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Small Amp Project (Tube/power stage info needed)

  1. Apr 23, 2016 #1
    Hey,
    Im in the process of building my final project for an engineering electronics course.

    The requirements:
    - 3 Inputs
    - mixer stage
    - driver stage 0.5 - 3watts ( my speaker is 3 watts)
    - output to 8 ohm speaker.

    Questions

    - Anyone have any good links regarding helping pick BJT, Jfet or Mosfet for pre-amp stage? I know from recording music they sound different but does anyone have any technical papers or good explanations?

    - I don't know which I should build for driver stage. Its recommended to do a BJT push pull a/b amp. Since I'm a recording/guitar guy I really like tubes. Anyone have any low wattage tube driver schems?
    Any recommendations would be appreciated.

    thanks
     
  2. jcsd
  3. Apr 24, 2016 #2
    - I don't know which I should build for driver stage. Its recommended to do a BJT push pull a/b amp. Since I'm a recording/guitar guy I really like tubes. Anyone have any low wattage tube driver schems?
    Any recommendations would be appreciated.

    When you say "driver stage" I think you mean output stage. Usually, a driver is the stage before the output.
    If you want to use a tube output, very good audio can be obtained for small powers from a single ended beam tetrode in Class A, such as the EL84, without using push-pull. But if you want to over-drive the output stage intentionally to obtain a traditional electric guitar sound, I believe the use of triodes is recommended so that the overload point is more gradual. Such tubes, however, involve much more cost and a more complex driver stage as the drive requirement is much greater. Incidentally, as an engineer, I am not prepared to accept that things sound different when I cannot see it on a CRO.
     
  4. Apr 24, 2016 #3

    jim hardy

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    RCA Tube Manual has a LOT of hobbyist audio circuits in last section. Below is a snippet from the RC30 edition of RCA Receiving Tube Manual which i downloaded from someplace. Google or Bing will find it...
    You will find the tubes and transformers pretty expensive nowadays .
    If you can find a trove of old not working tube radios and TV's to pillage it'd help.

    In high school(1963) i built this amplifier , used it for years .

    upload_2016-4-24_8-47-28.png

    Everybody uses integrated circuit amplifiers these days. Look up LM386.
    You can buy throwaway computer speakers for just a couple bucks in thrift stores.
    I learned a lot by taking them apart and looking up datasheets for the IC amplifiers inside.

    GE's transistor manual from about 1964 is a good "how to" book for bjt circuit design. It too is out there in pdf.

    Good luck in your studies,

    old jim
     
  5. Apr 24, 2016 #4
    The push pull amplifier shown will have less even order distortion than my suggestion for a single ended Class A beam tetrode. However, I would, be very surprised if this is audible or objectionable, and the additional complication and cost is very great.
    I think the LM386 does not give good audio, due to distortion and noise.
     
  6. Apr 24, 2016 #5

    jim hardy

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    All 3 of those points are true.

    That amplifier had wonderful sound.

    I have long believed that the highly controversial "Tube Sound" arises not so much from "distortion" attributed to tube amps
    but from the difference in speaker cone response when driven by a current controlled source versus a voltage controlled source.
    SInce amps are usually lab tested with a resistor load that difference gets missed because V = IR and R is constant.
    But for a speaker, V = IZ and Z includes inductance, mechanical inertia, spring constants of speaker cone and air in the enclosure.
    So the current through and voltage across a real speaker in a real enclosure in a real room will have different waveshapes.

    A pentode tube being basically a current source will deliver to a real speaker load a different voltage signal than to a pure resistive load. That would look like distortion on an o'scope . But they're usually tested with a resistor load so it doesn't show up in reviews.

    I am a "Transient Intermodulation Distortion" denier . It's the complex Z.

    Here is an article from a respectable engineering publication that goes further into the subject:
    http://www.edn.com/design/consumer/...periority-of-current-drive-over-voltage-drive

    And another from a hi-fi writer who did quite a bit of actual testing ,
    http://www.firstwatt.com/pdf/art_cs_amps.pdf
    He's a "true believer" in current source amps, see his site
    (CAVEAT - he appears to sell them and this is NOT an endorsement,
    i dont own one, just i find the concept interesting)

    upload_2016-4-24_12-17-28.png
     
    Last edited: Apr 24, 2016
  7. Apr 24, 2016 #6

    dlgoff

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    Indeed. Having been in a little rock band in the '60s and poor, we had to build our speaker enclosures; different speakers, different sounds. To bad these speakers didn't have today's magnets. A little off topics but, with tubes & speakers your can get an added dimension from the Leslie speaker ... most commonly associated with the Hammond organ, ...
     
  8. Apr 25, 2016 #7

    jim hardy

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  9. Apr 25, 2016 #8
    I worked out of a recording studio that had a working hammond and leslie speaker.
    I used the leslie once to give effect to an acoustic guitar part once. It was real neat.
    But I think moving that thing in the studio weighed more than my car.

    Thanks for all the information.

    If I stray from valve (since I have a time constraint)
    what is a backup plan for a power stage?
    I was thinking push-pull discrete transistor but am open to suggestions.
    (again simpler)


    -This information is amazing though because one of my summer projects is refurbishing a tube amp I ripped out of a old vinyl cabinet.
    If Its not really worth saving, I'll probably just build one. I am researching the math involved though as I'd like to learn tube design instead
    of just whipping a circuit together.


    Again thanks for replies.
     
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