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Amplifier Power Transformers.

Discussion in 'AV Receivers & Amplifiers' started by Donnie Smith, Mar 17, 2003.

  1. Donnie Smith

    Donnie Smith
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    Hello

    Could someone explain to me how Amplifier Power Transformers work? I´m confused from reading some posts on here as it appears that some amps/receivers output one set of figures but only consume a smaller amount of power. e.g an amp outputs 5 x 100 watts into 8 ohms but only consumes 400 watts of electricity.

    I assumed rightly or wrongly that power transformers transformed the electric current to give more power when needed. Some people say that an amp cannot output a certain wattage if it doesn´t consume that much in the first place.:confused:

    Very confused by the whole thing to be honest so if someone could put me right.

    Many thanks.:)
     
  2. NicolasB

    NicolasB
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    There are several possiblities.

    1) They're lying.

    2) Any one channel can inidividually be pushed to the maximum value, but not all of them at once.

    3) There's an important distinction between peak power output and sustained power output. A certain sharp peak (like a snare drum hit) may hit very high wattages for a tiny fraction of a second, but the overall power output over a period within which that hit happens will be much lower. The power supply only has to supply enough power for the overall demans, it doesn't have to be able to deliver the peak output.
     
  3. mjn

    mjn
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    there are a number of possibilities:

    That figure is NOT all 5 channels driven at the same time.

    The specs are incorrect or have been over inflated

    The power supply in the amplifier is budget/crap/cheap

    The "class" of the amplifier, as different classes have different efficencies.

    I'm sure somebody will come along with a very techincal explanation. :)
     
  4. Donnie Smith

    Donnie Smith
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    Thanks Gents.:) Still not 100% on it all I´m afraid, seems to be too many variables.:suicide:
     
  5. General Skanky

    General Skanky
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    A stereo contains a power supply that converts 110-volt alternating current into lower-voltage direct current. This direct current is ultimately when powers the speakers. The stereo's power supply first lowers the voltage with the help of a transformer. Alternating current from the power line flows back and forth through a coil of wire in this transformer, the primary coil, and causes that coil to become magnetic. Since the coil's magnetism reverses 120 times a second (60 full cycles of reversal each second), along with the alternating current, it produces an electric field--changing magnetic fields always produce electric fields. This electric field pushes current through a second coil of wire in the transformer, the secondary coil, and transfers power to that current. There are fewer turns of wire in the secondary coil than in the primary coil, so charges flowing in the secondary coil never reach the full 120 volts of the primary coil. Instead, more current flows in the secondary coil than in the primary coil, but that secondary current involves less energy per charge--less voltage. In this manner, power is transferred from a modest current of high voltage charges in the primary coil to a large current of low voltage charges in the secondary coil.

    Having used the transformer to produce lower voltage alternating current, the power supply than converts this alternating current into direct current with the help of four diodes and some capacitors. Diodes are one-way devices for electric current and, with four of them, it's possible to arrange it so that the alternating current leaving the transformer always flows in the same direction through the circuit beyond the diodes. The diodes act as switches, always directing the current in the same direction around the rest of the circuit. The capacitors are added to this circuit to store separated electric charge for the times while the alternating current is reversing and the diodes receive no current from the transformer. The capacitors store separated charge while there is plenty of it coming from the transformer and provide current while the alternating current is reversing. Overall, the stereo's power supply is a steady source of direct current.


    From a US website.
     
  6. Donnie Smith

    Donnie Smith
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    Wow! Thanks for that.:smashin: Got some reading and digesting to do to understand it thoroughly.:)
     
  7. Reiner

    Reiner
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    5x100 Watt does not mean the amp can drive that continously, in fact more likely it will only do 2x100 Watt but each channel on it's own is designed to handle 100 Watt.
    This is usually due to the limitation of the power supply section, a common restriction in integrated AV amps/receivers due to their size, cramped full with preamp and video section, DSP & decoders, power amps etc.

    As well it is not that straight forward to conclude the total output power based on the AC power consumption as peaks can be buffered with capacitors. But then a 500 Watt consumption does not mean you can just divide by 5 and get 100 Watt per channel since you need to consider the efficiency of the power supply seciton (around 85% I would estimate) and that you need a few Watt for video and DSP section, perhaps another 50-100 Watt or so.

    A good amp can double the power if the impedence is halved, a e.g. Mark Levinson No.33 manages to go from 300W @ 8 Ohm to 2400 Watt @ 1 Ohm (that's exactly 8 times and thus can be considered "perfect") - but you hardly find an AV amp which can deliver double at 4 Ohm, more likely it will only do 170 Watt instead of 200 (assuming 100 W @ 8 Ohm). And all that because of the limitations in the power supply.

    I assumed rightly or wrongly that power transformers transformed the electric current to give more power when needed.

    Transformer's first task is to down-convert the AC (alternate current) mains from e.g. 230V to e.g. 60V. This is feed into a circuit (some electronic components) which will turn it into DC (direct current).
    Transformers can provide a constant supply up to the max what they are designed for. If there is no demand the current output will be lower but note that even the equipment is not in use the transformer will consume some electricity unless completely disconnected from the AC mains (on the primary side).
    And if you short a transformer on the secondary side it will draw too much current and melt away. :devil:

    Some people say that an amp cannot output a certain wattage if it doesn´t consume that much in the first place.

    That is correct, if 500 Watt max go in you can't have more than that coming out, rather a bit less due to the losses explained above. You can "feel" this as heat dispersed, even it's idle.
     
  8. Flimber

    Flimber
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    There are some measured figures for amps with 5/6/7 channels operating here. Note that the Sony STR-DB1070 only managed 6 x 31 Watts (!), 30% of it's quoted figure (which was probably obtained with one channel operating but 'timesed' by six because they then bolted another five amps next to it within the case ;)).

    Mike.
     

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