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audio amp adjust

Discussion in 'Electronic Repair' started by Sofie, Sep 23, 2004.

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  1. Sofie

    Sofie Guest

    re: Hitachi SR-604 Receiver.
    On the driver board there are two pots R731L and R731R
    and beneath each pot are 2 test lugs labeled TP (+), TP(-),
    ...... obviously adjustments and test points for each channel.
    What is the factory specified of idle current or voltage?
    I can make it work fine by guessing but I really am curious about the
    correct setting.
    Thanks in advance.
    Dan
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  2. Norm Dresner

    Norm Dresner Guest

    It's likely that the adjustment should be done to get minimum distortion.
    Do you have an analyzer? You're not going to see the difference between,
    say, .5% and .1% at any frequency by eye on an oscilloscope or by ear.

    Norm
     
  3. Asimov

    Asimov Guest

    "Norm Dresner" bravely wrote to "All" (24 Sep 04 02:57:00)
    --- on the heady topic of "Re: audio amp adjust"

    ND> From: "Norm Dresner" <>
    ND> Subject: Re: audio amp adjust
    ND> Organization: AT&T Worldnet
    ND> Xref: aeinews sci.electronics.repair:39689

    ND> It's likely that the adjustment should be done to get minimum
    ND> distortion. Do you have an analyzer? You're not going to see the
    ND> difference between, say, .5% and .1% at any frequency by eye on an
    ND> oscilloscope or by ear.

    The idle bias current adjustment is best done at a medium low
    listening level because this is where crossover distortion is most
    audibly objectionable. At very low power the amp may still be
    operating in class A and it won't show. At the other extreme at a high
    level the distotion is dwarfed by the large signal and similarly won't
    show. A ballpark figure of 1 watt is a good level to test for
    crossover (10 watts depending on speaker efficiency). Start at an idle
    bias current of about 10-20mA and work up from there to a limit of
    about 60-100mA (per device if in parallel). Just watch for thermal
    stability. The current tends to increase as the heatsink warms up.

    Some will use a scope and distortion analyzer to get within specs.
    However it can be done by ear if you know what to listen for.
    Basically, crossover distortion makes the music sound gritty and
    grainy. A good frequency to listen to is the high bass say around 200
    to 300 Hertz. The grit will stand out better in the bass register. It
    could be done with a musical input but this makes it hard to hear the
    grit. Perhaps something like ELP's "Lucky Man" might do having long
    isolated and sustained bass notes. (ah the 70's!) Good luck.

    A*s*i*m*o*v

    .... A stereo system is the altar to the god of music.
     
  4. Digi Man

    Digi Man Guest

    All amplifiers have trimmers for adjustment on idle.
    Select minimum current coming from power supply when inputs are grounded and
    speakers connected.
     
  5. Sofie

    Sofie Guest

    Digi Man:
    Thank you for your quick response, however, AFAIK setting for minimum power
    supply current as your post suggests would, in most amp designs, produce
    some amount of cross-over distortion in excess of factory specs.....
    depending of the specific design of the amp and where the test points are in
    the circuit, the factory will have specified settings that will produce the
    least amount of distortion with satisfactory idle current that will safely
    operate the output semiconductors to the full specified power output.
    Since I am not interested in "engineering" this design and do not have a
    distortion analyzer I would be grateful if someone out there had the factory
    service manual and could provide me with the correct settings/procedure.
    Dan.



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  6. Sofie

    Sofie Guest

    Norm Dresner:
    Thank you so much for your quick reply. Yes, you are probably correct
    about the minimum distortion.... And, NO, I do not have a distortion
    analyzer.
    The factory service manual will usually indicate a specified amount of idle
    current or voltage using their test points that will keep the distortion at
    specified levels and allow proper operation of the output devices at full
    specified power.
    Does anyone have the factory manual?
    Dan.
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  7. jakdedert

    jakdedert Guest

    Just feed it a sine wave into a dummy load. Initially, adjust for minimum
    voltage with your DMM. Monitor the wave with your scope, paying particular
    attention to the zero crossing point (After verifying a clean waveform,
    magnify the display until that part of the trace almost fills the screen).
    Crossover distortion shows up as a discontinuity where the trace crosses the
    line. Adjust for minimum distortion.

    jak
     
  8. ddr0id

    ddr0id Guest

    If you don't get the proper info, you will find that if you set the
    current through the emitter resistors to about 20mA, you will be near
    enough, assuming bipolar transistors.
     
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