Amplifier Distortion, DC-Offset, and You!

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

  1. Chaulk61

    Chaulk61 New Member

    Messages:
    16
    Yamaha cr 820

    R: 11.8
    L: 11.6
    But I went to measure idling current, and get readings of 0 on both left and right. How can this be or what am I doing wrong? Red to tp1 - black to tp2 - receiver on for 10 mins, speakers off, aux selected, vol at Minimum
     
  2. desertrat748

    desertrat748 New Member

    Messages:
    16
    Purchased an refurbished Sansui 661 from local shop. Decent sound but decided to check the offset after warming it up.
    Before: R: 22~28 mV
    L: 55~58.5 mV

    After adj: R: 2.1 ~ 4.8 mV
    L: 0.8 ~ 2.6 mV

    I still love the sound of this old SS receiver and my mind says it sounds even better now. EDIT: I listened to the Sansui for hours yesterday. Didn't believe it could sound so much better! Unreal now. THANK YOU Echowars!

    Checked an old Kenwood KR-5060 that powers up the kitchen turntable over some vintage Sansui speakers (don't laugh, it's just for making coffee and toast in the mornings). This thing sounds really clean and clear, maybe it's because of decent DC offset?

    R: 2.2 mV to 4.5mV
    L: -1.9 mV to 2.6 mV


    Fisher CA-880 amp (has a Darlington pair and can't find where to adjust it). Still sounds really decent though. Clean.

    R: 48.2 mV
    L: 52.1 mV

    Only with the Sansui does the offset mV swing around and fluctuates a lot even when it is really warmed up but it looks like that's okay. This thread is huge and I've only read up to the first 160 pages or so although I jumped to the end to see what's been happening. Thanks Echo for starting and sharing your knowledge!
     
    Last edited: Dec 18, 2017
  3. 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...
     
  4. rmunson

    rmunson console destructor

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

    bmanone Active Member

    Messages:
    103
    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.
     
  6. LesB3

    LesB3 New Member

    Messages:
    29
    Location:
    Philadelphia, PA
    '76 Tandberg TR-2055
    L: -7.7mV
    R: -7.8mV
     
  7. starcopy

    starcopy AK Subscriber Subscriber

    Messages:
    145
    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
  8. redk9258

    redk9258 Super Member

    Messages:
    1,275
    Location:
    Illinois, close to St. Louis.
    3 places.
     
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  9. starcopy

    starcopy AK Subscriber Subscriber

    Messages:
    145
    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
  10. Bill Ferris

    Bill Ferris Well-Known Member

    Messages:
    818
    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
     
  11. Radfordman

    Radfordman Super Member

    Messages:
    1,362
    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.
     
    Bill Ferris likes this.
  12. 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).
     
  13. starcopy

    starcopy AK Subscriber Subscriber

    Messages:
    145
    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 at 1:31 PM
  14. Jimbo40

    Jimbo40 New Member

    Messages:
    8
    1974 pioneer qx949 original
    Right = 9.3 mv
    left = .3mv
     

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