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Need help understanding 70.7 Volt Line Audio

Discussion in 'General Electronics Discussion' started by Y2KEDDIE, Jan 8, 2020.

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  1. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    After reading about Low Impedance (8 Ohm) Audio Output vs.70.7 Volt systems, I still have questions.

    I know with an amplifier driving an 8 ohm speaker, a transformer is used to match the speaker (8 ohm) to the output impedance (load resistance) of the output stage/ driving tube or transistor. If the load (8 ohm) is changed, the reflected load on the output stage changes, either heavier or lighter depending on the respective change. This mismatch will cause undesirable overheating and/or distortion.

    The 70.7 Volt line amplifier I have, has an output transformer with terminals labeled 70.7 Volts, 50 Watts, 100 Ohms.

    I can connect a single 8 ohm/with matching transformer (say 10 Watts) or multiple speaker/transformers as long as I don’t exceed 50 Watts . I assume maintaining a correct match is still necessary.

    I can use only one transformer/speaker ( say 50 Watts) presenting 100 ohms, or I can put two 25 watt transformer /speakers in parallel across the 70 .7 Volt line, which would present 50 ohms (?) combined. This where I get confused how is the 100 ohm match achieved.

    Doesn’t paralleling multiple output/matching transformers cause the line impedance to change causing mismatch/distortion etc?

    Is changing the wattage taps on each speaker/transformer maintaining a constant load to the amplifier transformer? Regardless, wouldn’t each parallel load reduce the combined impedance?

    Can this be illustrated with a resistor network to clarify?


    Thanks,
     
  2. duke37

    duke37

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    Jan 9, 2011
    70.7 (US) or 100V (Europe) use an amplifier which can stand various loads. The output line has transformers to connect the speakers. The transformer ratio is chosen to give the correct volume out.The impedance shown to the amplifier will be high with only one small speaker and will be low when providing power to several small speakers or a few high powered spakers. The loading should not exceed what the amplifier can give without distortion.
     
  3. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    Duke, thanks for taking the time to reply. I’d like a little more in depth explanation.
    Are you saying the matching isn’t as critical as long as power limit is not exceeded?
    What is the impedance of a single 50 Watt transformer/speaker? 100 ohms?. Then by selecting appropriate taps, (5) 10 Watt transformer/speakers would present 500 ohms each?
     
  4. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    Still trying to understand 70.7V systems:

    I have an amplifier with output labeled 70.7 V. 50 Watt, 100 ohm.

    I believe the 100 ohm, is the minimum impedance the amplifier can deliver full 50 Watts power.

    If this is correct, and: P=5000/Z:

    5W=5000/1000, 10W= 5000/500, 25W=5000/200, 50W=5000/100 ohms

    If I connect different loads totaling 50 Watts;

    25+10+5+5+5 +50 Watts (each transformer/speaker load): 200,500,1000,1000,1000 in parallel would equal 100 Ohms.

    Even though the power distribution and impedance are correct, wouldn’t the individual ‘different’ impedance cause different amounts of distortion?

    Is this why 70.7 V systems don’t sound as good as standard 8 ohm output amplifiers?
     
  5. bertus

    bertus

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    Nov 8, 2019
    duke37 likes this.
  6. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    What happens when you connect two 10 watt (500 ohm) across the output of a 50 Watt power amp?

    Two, 10 Watt @500 ohms each is a 250 load , but doesn't the 50 Watt amp wants to see 100 ohms. I can see how you get 20 Watts to the speakers, but isn't there a serious mismatch causing distortion? What am I missing?
     
  7. duke37

    duke37

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    Jan 9, 2011
    The term match or mismatch means very different things from RF match where the load at the end of a cable is the same as the cable impedance. To get maximum output power, the losd has to be equal to the source impedance.
    The electrical distribution system would be interesting if the load was matched. The distribution amplifier is similat to the mains system where the load determines the power up to system failure. The amp has to provide a near constant voltage reguardless of load until it runs out of breath.
    Each speaker will be coupled to produce a desired output using a tapped transformer.
    The amp has to provide a low impedance output which will be done by strong negative feedback. This will be optimised for stability not low distortion.
    If the amp needs 500 ohms to get full 50W output, then using a 250 ohm load will overload it, giving distortion or fuse blowing depending on design.
     
  8. Audioguru

    Audioguru

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    Sep 24, 2016
    Modern hifi amplifiers do not use an output transformer. They can drive an 8 ohm speaker or deliver almost twice the power into a 4 ohm speaker.

    The output of an amplifier does not want to see its rated full power load. In fact, with an output power less than full power its heating and distortion are less.

    A 70.7V line is used for stores and malls. The transformers produce bandwidth limiting and distortion. Transformers are used so that some speakers can be louder than others by adjusting the tap on the transformer, and to reduce the size, weight and cost of the connecting wires to all the speakers since when the transformer on the amplifier increases the line voltage then the line current is reduced.
     
  9. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    My goal is trying to understand the effect of the changing load as speakers are added or subtracted on the 70.7V line.
    Starting to grasp the concept; I’m coming to the conclusion that with a fixed, finite number of speakers and power requirement, matching load to the source is of the upmost importance. Designers want to get as much power out of the source to utilize the fixed load to the max, (4 or 8 ohm system).

    Where as in a distributed system (70.7 V) flexibility is more of a concern. Mismatch (if you will) by impedance selection between source and load, controls the distribution of power/volume. The load can be changed to more or less speakers as well as wattages, as long as the mismatch does not exceed the source’s power output capacity. The load isn’t going to use any more power than selected.
    Wordy explanation but correct?
     
    Last edited: Jan 10, 2020
  10. Audioguru

    Audioguru

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    Sep 24, 2016
    There is no problem using an 8 ohm speaker on an amplifier rated for a 4 ohm speakers, the 8 ohm speaker will simply produce almost half the power which sounds only a little less loud.

    You needed to match the load only with antique vacuum tube amplifiers. Modern amplifiers that use transistors do not need to match the load, just do not exceed the maximum allowed output power. For a 70.7V distributed system, use up to 200 speakers, each set to 0.5W to an amplifier rated at 100W. The 100W amplifier can power one speaker set to only 20W if you want, but a 20W amplifier will cost a little less.
     
  11. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    I’ve been watching a few you-tube videos on impedance matching (mostly tube circuits) and allowed my self to be confused by the emphasis on matching; selection of output transformers. I know matching is required for maximum power transfer.

    In RF there are standing waves/reflections to be concerned with, etc., this adds to the confusion.
     
  12. Audioguru

    Audioguru

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    Sep 24, 2016
    Due to lots of negative feedback, a modern solid state audio amplifier has an output impedance of 0.04 ohms or less so that it can damp speaker resonances. But it feeds an 8 ohm speaker. Maximum power transfer is not even considered.
     
  13. BobK

    BobK

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    Jan 5, 2010
    You are confusing two concepts. The max power theorem applies when a source has a high impedance, same order as the load. In a modern amplifer as @Audioguru has been saying, the output impedance is far lower than the max load impedance that it can drive. If you tried to connect a matching load, it would trigger the amp’s protection circuitry or burn up trying to supply more current than it can.

    Your 70.7 V amp rated at 100 Ohms means it can drive any load with an impedance of 100 Ohms or more.

    Bob
     
  14. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    Got it! Thanks
     
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