Transformer for +-55V

Discussion in 'The Projects Forum' started by swinch25, Jan 8, 2012.

  1. swinch25

    Thread Starter New Member

    Jan 8, 2012

    I plan on making some more speakers at some point and want to build an amplifier into them.

    First I want to sort out a power supply, I have found a transformer that provides 2x 55V outputs at 2A each.

    I have emailed the company with this circuit diagram attached (bridge rectifiers and smoothing caps, pretty basic) and got "this circuit will not work, cannot offer design help" back from them :mad:.

    I don't see any reason why it shouldn't work so if anyone does can you let me know.

  2. Wendy


    Mar 24, 2008
    The Completed Projects Forum is for Completed Projects only. It is meant to allow members to show plans for projects they built so other members can duplicate them if desired. Your thread does not belong in this forum, so was moved to a more appropriate forum.
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    It won't work as drawn. You can not "stack" two bridge rectifier circuits across the windings of a center tapped transformer. The rectifiers will blow as two of them get turned on as a "short" across a winding. Just about everybody who plays with electronics has tried it........:D You need a transformer with two separate windings.

    You need to keep the didode bridges isolated on each Xformer secondary. Then, you can stack the two DC outputs on the filter cap side (output side). You also have some diodes reversed in the bridges. The attached schematic shows how to connect the diodes.
    Last edited: Jan 9, 2012
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Follow the diodes down the right side of your rectifiers, and you'll see why it won't work. :)
  5. Experimentonomen


    Feb 16, 2011
    You dont really even need dual bridge rectifiers, just wire the ends of the secondaries to the bridge rectifier and make the centertap ground, then the outputs of the birdge rectifier is your B+ and B- and your centertap is your ground(0v)

    And to get around 55VDC, you need a transformer whose secondaries put out approx 38VAC.
  6. wmodavis

    Well-Known Member

    Oct 23, 2010
    Diodes in the bridge are not oriented in the correct direction. If they were it might work.
  7. t06afre

    AAC Fanatic!

    May 11, 2009
    Use a dual complementary rectifier. How much DC current do you need in your setting?
  8. Adjuster

    Late Member

    Dec 26, 2010
    Note that with a 55V - 0 - 55V transformer you will end up with roughly ±55V*√2 -0.8V, that is about +77V and -77V DC. I think that's too big.

    An audio amplifierworking into a standard 8 ohm loudspeaker from such a supply voltage could deliver something approaching 10A peak, (over 700W!) but your 2A rated transformer could not come near sustaining this. Perhaps you need to think this out again.
  9. swinch25

    Thread Starter New Member

    Jan 8, 2012
    Thanks people,

    I thought it would be someting basic that I have just not seen, shorting out the rectifier with 110Vac and 2A wouldn't be pretty :eek:.

    I didn't even think of a single rectifier, that would be perfect.
    I think I did stuff up the calculation for supply rails, thanks, I'll look at +-24V which should come out to about 6A for 8 ohm speaker (working off 48V pd and just the power).

    Basically I have a integrated amplifier that I'm going to connect the speakers to, but it has a max of 40W per channel which won't be enough for the dual 3ways I plan on making at some point (have LC filter design and transistor/MOSFET amp for the crossover and input amplification).

    I'm going to make everything as different modules so that if I want to change something its easy (most probably the crossover if the sound isn't right). But thats a different topic so will post on that when I get to that :D.

    Thanks for all the help guys, I'm gonna look around and see if its worthwhile building the AC/DC PSU or just buy 2x24V ones (probably these the PBB-13C-24E model as they are powerful and small).
  10. swinch25

    Thread Starter New Member

    Jan 8, 2012
  11. Experimentonomen


    Feb 16, 2011
    I would not use switching power supplies in audio amplifiers, atleast not these cheap ones since they don't have much if any filtering on the output. The RF noise on the output tend to make audio amplifiers hiss quite a bit and in some cases even whistle.

    And you'd need two to get the split supplies needed and then your back at the price for the transformer based psu, and these sometimes if not always have the 0V on the output tied to mains ground ,and in some cases even to neutral so you cannot series two either.