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Find Fault in my built amplifier

Discussion in 'General Electronics Discussion' started by Andrew Borg, Aug 24, 2014.

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  1. Andrew Borg

    Andrew Borg

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    Aug 10, 2014
    Hi guys recently I've built this amplifier:
    http://www.circuitstoday.com/150-watt-amplifier-circuit

    The thing is that I notice that one of the legs of the 1.5k resistor was hooked wrongly to the joint of the 0.33ohm resistors.
    I am hearing a hum from the speakers now with no sound from input.
    Did I burn one of the BC558 transistors?
    Any help please?
    Thanks
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It's hard to say. Take a copy of the circuit diagram and show us on the circuit what was shorted or incorrectly connected.
     
  3. Andrew Borg

    Andrew Borg

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    Aug 10, 2014
    Hi Steve thanks for response here is the incorrect connection that I did shown on this pic:
    [​IMG]
     
  4. BobK

    BobK

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    Jan 5, 2010
    That resistor should have limited the current to about 30mA which would probably not be enough to blow anything out. Had you connected the other leg, it would have been a problem.

    I take it the amp was wired incorrectly from the start, so the amp was never working, right?

    For starters, I would put a signal in, and look at the signal on the collector of Q5, then at the collector of Q3, each should be an amplified signal of the input.

    Bob
     
  5. Merlin3189

    Merlin3189

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    Aug 4, 2011
    Is it just me who finds this circuit weird? DC Voltages left to right and signal bottom to top, took me a while to follow this circuit.
    Am I just old fashioned expecting circuits to read from left to right?
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    That schematic is extremely badly drawn. FYI I redrew it when it came up in an earlier thread: https://www.electronicspoint.com/threads/help-with-amp.262454/#post-1565548

    To answer your specific question, that error would have forced the output hard to the negative rail. This would not have damaged anything as long as the speaker wasn't connected. But there are many problems with that design. See my comments in the other thread.
     
    Last edited: Aug 24, 2014
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    No, it's not just you! I wonder why they drew it like that. It makes me wonder if they've copied it from somewhere else, and redrew it all messed up to disguise this fact. But it's such a poor design that I doubt it's appeared anywhere else. And "never attribute to malice what can be adequately explained as simple incompetence".
     
  8. Andrew Borg

    Andrew Borg

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    Aug 10, 2014
    Hi guys thanks for help. So this means that I did not harm the circuit. Why is there alot of hum and no sound amplification. Can this be cause I connected a small speaker?
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Well, it's a bad idea to connect a small speaker to an amplifier that's powered from ±45V - one little click or pop, and your speaker is toast.

    Upload a photo of your construction, and mark up a copy of the schematic (preferably my readable version in the thread I linked to earlier) with the voltages on all pins of all transistors.

    You don't seem to be concerned by my comments about the significant problems with that design. Are you expecting it to work properly in spite of these problems?
     
  10. Andrew Borg

    Andrew Borg

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    Aug 10, 2014
    Hi Kris I concerned about your comments.Ok maybe you are right about problems in the circuit but the thing is that on youtube there are guys who built this circuit and they showed that it works.
    Here is a pic of my build.
    Thanks

    http://postimg.org/image/p18xbhe0h/
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK, that construction looks reasonable. Can you post the voltages on all pins of all transistors? This should give us a clue what's wrong.
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    edit: This post has been copied from another thread because it (a) gives a readable circuit diagram, and (b) clearly outlines the shortcomings. -- (*Steve*)

    Yes. That circuit is a power amplifier. It accepts an input signal at "line level". This is also the signal level that a spring-line reverb or other reverb circuit will operate.

    You will probably also need a preamplifier, to provide variable gain, tone control, impedance matching, and amplification, to drive that power amplifier. Typically, the reverb section would be connected around the output of the preamplifier.

    You should have posted a link to the project page for that amplifier. It is at http://www.circuitstoday.com/150-watt-amplifier-circuit. The accompanying text states that the amplifier can deliver 150W into a 4 ohm load. So it seems that your 4 ohm speaker can be connected to it.

    I have several concerns about that design though. My first clue to the nature of the design is that it is extremely badly drawn. So I redrew it.

    [​IMG]

    The biasing of the output transistors is wrong and will cause nasty crossover distortion at low output signal levels. The transistors are probably not adequately rated to deliver 150W into any load, and a 4 ohm load is more likely to exceed their SOA (safe operating area) and destroy them. In any case they will need a large heatsink. Also the long tailed pair transistors are only rated for 30V and should be replaced with BC557s at least.

    In general I think the circuit is an amateur design and I do not recommend it. Go to a professional site with properly designed circuits. An IC amplifier is probably a much better option, because the device is designed by experienced, educated people and there is much less for a designer to get wrong.
     

    Attached Files:

    Last edited by a moderator: Aug 27, 2014
    (*steve*) likes this.
  13. Andrew Borg

    Andrew Borg

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    Aug 10, 2014
    Hi Kris here are the pin voltages:
    Tip41 B to C = 0.7v; B to E= 0.06v; C to E= 0.03v
    Tip 142 B to C = -17v; C to E= -0.5v; B to E =16v
    Tip 147 B to E= -1.2v; B to C= 0.14v; C to E= 0.13v
    Left BC558 B to E =3.8v; B to C=5.3v; C to E= -1.6v
    Right BC558 B to E= 0.5v; B to C = unstable voltage; C to E= -0.5v

    Thanks
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Also note that the components have designations like Q4 and Q5, so you shouldn't refer to them as the "left BC558", or the "right BC558".

    The reason becomes clear when you look at Kris' far better diagram. (and I have edited your notes below)


    [​IMG]

     
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    So both output transistors (the TIP142 and the TIP147) have collector-emitter voltages of nearly 0V? They are connected in series between the +45V rail and the -45V rail. Their collector-emitter voltages should add up to 90V. You must have connected at least one thing wrong.

    I actually want to know the voltages AT those points, not the voltages between them. Save this image and use an image editor to fill in the rectangles with the voltages you measure at the marked points, measured with the multimeter black lead connected to the 0V rail, which I've highlighted on the diagram.

    150wamp ready for voltages markup.png

    Edit: Don't connect any speaker to the amplifier.
     
    Last edited: Aug 27, 2014
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  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    edit: read Kris' post. Do what he says. But also measure the resistance of R8 and R9.

    These are a problem (or you have measured incorrectly.

    The voltage between the collector of Q1 and the collector of Q2 should be 90 volts.

    Please report the voltage from 0V to +45V and from 0V to -45V. <-- Check this second

    You show 0.63V. This means that the 0.33 ohm resistors (R8 and R9) have about 89.5 volts across them, thus a current of about 140A flowing through them (which I think is quite unlikely).

    Are you sure R8 and R9 are 0.33 ohm resistors and not 33k resistors (or something similar)? <--- check this first

    The Q1's VBE of 16 volts either means the device is in breakdown or you have measured poorly.

    It would be useful to read these voltages with no speaker connected (assuming it was connected when these readings were taken), and to measure the voltage across the output and 0V (where the speaker would go). However this can probably wait.

    The base to collector voltage again suggests that something is wrong with the output stage. I would be expecting around 45V here. These transistors have to withstand 45V (at the best of times), and up to 90V (in fault or clipping conditions) across the Base/Collector junction. The transistors listed there won't like that.

    The VBE suggests that Q4 is in cutoff (not a typical place for it to be in a long tailed pair like this). That would suggest the output is pulled fairly hard to one supply rail. This is not indicated by your voltage measurements for the output transistors, unless (as mentioned before) the resistors R8 and R9 are wrong, your power supply is very low, or (possibly) you have the speaker connected. And it would almost certainly require 2 out of three of those.

    Once you verify a correct DC operating point (0V across the speaker with no signal, approx ) you can measure the voltage across one of these resistors to determine the current (again, no speaker connected). It should be between 0.0007V and 0.016V. I suspect you won't have a meter capable of measuring this.

    Measuring the voltage across R8 and R9 will give you an idea of the quiescent current through the output transistors. Typically this is a small value (between a couple and maybe 50 mA). This design provides no way to adjust it and appears to result in one of the transistors being biased off.
     
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  17. Andrew Borg

    Andrew Borg

    42
    4
    Aug 10, 2014
    Hi Steve and Kris thanks for your help. I am an amateur in electronics but I built other circuits.
    I will measure those points indicated and by the way my output transformer is 18v not 45v. I saw on youtube that this amp works on 18v too.
    Thanks
     
  18. Andrew Borg

    Andrew Borg

    42
    4
    Aug 10, 2014
    Today I took these measurements: Sorry I skipped one :/
    Steve R8 and R9 are 0.33ohms sure.
     

    Attached Files:

  19. BobK

    BobK

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    Jan 5, 2010
    Q5 is blown. (BE junction is open.)

    (There may be other problems.)

    Bob
     
  20. Andrew Borg

    Andrew Borg

    42
    4
    Aug 10, 2014
    Thanks Bob I will change that and see. Q3 base =-10.8v
    Input trafo =22v
    + to 0v = 11v
    - to 0v = -11v

    Thanks guys
     
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