK Subscriber Subscriber

    Messages:
    656
    Location:
    Brisbane - Australia
    Great post.
    Many thanks.
     
  6. cferry1

    cferry1 Member

    Messages:
    65
    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
  7. jcm2012

    jcm2012 AK Subscriber Subscriber

    Messages:
    119
    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!
     
  8. 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
     
    Homer4beer likes this.
  9. cferry1

    cferry1 Member

    Messages:
    65
    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.
     
  10. 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.
     
  11. sootboots

    sootboots New Member

    Messages:
    28
    Recently acquired Sansui AU-9900. Left 6mv right 15mv. That is good enough for me. How often should this be checked?

    Jp
     
  12. LTD428CJ

    LTD428CJ New Member

    Messages:
    2
    Sansui 5000. 23.6 - left, 24.2 - right. These were "peak" numbers seen rarely.
     
  13. ilusndweller

    ilusndweller Addicted Member

    Messages:
    5,217
    I just retested. Nearly 4 months later and they are at -9.0 and -9.1. At some point Ill probably readjust, but for now this JVC A-X5 is sounding wonderful. In just 4 months it seems to have drifted (and hopefully settled by now) a good amount.
     
  14. ernestas0001

    ernestas0001 New Member

    Messages:
    2
    Hi !

    My Harman Kardon HK 3490 gives ok results:

    L 25
    R 21

    I have set trimpots as close as possible to 21.12 as manual stated. Probably I will need to look closer into hardware.
     
  15. redk9258

    redk9258 Super Member

    Messages:
    1,161
    Location:
    Illinois, close to St. Louis.
    Are you confusing bias with DC offset? Be careful if you aren't 100% sure of what you are adjusting. You WILL blow up the output transistors along with other stuff.
     
  16. ernestas0001

    ernestas0001 New Member

    Messages:
    2
    Yes, I think I might be mixing up the things ! Anyways, I haven`t played with them, simply adjusted to correct values according to the manual as they seemed to drift away.
    Now, how could I decrease milivolt values on speaker connectors on HK 3490 ?
     
  17. Eastham

    Eastham More Class A than ever! Subscriber

    Messages:
    1,536
    Location:
    Liverpool, UK
    Well it's been a few months since I recapped my Sherwood S-8910 and here's the DC offset readings as of today; 4.5mV left, 11mV right.
     
  18. Powertech

    Powertech New Member

    Messages:
    41
    Location:
    South Wales, U.K.
    My Rotel RB976 (6 channels) range from 4.1mV to 8.4 mV. They are running quiescent currents of about 90mA per channel. It runs slightly warm, but it does sound better than the factory recommendation.
     
  19. Olecranon

    Olecranon Active Member

    Messages:
    152
    Do you need a preamp connected to the amplifier to get a "true" reading, or can this be done with the preamp disconnected?

    I check my Rotel RB-980BX after leaving it on for 15 minutes.

    L - 1.1mV
    R - 8.6mV
     
    Last edited: Nov 18, 2017 at 1:09 PM
  20. Vaylo

    Vaylo New Member

    Messages:
    1
    My Onkyo A-22: left 11.5mV, right 12mV.
     

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