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Analysing Power Amplifier circuits-General discussion

Discussion in 'General Electronics Chat' started by fantabulous68, Feb 27, 2010.

  1. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It's the closest we've seen yet! :D

    Bootstrap R3, add a Vbe multiplier (instead of the diodes) and it wouldn't be bad. A zobel network on the output would be a good idea as well.

    But it certainly looks like a fairly practical design.
     
  2. unclejed613

    unclejed613 Well-Known Member

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    i was going with the simplest possible amp possible. there's a lot of room for improvement. just using a current source in place of R1 and a current mirror to balance the diff amp gets the distortion down to around -60db from the -40db of the unmodified version. a Vbe multiplier eeks out about another -6db (some of the distortion is crossover notch), and a current source in place of R3 brings the distortion down to -80db or so... the TIP31/32 drivers and the 2N3055/2955 outputs are also not the most linear transistors that could be used.

    this is a very "standard" amp design, and even without current sources and current mirrors is good enough that Denon, Yamaha, Pioneer and Sony use it currently for their mid-priced receivers with barely any difference from the way i drew it except for a Vbe multiplier in place of the diodes, and the driver transistors being built in to a pair of darlington output devices. also the output choke and zobel to keep it stable with speaker wire capacitances.
     
    Last edited: Mar 14, 2010
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    The most important difference on a commercial amp is R3, either bootstrapped or replaced with a constant current source (same thing really). The amp isn't really practical without it, as it severely restricts the power and substantially increases distortion.
     
  4. dave

    Dave New Member

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  5. fantabulous68

    fantabulous68 Member

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    Thanks Unclejed613. I will take your circuit into consideration when I do decide to build an amplifier(that would be after the Easter holidays:D).

    Just letting you'll know, that the circuits i post are just for analysis. Knowing the purpose of different stages, components and sub-circuits etc. What could be done to improve different stages and make it practical.



    Im at the analyzing stages...Designing will come soon:rolleyes:.
     
  6. unclejed613

    unclejed613 Well-Known Member

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    in addition to the link i provided, i also recommend you read the High-Power Audio Amplifier Construction Manual by G Randy Slone and the Audio Power Amplifier Design Handbook by Douglas Self. both authors provide very well thought out and logical step-by-step approaches to designing amplifiers. i also recommend you read http://www.electro-tech-online.com/custompdfs/2010/03/AN-A.pdf because audio amp and op amp design share a lot of common techniques and challenges.
     
  7. fantabulous68

    fantabulous68 Member

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    :eek:i spoke too soon its before Easter!, i have to design an amp. Well buh bye analyzing stages.
    50V single negative supply into an 8ohm load. Got my specs yesterday. Looking forward to posting "MY" design up and taking criticism. :DEspecially from MR GOODWIN:rolleyes:
    Let me get started:)

    I do have the book by Douglas:)
     
    Last edited: Mar 16, 2010
  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Just make sure you start with something that has a chance of working :D

    Don't worry about the single negative supply, it's pretty meaningless - it's just a single supply, negative or positive is just a question of where you measure from.
     
  9. unclejed613

    unclejed613 Well-Known Member

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    he's working with a 50V NEGATIVE single ended supply????? why???? nothing about this makes a whole lot of sense....


    would anybody be upset if i gave him a Wile E. Coyote Award?????

    on second thought, with the weird parameters given, it could be a school project... some prof wants their students to think outside the box (way, way outside the box)..
     
    Last edited: Mar 16, 2010
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Why?, it makes no difference whatsoever - it's only a modern convention to measure positive with respect to negative, in the early days of transistors the convention was to always measure negative with respect to positive.
     
  11. Hero999

    Hero999 Banned

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    In telecommunication systems, the positive is always bonded to earth so it's pretty normal to have negative voltages. If it offends you, just change the sign but that can get lead to confusion so it's better to keep it negative.
     
  12. fantabulous68

    fantabulous68 Member

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    SPECS given: 50v single negative supply, 8ohm load, single transistor input

    uhmmmm i tried:D.
     

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  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Many of the transistors will not survive a 50V supply. Select transistors with a higher voltage rating.
    C3 is not needed. Then R9 and R10 can be a single resistor.

    Q7 needs a load resistor which will help turn off Q6 quickly.
    The output transistors need emitter resistors.

    The value for C1 is far too small and for C6 is far too large.

    The circuit will probably oscillate at a high frequency because it is missing the compensation capacitor that is used on all audio amp and opamp circuits.
     
  14. fantabulous68

    fantabulous68 Member

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    yeah, i supposed to check the safe operating area limits for the bjt & see if it will work in the cct.
    The max collector current, max voltage it can handle and max power.

    Thanks Audioguru
     
    Last edited: Mar 16, 2010
  15. unclejed613

    unclejed613 Well-Known Member

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    Q6 needs a compensation cap to roll off the gain above 25khz or so as AG has said. Q7 needs to be able to sink the base current of Q6 to ground, and cannot do that as-is. your bias pot should be on the emitter side of the Vbe multiplier, not the collector side. if the pot should fail (not real common, but it does happen) by going open circuit or by the wiper going open from dirt or oxidation, the bias transistor will turn full on, shutting down the bias on the output devices. the amp will still work with some crossover distortion. if a collector connected pot goes open, the bias transistor goes open circuit, running the output devices at or beyond their max current and they will short, and take the driver transistors, and Q6 and possibly Q7 along with them.

    you might want to move C6 to where R1 is, R1 to where C1 is, and C1 to where C6 is. the bootstrap cap doesn't have to be very large, but the speaker coupling cap has to be large, and it's best not to have speaker terminals that are live with DC voltage (but designers of class D bridged amps do that all the time without it being much of a safety hazard).

    btw, this amp will have a pretty hefty turn-on thump through the speaker when it's turned on, so realize when it does that it's normal for this type of amp.
     
  16. Hero999

    Hero999 Banned

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    Don't use Word for schematics, use PNG.

    There doesn't need to be two capacitors in series with the speaker: C1 isn't needed and will just limit the frequency response to 2kHz.
     

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  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    C1 is needed to block the 25VDC from this single supply amplifier. It should be about 1000uF for -3dB (half power) at 20Hz into the 8 ohm speaker.
     
  18. fantabulous68

    fantabulous68 Member

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    The pic was in bmp and was too large to attach, i tried compressing it in microsoft office manager but the picture quality was poor. Thats why I put it in word.
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I use Microsoft Paint program to save and change any picture into a PNG.
     
  20. Hero999

    Hero999 Banned

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    Yes, you're right, I misread the schematic.
     
  21. mneary

    mneary New Member

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    The circuit will not work at all because the -50V and 0V lines have no DC connection to ground except through a pair of 200k resistors (R9, R12). The 8 ohm load is returned to ground so all its drive current must flow through these two resistors.

    It is not possible that 0V and ground are the same as this would short out R9.

    [edit] I didn't notice C3 and C4. There is NO connection between the power rails and GND.
     
    Last edited: Mar 17, 2010

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