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Extend freq audio xfmr

Discussion in 'Electronic Design' started by Conrad Ferrer, Jun 26, 2007.

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  1. Is there any way, through the use of external components, to extend
    the range at which an audio output transformer rolls off high
    frequencies?

    For example, when using an 8 ohm to 100V PA transformer for voltage
    step-up, amplitude falls off at around 25KHz.

    Conrad Ferrer
     
  2. whit3rd

    whit3rd Guest

    The low frequency performance of an audio transformer is maintained
    by using a mass of high-permeability iron. To get higher frequency
    performance, one uses thinner iron plates with insulating spacers
    (just oxide layer on the plates, really) to prevent internal eddy
    currents, or for very high frequency one can use powdered-iron
    or ceramic (ferrite) cores.

    But a ferrite that performs well into the RF range is more expensive
    (and larger) than laminated-iron for a given power level. And this
    is an OUTPUT transformer, power level is high. If you can
    use biamplification (split off the woofer or subwoofer to a
    separate output circuit) the low frequencies can be handled
    with iron core, and high frequencies with whatever is appropriate.
    If it all has to go through a single core, you might need to get
    a custom-designed unit, or use something meant for switchmode
    power supplies (and the specs won't include '8 ohm').
     
  3. Phil Allison

    Phil Allison Guest

    "whit3rd"
    Conrad Ferrer

    ** That is totally wrong.

    High frequency roll off in a wide band audio transformer is controlled by
    the winding method - NOT the damn core material !!

    A toroidal type or an E or C core shape where the primary and secondary
    windings are multi-layered and interleaved will have wider bandwidth due to
    reduced LEAKAGE INDUCTANCE .



    ......... Phil
     
  4. Phil Allison

    Phil Allison Guest

    "Conrad Ferrer"

    ** A judiciously chosen capacitor across the load may help extend the
    response a bit.

    You'll need a scope, square wave generator and drive amp to do the job.


    ........ Phil
     
  5. Paul Mathews

    Paul Mathews Guest

    You might find this useful:
    http://www.rane.com/note159.html
    Paul Mathews
     
  6. Phil Allison

    Phil Allison Guest

    "Paul Mathews"

    ** I see it is your own article.

    On a number of occasions, I have been was called on to diagnose amplifier
    failures where the PA system installer simply had no understanding of "core
    saturation " whatever. Nor at the time did a company called QSC.

    The handbook for their USA850 and similar models directed owners and
    installers to do what was almost certain to destroy the amplifier. Three
    such victims were delivered to me with a " WTF happened ? " complaint.

    The output device protection circuit fitted to most QSC amplifiers is not
    capable of saving the amps if ever driven into a saturating transformer
    load. The life expectancy reduces to a few seconds in this event.

    The eventual fix was to install a 12dB /oct HPF with -3dB POINT @ 100Hz
    internally in each channel and simultaneously make sure that no sub sonic
    thumps were generated by the amps under ANY operating condition.

    When contacted about the failures, QSC tech support were the OPPOSITE of
    helpful.



    ........ Phil
     
  7. Winfield

    Winfield Guest

    Correct, it's the use of special interleaved winding methods
    to create low leakage inductance that wins the day, not core
    materials, at high frequencies. But note, if we were talking
    resonating inductors and high-frequency energy storage, why
    then it'd be another matter entirely. Core losses would rule.

    To answer Conrad, because it _is_ a leakage-inductance issue,
    it's hard to externally correct the problem.
     
  8. MooseFET

    MooseFET Guest

    You can push it up a bit by boosting the level of the high frequency
    signals going into the transformer. For normal sound there is very
    little power above a few KHz so adding a boost starting at 25KHz won't
    be a problem.

    Are you driving the transformer from a low impedance such as a normal
    audio power amplifier or a higher impedance? If the drive impedance
    is low, the cut off will be where the leakage inductance as the same
    impedance as the load. If your drive signal has a higher impedance,
    you are likely to see a different cut off.
     
  9. Paul Mathews

    Paul Mathews Guest

    Forgive the plug that follows, for what is quite possibly the lowest
    leakage inductance PA transformer available commercially. It can be
    seen here, accurately specified and conservatively rated:
    http://www.rane.com/mt4.html
    Unlike many PA transformers, this one was NOT designed according to
    power transformer design rules. The balance of copper and core losses
    and the winding design were entirely driven by audio fidelity and
    power delivery considerations, including tolerance for amplifier
    output DC offset. (Power transformer design rules place the emphasis
    on minimizing cost within a temperature rise budget, which results in
    poor audio performance.) End of plug. (I have no financial interest in
    the manufacturer, a former client.)
    Paul Mathews
     
  10. Phil Allison

    Phil Allison Guest

    "Paul Mathews"

    ** At the specified 20mH, it must be the worst on the planet !!

    ** 20mH should be 20 uH - no ?


    ** OK - 500 mohms is about double the usual value, plus the extra turns
    pushes the saturation frequency lower. However, using a wound toroidal core
    means that the limit is quite dramatic when it arrives. Still very bad news
    for many power amplifiers - particularly QSC - that cannot take being
    effectively dead shorted ,twice each cycle, at some low frequency while
    delivering high power.

    Pffffffftttt.......



    ........ Phil
     
  11. colin

    colin Guest

    maybe you could have two transformers,
    for different bands,
    with the primaries and secondaries in series,
    say a ferrite for high frequency wich becomes very low impededance at low
    frequency
    and so has no effect at low frequency where the iron tranformer does it all,
    and maybe a capacitor accross the low frequency transformer prim and sec.

    Colin =^.^=
     
  12. Jasen

    Jasen Guest

    Negative feedback?
    And that's a problem?

    Bye.
    Jasen
     
  13. Paul Mathews

    Paul Mathews Guest

    Hence the low-cut filter option available for most commercial power
    amps. The Rane power amps monitor and limit their own volt-seconds
    output.
    Paul Mathews
     
  14. Phil Allison

    Phil Allison Guest

    "Paul Mathews"
    "Phil Allison"

    ** You really are totally clueless - aren't you ?

    You think in dumb clichés and post opinionated drivel.


    " > It can be seen here, accurately specified and conservatively rated:
    ** 20mH should be 20 uH - no ? "


    ** Cat got your tongue on this point ???




    ....... Phil
     
  15. whit3rd

    whit3rd Guest

    In fact, it's six of one, half dozen of the other.
    When the core material is slightly conductive, eddy currents
    exclude the flux (by Lenz's law) and that means the flux, since
    it is not in the core, is not constrained to link the primary and
    secondary windings. The presence of unlinked flux is the
    definition of leakage inductance.
     
  16. Paul Mathews

    Paul Mathews Guest

    Thanks for noticing that typo. Yes, 20 microHenry. Font error: should
    have gone to a symbol. Thanks for the insults, too, I'm honored. Can't
    go long without a good insult from crazy Phil.
    Paul Mathews
     
  17. I suspect a measurable fraction of that 20 uHy might be just
    in the loop area of the lead wires to the connector,
    especially if the wires happen to be spread out a bit more
    than in the pictures. I think that pair should be twisted.
     
  18. Phil Allison

    Phil Allison Guest

    "whit3rd"



    ** You are an witless ass

    - **** off .



    ........ Phil
     
  19. Phil Allison

    Phil Allison Guest

    "Paul Mathews"

    ** You really are totally clueless - aren't you ?

    You think in dumb clichés and post opinionated drivel.

    The world has far too may deadshits like you already.

    Go drop dead.



    ........ Phil
     
  20. Phil Allison

    Phil Allison Guest

    "John Popelish"


    ** Can you spell the word " F A N A T I C " for me - John ?



    ....... Phil
     
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