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Thread: Anyone here understand volt-amps?

  1. #1
    R.I.P. 2018-07-16 RoundEye's Avatar
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    Question Anyone here understand volt-amps?

    I hate to admit it, but it’s something I don’t have a firm grasp of. I bought a 2.1 Yuanjing amp kit. I think it’s 28 X 2 and 68 X 1. I want to use a toroidal transformer for power.

    I don’t have much info on the amp, I want to use the right transformer.

    Well ……………………………….
    Sliding down the banister of life ..........................

  2. #2
    SG Enthusiast The Dude's Avatar
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    I may be a bit rusty on this but I believe they rate transformers in volt-amps instead of watts because the voltage can be out of phase with the current. In that case P = I X E wouldn't be accurate. In your case if its feed by your local hydo, current and voltage should be in phase and you could just use P = I x E. Power (Watts)= Current (Amps)x Voltage (Volts). Power in equals Power out in a transformer, ignoring the little bit of lose. I hope that helps.
    I don't know the same things you don't know.

  3. #3
    R.I.P. 2018-07-16 RoundEye's Avatar
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    Dude thanks for the answer.Something static like a light builb watts roughly equals volt/amps, something dynamic like the amp where the voltage and current is always changing it has a lot to do with phase. A/C isn't my forte, DC I know really well. I could stick a big one in there to cover my ass but I'm trying to keep the amp as small as possible. I guess I need to look around the net and study some more.
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  4. #4
    SG Enthusiast The Dude's Avatar
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    If I read your post correctly you're going to use this transformer to step the AC mains down and then convert that to DC to run your amp. You'll need a bridge rectifier that can handle what ever current is needed by your amp and a big honking filter capacitor. Some kind of DC voltage regulation would be good too. If you know the DC voltage output of the bridge rectifier there is a formula to figure out what AC voltage the transformer needs. Once you have the AC voltage and current needed on the secondary of the transformer you have your volt-amps. You'll want some extra room to play with so the transformer should be rated higher than your calculated values. Like I said though, I'm pretty rusty with this stuff, its been a long time since I ever had to sit down and design a power supply circuit. I've built a few low power units to replace dead power packs and pretty well just used what I had in my electronics junk drawer.
    I don't know the same things you don't know.

  5. #5
    R.I.P. 2018-07-16 RoundEye's Avatar
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    It has a 25 amp bridge rectifier on board. The amp will operate from 5 to 26 volts. I'm looking for a 26 volt transformer with a center tap so I can have the positive and negitive rails. This is the amp, Yuanjing I paid around $26.00 from ebay.
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  6. #6
    SG Enthusiast The Dude's Avatar
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    OK, there you go, most of your work is done for you. All you have to do is look though the specs for the maximum current it might draw.
    I don't know the same things you don't know.

  7. #7
    R.I.P. 2018-07-16 RoundEye's Avatar
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    All you get is the amp. No schematic, no specs. no wiring diagram, no box.
    Oh well.
    Sliding down the banister of life ..........................

  8. #8
    SG Enthusiast The Dude's Avatar
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    Looking at the picture, I'm thinking you may want to put some heat sinks on those IC's mounted vertically along the back side.
    I don't know the same things you don't know.

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