Amplifier Distortion, DC-Offset, and You!

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

  1. woofmytweets

    woofmytweets AK Subscriber Subscriber

    Messages:
    1,471
    Location:
    Toronto, Canada
    well there you go - at least i was correct when i said i didn't know what i was talking about.

    perhaps i should consider running for office?
     
  2. ShadedDawg

    ShadedDawg New Member

    Messages:
    1
    Thanks for the challenge and test instructions. I just joined and I am most grateful to be among other audio enthusiasts.

    When my neighbors moved, their son handed me this Marantz and said thanks. I didn't think it was anything but an forgotten ancient amp that would be of no use to me. But it was free. I couldn't complain. It was sad looking, with broken feet, and really noisy when looked at, with lights out. But with all that, on a set of Infinity Beta 20's, this unit is delicious sounding once you left the knobs alone. Some Deoxit and some speaker relay and interior and cleaning, and this unit is a really sweet sounding amp.

    MARANTZ 2238b
    RIGHT: .3mV
    LEFT: .3mV

    WooHoo!

    I am awaiting the arrival of a new pair of Q-Acoustics 2050i's, and I was wondering if this Marantz would be compatible with them?
    and…
    What Class amp is the Marantz 2238b? I saw in a review of the 2050i, and they mentioned that "Class D" amps need not apply.

    Thanks and Happy listening.

    Adam
     
    Last edited: Jun 15, 2017
  3. ilusndweller

    ilusndweller Super Member

    Messages:
    4,898
    Yamaha RX-530 (1987-1989). -2.4 mV L / -1.4 mV R. Nice!
     
  4. LWB

    LWB AK Subscriber Subscriber

    Messages:
    295
    Location:
    Calgary, Alberta, Canada
    Re. Replaced differential pair in ADCOM GFA 545 (post 4416):
    I've not been able to find substitutes that would have lower Vbe, so I think I'm at a standstill on this one. Suggestions welcome: the original transistors in the pair were 2sc2362, I replaced them with KSC1845 (Hfe about 415, Vbe .76V).

    Later edit: After installing the KSC1845's and resetting Bias, resultant DC offset was: Right channel 26.5 mV (was 57) & Left channel 28.5 mV (was 35). Googling "GFA545 DC offset" turned up numerous references that seem to indicate these GFA545 amps often exhibit widely varying DC offset (over 100 mV not uncommon), and replacing the differential pair in one other case similar to mine dropped the offset down to about 25mV. I also found a reference from Jim Williams (who apparently offers [offered?] a commercial refurbishment of these amps) that mentioned "A manual DC offset trim could also be installed if this (DC offset) is a major problem." Now that sounds interesting and is perhaps what this amp needs to bring DC offset down to nothing.
     
    Last edited: Jun 16, 2017
  5. biw1428

    biw1428 New Member

    Messages:
    20
    Pioneer SX-3500
    L: 22.6Mv
    R:27.2Mv

    Guess I've got some digging to do...
     
  6. ilusndweller

    ilusndweller Super Member

    Messages:
    4,898
    Can someone confirm this statement(generally speaking, but feel free to point out exceptions)? "Aging capacitors do not affect DC offset". This is what I have gathered.
     
  7. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    7,828
    Location:
    New Jersey
    Tough to make a blanket statement. Components can change value over time which is why things need to be adjusted. Are you talking power supply , decoupling caps?
    It's too broad question I believe. Someone more knowledgeable please feel free to correct me or add to what i said
     
  8. ilusndweller

    ilusndweller Super Member

    Messages:
    4,898
    Im talking any and all caps that someone replaces with they do a thorough "recap" on their unit.
     
  9. BobHol

    BobHol Old fart Subscriber

    Messages:
    492
    Location:
    Fort wayne IN
    If the unit sounds good to you may be better off leaving it as is. As Steve Goodman sang "don't go looking for trouble, trouble will find you". Depends how comfortable you are getting into old electronics.
     
    kjello likes this.
  10. ilusndweller

    ilusndweller Super Member

    Messages:
    4,898
    I got the service manual online and was able to dial both to within <1mV by turning a pot for each one. Sounds fantastic. Wish they were all this simple to adjust!
     
  11. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    7,828
    Location:
    New Jersey
    Short answer caps shouldn't be what making DC offset off
     
    ilusndweller likes this.
  12. Alan0354

    Alan0354 Super Member

    Messages:
    2,090
    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.
     
    I LIKE MUSIC and ilusndweller like this.
  13. xschop

    xschop New Member

    Messages:
    36
    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
     
  14. 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
     
  15. beeluv

    beeluv New Member

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

    starcopy Member

    Messages:
    83
    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.
     
  17. Dylanl

    Dylanl Active Member

    Messages:
    117
    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?
     

    Attached Files:

    Last edited: Aug 12, 2017
  18. Alan0354

    Alan0354 Super Member

    Messages:
    2,090
    Location:
    Bay Area
    Yes, I would adjust to below 30mV.
     
  19. Koppa

    Koppa New Member

    Messages:
    1
    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.
     
  20. Zolton

    Zolton Well-Known Member

    Messages:
    814
    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.
     

Share This Page