6.15 BJT amplifier

Discussion in 'The Projects Forum' started by Nicholas Olcan, Apr 3, 2013.

  1. Nicholas Olcan

    Thread Starter New Member

    Feb 22, 2013
    9
    1
    Hi everyone,
    I'm new to the forum,and I need some help.
    If anyone knows Randy Stone "The Audiophile's Project Sourcebook",I built a BJT amplifier (Figure 6.15) ...which doesn't work.So far I burned three speakers,till I learned to check for DC current at the speaker.
    Removing transistors from Q16 to Q23 brings the DC voltage down to .5V,but one I put back the Q16 and Q17 the voltage at the speaker goes back to +48V.
    Also the D6 diode goes so hot after 1-2 minutes,you can smell it.
    For the PCB I used the artwork from the book,so I can't make any mistake designing it.
    Beeing my first posting,I don't know how to attach a picture of the electrical diagram of the amplifier.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,414
    3,353
    Without a schematic it is difficult to comment.
    However, unless a DC coupled amplifier is properly balanced you are going to blow your speakers.
    Use a 4700μF capacitor in series with the loudspeaker.
     
  3. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    If are ARE using a DC coupled amplifier, test it with an 8-ohm resistor before connecting to a speaker. You can get resistors in 5 - 150 watt range, maybe higher.
     
  4. #12

    Expert

    Nov 30, 2010
    16,248
    6,744
    go advanced, manage attachments, browse, open, upload.
     
  5. Nicholas Olcan

    Thread Starter New Member

    Feb 22, 2013
    9
    1
    I have a schematic in My Documents,but I don't know how to attach it to the text.
     
  6. Nicholas Olcan

    Thread Starter New Member

    Feb 22, 2013
    9
    1
    I'm trying to post some pictures.
     
  7. patricktoday

    Member

    Feb 12, 2013
    157
    42
    D6 gets hot? The one paired with D7 near the center of the diagram? But it appears you removed D(9??? sorry, hard to read the numbers) the one on the far right of the page towards the top. Was D9 installed in the correct 'reverse' direction with the white band towards the positive rail? Perhaps it would be helpful to take more voltage measurements and mark them on the schematic to help determine which parts of the circuit look correct and which do not.
     
  8. Nicholas Olcan

    Thread Starter New Member

    Feb 22, 2013
    9
    1
    Hi Pat,
    The D9 you see,I removed just to see if something will change.Practically I can't power the amp for more than 1-2 minutes,because D6( 1N4148) gets so hot you can smell it.Also,R28 gets pretty hot too.
    I've built two identical amps and both have the same problem.I've read on the Google that some amplifiers from those books work...and some don't.Thanks for help.
     
  9. Nicholas Olcan

    Thread Starter New Member

    Feb 22, 2013
    9
    1
    I'm attaching more information.
     
  10. #12

    Expert

    Nov 30, 2010
    16,248
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    This can be very difficult. It is basically an operational amplifier, closed loop, DC coupled. Anything that pegs an op amp will peg this design. Anything from a missing bias on an input transistor to a resistor in the feedback loop can cause this problem. The normal failure mode is to lock up at the + or- rail.

    This takes me all the way back to Q2 and Q3 which dump base current from the positive rail into the input circuits, and there is no ground reference on them. This one looks like it depends on the Q16 string to set a DC point. My first move would be: Short the input to ground, short the "shield" input to ground, disconnect the power from everything after the Q16 string and see if it will zero from the middle of the Q16 string through R17.
     
    Last edited: Apr 5, 2013
  11. Nicholas Olcan

    Thread Starter New Member

    Feb 22, 2013
    9
    1
    Hi #12,
    I followed your idea to check Q16,17,the first one show an inconstant beta on the tester,and the second one a low beta,around 80.I also checked Q14,15,and replaced them because of a low beta.Rechecking the DC at the speaker,reads 0.00V!!!Hooked up the speaker and the CD player and I got a nice warm sound!
    Thank you very much,it looks like you really know this stuff.
     
    absf likes this.
  12. #12

    Expert

    Nov 30, 2010
    16,248
    6,744
    (Complete jerk struts and preens.)

    "Some days the magic works and some days it doesn't."
    (Chief Dan George in Little Big Man)
     
    absf likes this.
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