Amplifier Distortion, DC-Offset, and You!

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

  1. Ilikevinyl

    Ilikevinyl Cool Breeze Subscriber

    I adjusted the DC Offset the best I could. Both channels now fluctuate between 26-39. The meter keeps going up and down between these two numbers on both channels at the best setting. There's no way the numbers will be close to 0! The amp does feel cooler to the touch now. The bass does seem a bit fuller than before. I'm a happy camper with my NAD 3140 more so now! :music:
     
    Beau Geste, jgannon and Eastham like this.
  2. rshep

    rshep Working my way to 1000 posts Subscriber

    Messages:
    833
    Location:
    Sunny Phoenix, AZ
    I have a Hafler DH200 (original owner) and measured the DC offset at 46mv/26mv. Of course, there isn’t any offset adjustment on the amp, you have to match the driver transistor pairs to minimize the offset.

    I read some posts by EchoWars and purchased a dozen TX694B AND TX795A’s which he recommended from Mouser. Using my handy chinese universal component tester, I matched beta and actually go 4 well-matched pairs of each type out of the batch.

    Substituting the 2N5550’s (Q1/Q2) with TX694B and the 2N5401’s with TX795A (Q5/Q6) with the best matched pairs from the dozen that I purchased. Since I have more transistors, I decided to swap all the remaining 2N5550’s and 2N5401’s.

    I powered up the DH200 and let it warm up for half an hour. Both channels were down to 1.1 mV!

    Listening to the amp, I think it does sound better. All the small caps have been replaced, now I just need to upgrade the power supply and renew the output transistor thermal paste.
     
  3. zepantufa

    zepantufa New Member

    Messages:
    5
    Hi all,

    Does anyone cares about this?

    Any help well appreciate.

    Thanks
     
  4. ilusndweller

    ilusndweller Super Member

    Messages:
    4,898
    NAD C326BEE (used). Left 2.0 mV, right 0.2 mV
     
    Last edited: May 21, 2017
    kjello likes this.
  5. zenithfan1

    zenithfan1 Super Member

    Messages:
    2,038
    Location:
    Winthrop Harbor, Il
    I was given a Pioneer SX-939 today, it's in good shape but is full of cigarette smoke. Uggghhh. I measured 18.5 and 22.something...not bad for an all original survivor that's never been taken out of use in 42 years. I was going to sell it, but after an intense (cop calling loud) listening session, I may keep this one. After I go through it and clean it and all pots and controls it'll be like new again...minus that one scratch ontop the faceplate that really isn't bothering me right now...
     
  6. Trojan4Life

    Trojan4Life Active Member

    Messages:
    377
    You might want to read up on Vbe matching differential amplifier pairs, rather than matching beta on the diff pair. The Peak Atlas DCM55 will give you a Vbe measurement reading.
     
  7. LWB

    LWB AK Subscriber Subscriber

    Messages:
    295
    Location:
    Calgary, Alberta, Canada
    Re. Post 4448: Vbe matched as well, both measured 0.76.
     
  8. Trojan4Life

    Trojan4Life Active Member

    Messages:
    377
    Read this post: http://audiokarma.org/forums/index....or-2270-high-bias-early-serial-number.770102/

    You'll need to find a transistor replacement for your amp that has a lower Vbe measurement for use as the diff pair. You can also read about what the differential amplifier pair is used for in the circuit: https://en.wikipedia.org/wiki/Differential_amplifier

    Biasing

    In contrast with classic amplifying stages that are biased from the side of the base (and so they are highly β-dependent), the differential pair is directly biased from the side of the emitters by sinking/injecting the total quiescent current. The series negative feedback (the emitter degeneration) makes the transistors act as voltage stabilizers; it forces them to adjust their VBE voltages (base currents) to pass the quiescent current through their collector-emitter junctions. So, due to the negative feedback, the quiescent current depends only slightly on the transistor's β.

    The biasing base currents needed to evoke the quiescent collector currents usually come from the ground, pass through the input sources and enter the bases. So, the sources have to be galvanic (DC) to ensure paths for the biasing current and low resistive enough to not create significant voltage drops across them. Otherwise, additional DC elements should be connected between the bases and the ground (or the positive power supply).
    [​IMG]
     
  9. ilusndweller

    ilusndweller Super Member

    Messages:
    4,898
    IMG_2442.JPG Nakamichi 530(1978) 6.0mV L/ -20.0 mVR
     
    Last edited: Jun 4, 2017
  10. ilusndweller

    ilusndweller Super Member

    Messages:
    4,898
    NEC A-610 -22 mV L / 6.7 mV R

    IMG_2444.JPG
     
  11. bikehomero

    bikehomero Active Member

    Messages:
    124
    Location:
    Frankfurt am Main - Germany
    Very nice tread! Havn't read all the posts here, but some of them. I remember to have a SONY TA-3140F in storage with a "little" offset problem. It has about 196mV at the right channel. Left is at 43mV DC.

    Offset is not adjustable, so i have also to change some transistors. Can anyone tell me which transistors are affacted? Is it Q502/3? Usually the original types are not longer available, so i woud need recommendations for substutes as well.

    Thank you in advance!
     

    Attached Files:

    Last edited: Jun 4, 2017
  12. bikehomero

    bikehomero Active Member

    Messages:
    124
    Location:
    Frankfurt am Main - Germany
    I think it's better to post a fullsize picture of the plan. BTW: D501 is a termal bias compensator located at the power transistors heat sink. The circuit braker CB-1 is a lamp and a bimetal switch together.

    Schaltplan TA-3140.PNG
     
  13. bolero

    bolero Well-Known Member

    Messages:
    600
    Location:
    Ontario
    just picked up a Kenwood KA 4006

    R: 9.5 mV ( started at 12 mV but stabilized @9.5 mV after being on 10 mins )
    L: 1.2 mV ( started at .8 mV and even bounced around 0 mV for bit, before stabilizing at 1.2 mV )

    I plugged in a tuner & checked the headphone output, speakers off, and everything sounded fine. hooking speakers up now...
     
  14. ilusndweller

    ilusndweller Super Member

    Messages:
    4,898
    JVC A-X5 48.4 mV left, 431mV right. Hrummmph. Gonna need to start checking this before buying whenever possible. Didnt think it was sounding that great, now I know why.

    // 77 mV left, 457 mV right. I think this thing is ready to take out speakers.
     
    Last edited: Jun 12, 2017
  15. ilusndweller

    ilusndweller Super Member

    Messages:
    4,898
    From original post

    "100mV to ?: A high enough voltage will cause the DC protection to kick in. This happens at a level determined by the designer, but is usually equivalent to about a diode drop (600mV)or so. Needless to say, if you are listening to an amp with 100mV or more of DC offset, you have no idea what the amp really is supposed to sound like. Indeed, some amps without a differential input are actually designed to have a bit of DC at the outputs, but this is triple-rare, and I don't think anyone here owns one. (in my book it's piss-poor design, but if you can sell it WTH...)"

    Im listening to the JVC A-X5 with DC offsets of 77mV left and 457mV right. It sounds fine. At least my ears cant tell that anything is off (most mid fi amplification sounds about the same to me). I dont think I could tell a difference in sound bw this JVC and my current model NAD C326BEE with dc offsets of 2.0 mV left and 0.2 mV right. :confused:.
     
  16. woofmytweets

    woofmytweets AK Subscriber Subscriber

    Messages:
    1,471
    Location:
    Toronto, Canada
    it was my understanding that too much dc is going to heat up and put excess wear (or damage) to the speaker's voice coil....not necessarily something you'd hear
     
  17. ilusndweller

    ilusndweller Super Member

    Messages:
    4,898
    The original post seems (to me at least) to suggest a correlation bw DC offset and sound quality.

    So will this (high dc offsets) mostly be taken care of by replacing all the transisters?
     
    Last edited: Jun 13, 2017
  18. woofmytweets

    woofmytweets AK Subscriber Subscriber

    Messages:
    1,471
    Location:
    Toronto, Canada
    i'm not really a tech, but from the service manuals I've read, there's usually independent bias adjustment pots to take care of the L&R offset....although that's not something I've ever done. I can say for sure, that just because you can't hear it, does not mean that it's not a problem.
     
  19. ilusndweller

    ilusndweller Super Member

    Messages:
    4,898
    Gotcha, thanks! Will look into finding the service manual. As I listen to this jvc, especially at higher volumes, it does not sound so good.
     
    Last edited: Jun 13, 2017
  20. redk9258

    redk9258 Super Member

    Messages:
    1,017
    Location:
    Illinois, close to St. Louis.
    Bias and offset are two things. Many amps do not have offset adjustments. The offset is 'adjusted' by matching the differential transistors.
     

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