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Audio amplifier.

Discussion in 'Electronic Basics' started by [email protected], Aug 7, 2005.

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  1. Guest

    I want to build an amplifier for listening music on my computer.
    Can anyone suggest where to start? I have very basic knowledge of
    electronics but probably need some theory. Any good books?

    I searched around on the net and the only thing I can find are complete
    schematics and books on how to build tube amplifiers. I'm really not
    intersted in tube amps, but since the only information on the net is
    about tube amps I dont even know what is the alternative!

    Any help is greatly apprciated.
    Thanks in advance.
     
  2. Is this to connect to the audio output of your sound board or sound
    connector on your PC? (There are often outputs both designed to
    directly connect to headphones and also to a pre-amp input of an
    amplifier system. Which do you plan to use?)

    Are you trying to make this a learning experience and not just find a
    cheap solution? (There are pretty cheap amplifiers available.)

    An amplifier system may first pre-condition the input source, then
    send the result of that through one or more voltage amplification
    stages, then to a power amplification stage (or two.) Are you
    interested in learning how to design a single stage of this with
    transistors and then expand upon that knowledge? Or?

    Jon
     
  3. pacemkr

    pacemkr Guest

    I have a Sound Blaster Live 5.1 to connect to.

    I am deffinitely trying to make a learnign experience out of it.
    However, I think I'm better off geting some basic electronics knowledge
    first.

    So to change the topic a little is there an amp you would recommend. I
    heard that older/used amps are cheaper and better. Is that the case? I
    would like to preserve the 5.1 capabiliy if possible.
     
  4. Tom LeMense

    Tom LeMense Guest

    If you haven't done so already, you should spend some time figuring out what
    your design goals are. For example, how many Watts output are you shooting
    for? A good answer is probably something less than 20 - more than that and
    things get complicated real fast. That drives a lot of other factors -
    given the output power goal, you can then determine how much voltage you'll
    need to swing across the load, which in turn will give you an idea of the
    power supply rail voltage and current you'll need to have available. That
    will also help you determine how much voltage gain you'll need from input to
    output.
    A "boosted op-amp" topology is one approach that yields good results, here's
    a schematic that I found with a *very brief* Google search that is very
    similar in nature to an amplifier design I used many years ago:

    http://www.electronics-lab.com/projects/audio/012/

    This is a class AB current gain stage (Q1-Q4) attached to a differential
    voltage gain stage (IC1) so the two stages together provide power gain.
    This design uses negative feedback (R5 and R1+C2) to flatten out the
    frequency response and reduce distortion. You can adjust the amount of
    voltage gain by varying R5 and R1+C2.

    A circuit like this will help you get your feet wet with audio amplifiers,
    yet is pretty simple to build and straightforward to "debug". It also
    doesn't require any particularly hard to find components, matched transistor
    pairs, &c.

    If you're not already familiar with Opamps, you might see about getting a
    hold of a copy of _The Art of Electronics_ by Horowitz & Hill which covers a
    whole range of electronics topics. Walt Jung's _Opamp Cookbook_ is an
    excellent in-depth treatment of opamps.

    Some fine authors that have written on audio electronics are Douglas Self
    and John L. Hood. Search for them on Amazon.com and you'll come up with a
    number of titles.
    Tube amplifiers are a lot of fun to work with, and I've built several and
    even tried my hand at designing a couple on my own, just for fun. I can
    say, however, that the components for a tube amp tend to be expensive and
    harder to obtain; e.g. the tubes themselves, output transformers, power
    transformers with HV secondary windings, good quality tube sockets, high
    voltage 'lytic and film caps, &c. Because of this, solid state audio
    amplifiers are, IMHO, a more natural starting point for someone setting out
    to learn about audio electronics.
    Good luck and have fun!

    TJL
     
  5. pacemkr

    pacemkr Guest

    Thank you very much for an insightfull reply.
    "The Art of Electronics" is actually the book I wanted to buy anyway,
    just to get some theory on electronics. You are the second person to
    recommend it, so I am deffinitly getting it.

    I was thinking of starting my own amp right away. But as I move further
    into the actual process, I think that I'm better off building one from
    a schematic just to "get my feet wet" so to speak. Thanks for
    mentioning the authors too, it will make my search a lot easier.

    As for tube amps cost and reliability was my main concern, so for the
    time being I am going to stay clear of tube amps. Speaking of cost, I
    know this greatly depends on components, but in general what price
    range am I looking at. To begin with I will probably make a 2 chanel
    amp. But I REALLY want to have 5.1 capability.
     
  6. I'll add my voice to this, but really *do* try and secure the student
    manual for it, as well. It fills in a lot of the kinds of details you
    will want to read about, that is NOT present in as clear fashion in
    the book. It supplements the book in good fashion.

    Jon
     
  7. Tom LeMense

    Tom LeMense Guest

    You're most welcome.
    Audio is one of the first applications of electronics (up there with
    lighting, motors, &c) and so there is a tremendous body of knowledge out
    there, some of it dating back to the turn of the century - the last one,
    that is! While I am certainly not the type to tell someone NOT to go ahead
    and try something, it's almost always a good idea to see how others have
    done something before setting off on your own. After you understand and
    weigh the merits and drawbacks of what you've found, your own design will be
    that much better.

    Another great way to learn is through kit building - a whole bunch of people
    (myself included) got started with electronics by way of building Heathkits
    or Knight Kits. Heathkit is centered on education now, and no longer makes
    kits, but Velleman is one company that makes kits that appear to be high
    quality and I consistently hear good things about them. They have a large
    number of audio amplifier and preamplifier kits, both solid state and tube:

    http://www.vellemanusa.com/
    If you go the boosted opamp route (my previous followup post) you can cost
    out the components from somewhere like DigiKey or Mouser. Your costs
    *still* all depend on how powerful an amplifier you want to build, and how
    well stocked your "junk box" is. I'd venture to say that the power
    transformer(s), the power supply reservoir capacitors, and the chassis will
    be your major costs. Some folks make one power supply circuit that feeds all
    the channels, and others prefer to seperate them. Separating the supplies
    is more expensive, but then the loading of one channel affect the other(s).

    I think that it's not unreasonable to expect that the amplifier you build
    first won't be perfect. Opamp selection is one opportunity for improvement.
    A regulated power supply for the opamp would be yet another one. How about
    a volume control? Mute feature? Tone controls? Input selection circutry?
    Remote control? You're only limited by what you want to do, and feel like
    doing...

    TJL
     
  8. In thinking about all this and placing it into my own context a bit
    (again, I do NOT do much electronics stuff as it is only a hobby and I
    have my life filled with other responsibilities), I notice that I
    learned a great deal more when trying to design a simple, single stage
    voltage amplifier out of one BJT than I did playing around with an
    opamp.

    Understanding things like why a bypass capacitor might be used in the
    emitter leg in order to allow setting a decent AC gain while at the
    same time providing a usable DC bias point for the emitter to set some
    temperature stability; or how capacitive bootstrapping from emitter
    back to base actually works to stiffen the input impedance; etc. But
    then actually playing with building an actual stage and seeing it
    operate, too...

    Sometimes, I think that building with opamps teaches how things work
    in electronics about as well as assembling a prefab bookshelf teaches
    you about woodworking.

    Jon
     
  9. To be more precise about it, I'm thinking along these lines when
    talking about bootstrapping, etc.

    : Vcc
    : |
    : |
    : Vcc |
    : | \
    : | / R2
    : | \
    : \ /
    : / R3 |
    : \ |
    : / +--------------Vout
    : C1 | |
    : || | |
    : ----||---------------------, |
    : || | | |
    : | R6 | |/c Q1
    : +-------/\/\-----+-----|
    : | |>e
    : | |
    : | C3 |
    : | || +-------,
    : +--------------||--------+ |
    : | || | |
    : \ \ \
    : / R4 / R1 / R5
    : \ \ \
    : / / /
    : | | |
    : | | |
    : | | --- C2
    : gnd gnd ---
    : |
    : |
    : |
    : gnd

    Jon
     
  10. Ban

    Ban Guest

    I wonder what this stage has to do with an audio power amplifier. And then
    opamps are usually *not* used for these amps, but specialized integrated
    power ICs like TDA7293. It will take a heck of knowledge and experience to
    make something better (for 5 bucks).
    I also have the impression that you will need first yourself to get
    accoustomed with circuit design before giving advise.
     
  11. I was thinking in terms of an intermediate voltage amp stage. Not a
    final.
    Agreed. But I'm thinking of the learning experience.
    You are probably right. I won't argue with a professional on that
    score.

    Jon
     
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