Pin < Pout ?

Discussion in 'General Electronics Chat' started by ariemeir, Apr 9, 2013.

  1. ariemeir

    Thread Starter Member

    Jun 24, 2011
    68
    1
    Hi all,

    I am trying to understand power ratings of an audio receiver : pioneer vsx-305.
    Its specs are attached.

    Now, the power consumption specified is 185w,
    but it claims to output 60rms per channel (5 channels).

    So it seems as if i can get 300w output from 185w input.
    Where am i wrong ?

    Please advise,
    thanks,
    Lenny
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    804
    You are wrong in trying to trust the numbers that manufaturers print on their products.
    In your case, 60rms per channel means just that. Each channel on its own can produce 60w. Nowhere it says what happens when you ask all the channels to do that at the same time - typically the supply voltage collapses and you get much less per channel.
     
  3. ariemeir

    Thread Starter Member

    Jun 24, 2011
    68
    1
    thanks Kubeek !
     
  4. ariemeir

    Thread Starter Member

    Jun 24, 2011
    68
    1
    In the same direction , i am trying to tie all the channels together (all 5)
    to get a single channel with the most power i can get away with.

    I have perused the pioneer vsx-305 specs (attached in the original post)
    and it doesn't mention the word bridgeable at no place.

    If i just connect all the (-) together from all channels and the same for the (+),
    what are the potential pitfalls i might run into ?

    Thanks,
    lenny
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    All the power stages look identical, even though i see only four?
    Anyway they should be bridgeable, and you might even get away with paralleling two of each together if they are well matched, but that might bring more problems than it would solve.
    Still, you are limited by the heatsinks on the main transitors to dissipate the heat, and mainly the rating of the supply tansformer which could melt if you abuse it too much and the transistors hold.
     
  6. ariemeir

    Thread Starter Member

    Jun 24, 2011
    68
    1
    Thanks kubeek, this is insightful and it might work for my purposes.

    I am using this amp for a scientific experiment where i really operate it for a tiny fraction of the time at a time, so heating shouldn't be a problem.

    Here is my last question:
    i want to turn the output on for only a short time (100-200 msec)
    an i am thinking what would be
    the best place to interject the on/off switch:

    i have a signal generator feeding into my pioneer amp feeding into my load.

    i am thinking of putting an opamp voltage buffer after the signal generator
    and control the power of the opamp using a microcontroller via a mosfet.
    I do remember however than even when opamps don't have power, they sometimes generate output noise, so this not work perfectly.

    Could you share your thoughts on this ?

    Thanks.
    Lenny
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I would inject and switch the signal at the input to the power stages. For example a TL074 quad opamp could be used for the buffer amplifier, but I wouldn´t rely on switching off the power to it to mute the amp. For example a good old 74hc4066 analog switch should be able to do the job, or any similar chip.
    See the required AC coupling and biasing for the switch here: http://electroschematics.com/wp-content/uploads/2009/04/audio-stereo-switch.gif
    The signal needs to stay between the supply rails of the switch IC, so you migt need to use lower input amplitude and amplify it back after the switch.
     
  8. ariemeir

    Thread Starter Member

    Jun 24, 2011
    68
    1
    Great !

    děkuji !
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    You´re welcome :) Please do come back with the results or if you get into any problems.
     
  10. ariemeir

    Thread Starter Member

    Jun 24, 2011
    68
    1
    Kubeek , your suggestion worked awesome.
    I blocked the signal at the entrance using a 10k ground pull resistor,
    worked great.

    Thanks.
    Lenny.
     
    kubeek likes this.
  11. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    Actually,there is no such thing as RMS power------RMS voltage,RMS current,yes,but not RMS power!

    If V(peak)xI(peak)=P(peak).

    where Vrms =0.707 x V(peak)

    & Irms =0.707x I (peak)

    The product of I(peak) & V(peak)= approx. 0.5x P(Peak)


    RMS power if it existed,would be 0.707 x P (peak)
     
  12. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Well to be exact you are right, but generally what people mean by RMS power is power obtained by mutliplying RMS voltage times RMS current. Your last equation would hold only for sine waves, but this "RMS" power definition should hold for all wave shapes to get you eqivalent DC power.
     
  13. vk6zgo

    Active Member

    Jul 21, 2012
    677
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    Generally,people are idiots!!
     
  14. richard.cs

    Member

    Mar 3, 2012
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    31
    RMS is simply a mathematical function that can be defined for any waveform, including dc, and for any quantity. In that sense it exists as much for power as for anything else.

    It is not however a useful concept in relation to power.
     
  15. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    Which is the point I was making!:rolleyes:
     
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