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Audio amp...Phase splitter...Why?

Discussion in 'Electronic Basics' started by Xtrchessreal, Jan 17, 2006.

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  1. Xtrchessreal

    Xtrchessreal Guest

    I have been racking my brain to learn about audio amlpifiers. My EE
    courses never covered them. I can't find any books in the library.

    Audio is apparently either difficult to understand or intentionally
    cryptic to make it seem so.

    I have a question for anyone that can explain it.

    In a Class AB Tube push pull amplifier the OT output Transformer is fed
    a signal and an inverted signal. WHY?

    What is the purpose of this two signal input to the OT?

    Here is my guess: Because the inverted AC signal causes a larger
    potential difference across the OT gaining as much power as possible
    for the output.

    How is the split signal 0 degree phase and 180 degree phase then
    reconstructed on the secondary into one signal across the speakers.
     
  2. Xtrchessreal wrote:
    (snip)
    The output transformer is a sort of electrical see saw. Tubes pull
    down, only (draw current from positive supplies) so to get an audio
    that swings both positive and negative, one way is to rock the "see
    saw" one way with one tube and the other way with the other.
    It is a way to combine the power from two tubes into a single signal
    with low distortion, since some of the distortion since the positive
    and negative swings are produced symmetrically.
    The two signals are combined as a single magnetic field in the core of
    the transformer (with one tube magnetizing it one way, and the other
    tube magnetizing it the other way) and the secondary extracts power
    from this alternating field.

    Here is an example of a simple push pull (actually pull pull, but on
    opposite sides of the see saw) amplifier.
    http://members.aol.com/sbench/norcpp.html
    and the page with it and more examples:
    http://members.aol.com/sbench101/#PowerAmps
     
  3. Here is another basic tutorial on audio tube amplifiers:
    http://www.hans-egebo.dk/Tutorial/amplifiers.htm
     
  4. Phil Allison

    Phil Allison Guest

    "Xtrchessreal"

    ** Audio is far from simple.

    But it looks so simple.

    That is why there are soooo many arguments.


    ** If you take the *whole* primary winding, there is really no such
    inversion.


    ** The use of two tubes with a *centre tapped transformer* means DC bias
    current in the tubes flows in opposite directions in each half of the
    primary winding. Hence the DC magnetic field cancels out and the iron core
    is NOT magnetised as it would be otherwise.

    With a single tube design this cannot happen, the iron core has to be much
    larger and fitted with a small air gap to prevent magnetic saturation. When
    a single tube amp is at idle - the iron core is always half fully
    magnetised.

    Using two tubes in so called "push- pull" mode also means tube non linearity
    tends to cancel as well.

    The two tubes are driven with identical but antiphase signals so that AC
    current will flow in the primary in one direction at any moment - hence
    generating a voltage at the secondary.

    Googling with "push pull amp"' get lots of good hits.




    ......... Phil
     
  5. Pooh Bear

    Pooh Bear Guest

    Only if taught badly. Audio is gneric 'low frequency' electronics really.
    So that one tube handles positive going outputs whilst another handles the
    negative.
    That's what a centre tapped transformer does by its nature !

    Graham
     
  6. John  Larkin

    John Larkin Guest

    Not in the last, say, 40 years.

    Actually, neither.
    DSP is complex. Differential equations are complex. Control theory and
    Laplace transforms are complex. Filter design is complex. But not much
    argument is provoked by them.

    So something else must be going on.

    John
     
  7. Xtrchessreal

    Xtrchessreal Guest

    Thanks JP,PA, and PB for your time to answer my questions. I spent a
    few hours going over the Tube in general getting a feel for the actions
    taking place inside.

    So, now I feel like I know why there is a phase splitter in a Push Pull
    type output stage. Basically the tube has to operate in a positive
    current flow from cathode to plate using a grid to control the
    flow/current inside the tube. The grid is biased with a less positive
    voltage in order to control the current from cathode to plate. That
    control bias can be designed in several ways. The main idea is that in
    a push pull type amplifier the two ouput tubes must run in a positive
    current from cathode to plate. As the AC signal is flowing in the
    positive to negative cycle the tubes (naturally to the circuit) take
    the positive portion of the signal and amplify it to the output
    transformer. Hence the transformer is center tapped and the signal is
    fed from each output tube 180 degrees at a time. There may be some
    signal loss during the crossover from each output tube or at least some
    noise or distortion introduced to the output.

    I am guessing that the transformer is inverse wound from the center tap
    to the bottom such the the negative portion of the signal causes flux
    to flow in the same direction as the top half with the positive portion
    of the signal. Thus the entire cycle is reconstructed on the
    secondary. Also, that each tube power output is additive to the total
    output less any losses from noise etc in the transformer. So If my amp
    is running at 29 watts per tube plate dissapation then the total output
    of the amp is near 58 watts RMS depending on the efficiency of the
    transformer.

    I was initially confused with a few different things. 1st that the
    inverted signal was adding in a delayed signal 180 degrees behind the
    initial signal. 2nd that the inverted signal was adding to the
    potential difference or subtracting depending on the phase you are
    looking at. 3rd that both tubes were in operation during the whole
    cycle of the signal.

    I am now wondering about the phase splitter circuit.

    In my amp Marshall 2204 a single tube is used to split the signal and
    there is a feedback loop fed to the same phase splitter circuit.

    Without the feedback the tube still produces two signals inverted from
    each other - that is right yes or no?

    The Feedback loop I know is canceling some portion of the input signal
    but I don't know why it is there. If that could be explained that
    would be very nice.

    I appreciate your input and your time. Appropriate books are hard to
    find in stores and libraries so please accept my apologies for asking
    too much.

    Thanks for your help.

    X
     
  8. Guest

    Push pull is used because it is more efficient, you get a lot more
    power out for the same sized components. The transformer is normally
    wound it just has a tap halfway down the winding. Crossover distortion
    is avoided by ensuring that both tubes conduct near the crossover
    point. Negative feedback is used to reduce distortion, you'll find lots
    of full explainations about it on the web or in text books.
     
  9. Phil Allison

    Phil Allison Guest


    ** Efficiency is simply the ratio of power input to power output.

    Not the "size" of the bits used.

    BTW The number of components in a PP stage is way higher.



    ** Really ? Been Googling have we ??

    Push pull *tube* amps generally operate in class A up to half power or
    more.

    Crossover problems are impossible.



    ** Improvements in frequency response and damping are irrelevant then ?


    ** You must take you on advice some day.




    ......... Phil
     
  10. (snip)

    It looks like you have made considerable progress.
    (snip)

    Or there may be extra gain at the cross over, when both tubes are
    contributing to the output at the same time. The exact case depends
    on how the tube gain varies near cut off.
    Almost right. Tube plate dissipation is a loss. So the output is the
    supply power minus plate dissipation and all other losses.
    An inversion is not actually a time delay.
    Now you have the idea. Inversion of input and output allows the
    possibility of subtraction, so that the plate current can be rising on
    one tube or the other as the signal approaches either positive or
    negative peaks.
    In some push pull designs, they are. This is a push pull class A
    amplifier. They have high losses, but cancel the even harmonic
    distortion of a single ended class A design, and also the DC net
    current in the transformer primary.
    So this tube is in between a plate resistor and a cathode resistor of
    equal values. As it varies its current, the drop across those two
    resistors rise and fall similarly, except that one resistor has
    voltage varying on its most positive end, and one at its most negative
    end. So the voltages at those ends varies in the opposite direction
    from each other.
    Yes. But both those signal outputs have more distortion.
    (snip)

    The more gain, the more cancellation. So throwing away some of the
    gain with negative feedback makes the lower net gain more stable
    throughout the signal swing.
     
  11. Rich Grise

    Rich Grise Guest

    No average DC on the transformer primary, ergo, it (theoretically) doesn't
    saturate. (It might still on the peaks, but that will just distort the
    signal, not let the smoke out.)
    That, too, and the fact that you can run class AB which is much more
    efficient than class A.

    PLEASE!! PLEASE!!!! It is _NOT_ "180 degrees out of phase", it is
    INVERTED!!!!!!!

    One common way I've seen it is to use a triode, that's half ordinary
    amplifier, and half cathode-follower, with equal cathode and plate
    resistors - the signals will be mirror images of each other. (i.e., one
    goes up when the other goes down.) NOT 180 degrees out of phase!!!!!

    Admittedly, at any given frequency, a sine wave shifted by 180 deg.
    is indistinguishable from its counterpart that's only inverted, but
    if the frequency changes, the phase will probably change because
    there's something in the circuit that delays the signal, like an
    inductance or capacitance. But merely inverting it is not the same
    thing - it's not frequency-dependent.

    Hope This Helps!
    Rich
     
  12. Phil Allison

    Phil Allison Guest

    "Rich Grise"

    ** If transformer core saturation occurs at all - it is invariably at the
    lowest frequencies an amp is capable of AND squashes the waveform at or
    near ZERO crossings.

    On a scope, it looks like LF crossover distortion.

    On a Marshall, the first signs appear at about 45Hz and full output.



    ......... Phil
     
  13. Phil Allison

    Phil Allison Guest

    "John Popelish"

    ** Nope.

    That "single tube" is a 12AX7 twin triode.

    Marshalls use a differential ( cathode coupled) phase splitter.



    .......... Phil
     
  14. Phil, do you know where I can see a schematic, so I don't have to keep
    guessing for this poster.
     
  15. Xtrchessreal

    Xtrchessreal Guest

    You guys are very helpful here. Thank you for your input.

    I will be more thorough of circuit descriptions in the future. The
    Marshall does have a 12xa7 in the so called phase splitter circuit
    though in some schematics it is written as ECC83. I do not have enough
    tube knowledge to know if that is the same thing or if one is a
    specific tube name while the other is a general tube function name or
    they are completely different triodes.

    I do know that schematics do not always have the actual components
    listed or circuit diagram as the device was released out the door.
    Sometimes components change due to inventory the day it was assembled
    as opposed to what it was supposed to have according to schematic or
    BOM.

    The term "Phase splitter circuit" in this amplifier is not a well
    chosen name because it does lead a person, at least a new person in
    electronics, to believe that the signal is 180 degrees out of phase.
    This I was struggling with initially believing the signal was delayed.
    You are taught this idea of lagging and leading Eli when learning about
    phase and phase diagrams etc. In this Marshall the circuit is actually
    producing a normal and an inverted signal so I see why using the phase
    term at all is not a good idea. Perhaps it should simply be called
    "Inverted signal splitter circuit" though the schematic does call it as
    I said above.

    Is this a difference of European vs American terminology perhaps?
    Whatever the case may be this is another example of why I feel that so
    much of Audio Electronics and Electrical Engineering seems so cryptic
    almost as though to make it harder than it really is. The real issue
    for me in that regard is I can't find any books on audio electronics
    that explain this stuff independently of the rest of Electronics. All
    of my text books completely miss Tubes and the only mention of audio is
    in the form of a radio amplifier and receiver which gets one half page
    out of 890.

    Why has the electrical engineering curriculum so completely stopped
    teaching audio? Last I looked everyone has a stereo, mp3 player, 5.1
    audio card for the PC, a Home theater, TVs, and what about musicians,
    guitar players, keyboardists, Disc Jockeys and rappers, singers, etc.
    why am I not taught audio in all of these areas? I will ask my Dean.

    If anyone out there has enough knowledge to write a book on audio you
    may make some money on it. For me understanding the signal the
    electrical/physical signal as it propagates through the audio amplifier
    would be very helpful to me. Seeing the various voltage dividers and
    tube electrodes in motion would give me something real I can picture in
    my head. Math by itself just doesn't do anything for me.

    Thanks for all of your help
     
  16. Phil Allison

    Phil Allison Guest

  17. Pooh Bear

    Pooh Bear Guest

    It is the same thing. ECC83 is the European name for an American 12AX7.

    Graham
     
  18. Xtrchessreal wrote:
    (snip)
    (snip)

    I think, since the earliest active devices were tubes, anything having
    to do with them has a lot of historical baggage.
    I wouldn't bet on it. At least not unless it is written in Chinese.
    I agree. Until I can wave my arms around and talk about an electronic
    effect in conversational terms with almost no math, and make sense to
    myself, I don't feel like I have any real understanding of a mechanism
    or process.

    By the way, in case you haven't heard, when doing Google searches for
    understanding (as opposed to sales or patents or whatever), a very
    fine word to add is "tutorial".
    E.g. ["tube amplifier" tutorial], ["audio amplifier" tutorial],
    "single ended amplifier" tutorial], etc.
     
  19. Jasen Betts

    Jasen Betts Guest

    not true, the earlest active devices were spark gaps.

    Bye.
    Jasen
     
  20. Rich Grise

    Rich Grise Guest

    They're the same tube, but numbered 12AX7 in the US and ECC83 in Europe.

    Cheers!
    Rich
     
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