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70.7V line transformer

Discussion in 'General Electronics Discussion' started by Y2KEDDIE, Mar 20, 2013.

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  1. Y2KEDDIE

    Y2KEDDIE

    259
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    Sep 23, 2012
    Ok, I understand how to match a 8 ohm speaker to an output tube. I match the plate resistance to the primary. I can calculate turns ratio and thus calculate impedance.

    What I can't grasp is how do you match a 70.7 volt line to speaker transformer. It must have a primary impedance. How would one design a output stage using such a transformer?
     
  2. john monks

    john monks

    693
    2
    Mar 9, 2012
    The 70.7 volts refers to a standard driving source for speakers in RMS.
    So, for example, if you want to drive a 100 ohm speaker with a 70.7 volt line you the formula

    Z = (70.7^2) / 100 = 50 ohms.

    The output stage would be designed as a constant voltage source not exceeding 70.7 volts RMS.

    This may help
    http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf
     
    Last edited: Mar 21, 2013
  3. Y2KEDDIE

    Y2KEDDIE

    259
    15
    Sep 23, 2012
    The Crown link was helpful. I've never seen circuitry for a tube amplifier using 70.7 line. I can see it would be fairly easy to build a solid state output that has realitvely low output impedance. I'm thinking this could be accomplished without a tranformer on the amplifier itself.

    With a 100 watt tube amplifier there must have to be a transformer matching the tube plate resistance to what ever speaker transformer connected to a speaker.
     
  4. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    In the UK, the PA system uses 100V line, perhaps this is peak and the same as 70.7V line.

    Valve amplifiers can produce a low output impedance with negative feedback so that the loading can be varied as required.

    The speakers are coupled to the line with transformers. These have multiple primary taps to adjust the power and multiple secondary taps to match the speaker impedance.
     
  5. john monks

    john monks

    693
    2
    Mar 9, 2012
  6. davenn

    davenn Moderator

    13,837
    1,952
    Sep 5, 2009
    no they are different

    coming from PA installation work, many of our amplifiers would have multiple outputs

    8 Ohms, 50V, 70V or 100V for feeding different styles of lines
    we mainly used the 100V feed but some times if there was a fault on the 100V speaker line that lowered its resistance, we would drive it with the 70V output to keep the system running till we could find the fault.

    for example a normal load ( non fault condition) of 100W, a 70V line would be 47 Ohms and a 100V line would be 100 Ohms


    Dave
     
    Last edited: Mar 21, 2013
  7. Y2KEDDIE

    Y2KEDDIE

    259
    15
    Sep 23, 2012
    If I understand: Suppose it is a 100 watt tube amplifier. It's output transformer matches the Push pull output to 70,7 line. (10000 : 50 ohms Z for 100 watt load). Now if I want to power a 10 Watt 8 ohm speaker a 500 : 8 ohm matching transformer would be needed.
    Z= 70.7 sq'd / 10 watts = 500 ohms.

    Is this correct?
     
  8. john monks

    john monks

    693
    2
    Mar 9, 2012
    Your math looks totally correct to me.
     
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