Amplifier Distortion, DC-Offset, and You!

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

  1. M-One

    M-One New Member

    Messages:
    9
    Had to go up a scale - 0.176/0.230 on the little SA-5300 :eek:

    Well, looks like i'm transistor buying. Ain't no trim pots on this baby. Actually, there are two wires you cut when replacing them to "align" the amp - Mine have been cut (before I got it) - Any idea what this is/does? The procedure is in the service manual...
     

     

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  2. rmunson

    rmunson console destructor

    Pioneer sx-727
    left channel measured 1.2 on the 200m setting
    right channel measured .2
     
  3. bmanone

    bmanone Active Member

    Messages:
    130
    Location:
    Calico Rock, Arkansas
    Yamaha P-2200
    Left channel is 8.6 to 9.0 mOhms; Right channel is 9.6 to 10.1 mOhms. Was about 30 mOhms both channels prior to changing out input pair transistors.
     
  4. LesB3

    LesB3 Member

    Messages:
    50
    Location:
    Philadelphia, PA
    '76 Tandberg TR-2055
    L: -7.7mV
    R: -7.8mV
     
  5. starcopy

    starcopy AK Subscriber Subscriber

    Messages:
    201
    Location:
    NE Ohio USA
    A very warmed-up 1989 Pioneer VSX-4500S receiver.
    Set my Radio Shack multi-tester to 2V -- the lowest DCV setting I have.
    L: .067
    R: .072
    I'm no math major, but would I move the decimal point by three places to get 67mV and 72mV, respectively? Or move it two places to get 6.7 and 7.2? Thanks for any guidance.
     
    Last edited: Jan 8, 2018
  6. redk9258

    redk9258 Super Member

    Messages:
    1,483
    Location:
    Illinois, close to St. Louis.
    3 places.
     
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  7. starcopy

    starcopy AK Subscriber Subscriber

    Messages:
    201
    Location:
    NE Ohio USA
    Thanks, redk9258. So, just to confirm: I take my readings with my DMM set to my lowest available DCV setting of 2V. Then I take any readings I get and move the decimal point three places to the right to get my "mV" settings. So, in my post (two posts earlier) my Pioneer's left channel read .067, which means my DC offset for that channel is actually 67mV -- not great. (.067 DC volts = 67 millivolts.) And the right channel measured .072 DC volts, or 72mV, not great again. With these settings, "Something is amiss" according to EchoWars who started this iconic thread.

    BTW, some posters simply list/leave their actual readings as DC volts (which would be 0.67 and .072 in my case).

    Sorry for the silly, basic math questions. But EchoWars recommended that we set our meters "...to a low scale (300mV scale is common)." My DMM doesn't have a scale that low, so I wanted to make sure my math was in line. Thanks, again.
     
    Last edited: Jan 15, 2018
  8. Bill Ferris

    Bill Ferris Super Member

    Messages:
    1,495
    Location:
    NE. FL.
    FWIW All my 8 Crown DC 300/D150 & Phase Linear 400/700 amps that I own, have much less than 3 mV DC offset after full warm up/stabilize per channel.

    Sometimes 10 turn replacement/upgrade pots can help dialing in the lowest offset/bias, as original 270 degree rotation OEM controls are usually too course and can be too touchy to dial in unless your a safe cracker with Zen breathing discipline..

    But DC coupled amps must be functioning proper first and stabilized..
    Did same with the bias pots as well..

    Speakers tend to sound best when not "DC biased" IMHO/E !! No grit.

    Often the specs indicate +/- 10mV offset, I say much lower is better..

    But that`s just me and my OCD..

    Enjoy the music as it best fits your ears and tastes.

    Kind regards, OKB
     
  9. Radfordman

    Radfordman Super Member

    Messages:
    1,372
    Location:
    Middlesex
    Quad 405-2, in fact a Quad 405 bought new by me in 1976 and upgraded using Quad conversion kit around 1987.

    Right 0.7 mV

    Left 0.6 mV

    I'm quite impressed.
     
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  10. JLmoore3rd

    JLmoore3rd New Member

    Messages:
    8
    Location:
    Boise, ID
    B&K ST-140 #1 = Left .009 Right .047
    B&K ST-140 #2 = Left .010 Right .040
    Both of these have been refurbished within the last year. Interesting that they're so similar L to R.

    APT Corporation Power Amp One #1 = Left .009 Right .020
    APT Corporation Power Amp One #2 = Left .002 Right .007
    Neither of these have been refurbished, and they sound almost as nice as the ST-140 (although the ST-140 has the better bottom end).
     
  11. starcopy

    starcopy AK Subscriber Subscriber

    Messages:
    201
    Location:
    NE Ohio USA
    Onkyo TX-26 receiver. Left is .001. Right is .007.
    Sony STR-D665 AV receiver. Left is .000. Right is .002.
    Onkyo A-8170 amp. Left is mostly .002, but once varied from -.005 to +.006. Right is .002.

    Question: Am I (finally) getting lucky with finding older/vintage equipment that has respectable DC offset measurements? Or am I using the wrong DMM/process? In every case above: Volume is minimum. Bass/treble/balance are all centered. CD input selected -- but no CD player or other equipment connected. No speakers connected. Vintage Radio Shack DMM set to 2VDC (my lowest available DC setting). Red positive; black negative. When I disconnect the DMM its reading goes to zero.

    I'm sort of shocked by the good readings on all three. Lucky? Should I buy a lottery ticket? Any thoughts would be welcomed and appreciated. TIA.
     
    Last edited: Jan 17, 2018
    jritchie1980 likes this.

     

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  12. Jimbo40

    Jimbo40 Member

    Messages:
    90
    1974 pioneer qx949 original
    Right = 9.3 mv
    left = .3mv
     
  13. Ulysse

    Ulysse New Member

    Messages:
    39
    Harman Kardon 930
    Right = 113mv
    Left = 57mv

    The readings are high, and the right speaker fuse just blew out. What should I do next? A complete recap first? Or is there something else I should try?
    Thanks a lot!
     
  14. awillia6

    awillia6 Super Member

    Messages:
    1,126
    B&K Reference 7250 Series ii
    Ch. 1 = 0.0mV
    Ch. 2 = 0.0mV
    Ch. 3 = 0.0mV
    Ch. 4 = 0.0mV
    Ch. 5 = 0.0mV
     
  15. restorer-john

    restorer-john Addicted Member

    Messages:
    5,856
    Location:
    Australia
    You gotta turn it on ;)
     
  16. Jimbo40

    Jimbo40 Member

    Messages:
    90
    LOL, By George I think you have something.
     

     

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  17. jritchie1980

    jritchie1980 Audiologist Subscriber

    Messages:
    76
    Location:
    Chicago
    Bryston 2b

    Right - 6.2 mv
    Left - 11.1mv

    Does anyone know if/where the trimpots on this are?
     
  18. survey_sez

    survey_sez New Member

    Messages:
    12
    Location:
    Texas
    I've been struggling with this for way to long. My Anthem MRX300 powers up with Front Left DC offset of about 100mV which causes it to power down into protection mode a second or so later. Gut told me it should be a problem with the driver, so I proceeded to trace the schematic. The latter takes many hours...So far I've checked every transistor (OK) and replaced all electrolytics. The only thing I've found that's remotely suspicious is the differential pair gains are not precisely the same (i.e. Q4001/Gain 431 versus Q4002/Gain 505). My initial thoughts were that the swamping resistors would have negated small differences in gain. Regardless, I have swapped Q4001 with Q4024 (Gain 499) but have not powered it up yet. It sure would be nice if I had the Power Amp schematic...Any ideas?
     
    Last edited: Jan 29, 2018
  19. madspec

    madspec New Member

    Messages:
    36
    Location:
    Montreal, Canada
    Geez. .

    Sansui G-6000

    R: - 250mv
    L: - 90mv

    Have to do something about this lol, no wonder it goes into protection mode for nothing. ..
     
  20. FidgetMaster

    FidgetMaster Active Member

    Messages:
    433
    It don't help with capacitor distortion, maybe in a instrument amplifier even then a lot of it's not that good, thanks to when mettalized stuff came around and china making like some of the awfulest sounding ones.
     

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