Calculating the VA rating for a transformer!? (Audio amplifier)

Discussion in 'General Electronics Chat' started by TheLaw, Mar 14, 2011.

  1. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Hi,

    I'm putting together an audio amp comprised of 2x LM3886 @ ~ 60W (2x ~30W) and a LM4780 bridged @ ~100W. I need a 25V transformer.

    The problem I am faced with is I have no idea on how to pick out a transformer with the right VA rating.

    How is one supposed to go about finding this? Is there a simple-ish textbook method.

    Originally I was thinking a 330VA, but I think that may not be enough, so I was looking at a 625VA version. (These are dual output toroid transformers).

    Anyone have anything thoughts.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Most LM3886 amplifiers use a plus and minus supply that is produced by a center-tapped
     
  3. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    I'm talking about the VA rating...
     
  4. Audioguru

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    Dec 20, 2007
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    I worked for a pro-sound manufacturer. Its amplifiers could run at continuous full rated power all day long. The guarantee said that no questions are asked if there is a problem. Hundreds were sold and I never saw a defective one.

    I was asked to test a competitor's amplifier. I ran it at its rated output but it smoked then failed after a few minutes. Some people say that amplifier is fine playing music that averages 10% of full power when the peaks of music cause clipping.

    So it depends on how much of a gambler you are and what kind of music you play. Do you play "acid rock" at full blast all the time? Then you need a huge heatsink and a fan for the amplifier's output transistors and the transformer must be able to provide the power for the full continuous output power plus the power the amplifier wastes as heat.

    Or do you want to use a cheaper transformer that might supply 75% of max continuous power? Or only 50% of the max continuous power which might be fine?
    Of course an overloaded power transformer gets hot and might fail.
    It is your gamble.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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  6. floomdoggle

    Senior Member

    Sep 1, 2008
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    I've always read to take the peak power and double it.
     
  7. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Yeah I'm using huge computer heatsinks with multiple heatpipes. I haven't actually tried them out yet, but I am assuming they will do quite well. The subwoofer heatsink is even larger.

    The thing is: I am using these for my computer, so they are at close proximity. Therefore 95% of the time I am using them, I will probably only be using them at ~ 15-20%. If I'm having a party or something, I might hit peak, but most of the time not. I don't have a problem spending the money where it should be spent. I just don't want to have too little power, yet I don't want to go overkill either.
     
  8. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    To start you off, assume you are using a split-suppply (+/- voltages) and 8 ohm speakers.

    As a rule of thumb -- take 1/2 of half the split supply (squared) and divide by the load impedance to get the voltage you need for RMS power with pretty good headroom.

    Say you have +/-32V rails: 32V/2 = 16V; (16V^2) / 8 ohms = ~32WRMS

    So, using a little algebra: V = sqrt(P*R); V = sqrt(30W*8) = ~15.5V (or +/-31V).

    The current is I = V/R; 16V/8 = 2ARMS (in a perfect world), with peaks of 32V/8 = 4A.

    As a rule of thumb, double the RMS current required: 2A x 2 = 4A.

    Now double it again because you need 2 amplifiers = 8A.

    A full-wave bridge with a large filter capacitance acts as a peak detector, so the xfmr would need to be 32V * .707 = ~22V; a dual secondary 24V 10A x 2 (480VA), would be about right. The secondaries are in series (with a grounded center point) so the current is the same. Note that 24V * 1.414 = ~34V (or +/- ~34V).

    However, there are many aspects of amplifier and PS design that can affect these numbers due to losses and various voltage drops (efficiency). Much of this depends on how much headroom you want to build in, and how much heat dissipation you can allow (or tolerate) in various parts of the circuit; which goes to Audioguru's comments.

    If you use 4 ohm speakers, you can get by with less voltage but the current requirements increase (do the math).

    The above total does not account for an LM4780 BTL amplifer (which is similar, but a little more tricky).
     
    Last edited: Mar 15, 2011
  9. Audioguru

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    Dec 20, 2007
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    Cheap amplifiers have "peak power" ratings. They are a phoney number that is double the amount of real power. Sometimes peak power is only for a short duration which makes its number much higher than the amount of real continuous power.
     
  10. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Thanks for the great explanation! If I did the calculations right, it seems that a 500VA+ transformer would be appropriate. I should check once more though...
     
  11. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Thanks for the great explanation!
     
  12. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    BTW -- 1WRMS of music power into 8 ohms (stereo) can be fairly loud in a confined space (apartment living loud). You then have to double the power to 2W just to hear a difference in loudness, then 4W, etc. By the time you get around 10W or so, the neighbors be banging da walls.
     
  13. Audioguru

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    You don't show where in the universe you live so maybe your walls are just rice paper and you are in an apartment building with neighbours living on the other side of the paper. Then 1W might be too loud.

    Many or most of us live in detached homes. We play our music indoors as loud as we want (hundreds of Watts) without the neighbours hearing anything. The sound system in my car also produces hundreds of Watts so I can feel the bass.

    My clock radio produces about only 1W.
     
  14. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    The houses in my neighborhood are only about 5-6 feet apart. But both neighbors blast Reggaeton at 4AM so they have no business to tell me what to do...

    I'll probably NEVER use the full 150W+ of this system, mindless of the fact that my satellites won't even be able to handle that much current.


    By the way. If I have a satellite that is comprised of an 8-ohm midrange and a 5-ohm tweeter, how do you calculate the impedance of that speaker? Is it 13, 7, 2?

    So when I buy the transformer I can buy the correct voltage. Higher impedance, more voltage. Lower impedance, more current output.
     
  15. Audioguru

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    I am sorry for you that your neighbours are animals.

    Tweeters use full power only for a moment before they get too hot.

    The crossover network separates the frequencies used by the woofer and tweeter.
    The impedance of your speaker is the woofer's 8 ohms because the tweeter is only occasionaly used.


    A power transformer needs to provide the max continuous current for as long a duration you need. Its voltage must be correct to provide the correct DC voltage.
    Its rated current must be enough so that it doesn't get too hot.
     
  16. nomurphy

    AAC Fanatic!

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    So this is why you don't hear so well.
     
  17. Audioguru

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    I am 65 years old and still hear perfectly. Why? Because I didn't shoot guns like Americans who were deafened by their guns.

    Also, I did not listen to awful acid rock at full blast all the time like people who were deafened by it.
     
  18. t06afre

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    And you did not have any friends in your younger days. So got few or no invitations to parties:p
     
  19. Audioguru

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    I helped all my friends buy hi-fi systems that had a flat frequency response, lots of power and low distortion so we could have many parties.
     
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