From 12vdc to 14.5vac

Discussion in 'The Projects Forum' started by Edmunds, Oct 2, 2010.

  1. Edmunds

    Thread Starter Active Member

    Sep 27, 2010
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    0
    Hi all,

    The previous post turned out to be confusing, so I decided to start another one.

    What I'm after is a converter/inverter for running a preamp of what was originally a 230VAC 5.1 surround system from a car battery. I seem to have sorted the power for the amp itself, so no issue with that at the moment.

    The preamp was originally powered from a separate secondary of the transformer with an output of 14.5VAC. This should become approx 20VDC after the rectifier. I have tried applying 12VAC and while it did work, I would be happier with keeping as close to the originally as possible.

    So, basically, I need to convert 12VDC to 12-14.5VAC. It is difficult to know the exact power requirement, but I would rather count high - 5A? Does not feel like a massively difficult circuit to build. Any suggestions?

    Thank you for you time,

    /Edmunds
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    Are you sure? I'll bet the first thing on the board is a rectifier to take the AC to DC, which it'll then clean up and filter to give a nice quiet pre-amp. If you can find the DC side of the rectifier, you might be able to skip a conversion to AC and back again.

    Ps. Don't forget that a car battery spends most of it's time above 12v, usually between 13.5 and 15.5, give or take.
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    The transformer's secondary outputs 14.5 VAC but what is the voltage out of the regulator after the bridge rectifier.
     
  4. Edmunds

    Thread Starter Active Member

    Sep 27, 2010
    85
    0
    Hi guys,

    Thank you for your replies.

    Well, maybe I should have started with if anyone can help me understand this setup here:

    http://www.edmundsworld.net/Edmunds_World/Preamp_Rectifier_Circuit.html (sorry, but the picture link did not work for some reason).

    Unfortunately, I cannot access the bottom of the board, so I could see what connects where - going there would be very messy, because of the amplifier chips being covered in some white, sticky, useless mud, I would have to deal with in that case. If you guys, can help me figure out where do I connect after the rectifier and before or after the filters, I am keenly interested. I need to solder the new power to the surface, though as well. Just an idea - isn't the big cap the last thing connected in parallel in the output of the rectifier/filter thing? If I use a bit of violence and tear it apart, couldn't I just connect my new power to the cap leads now sticking up from the board?

    Yes, as explained above, I would very much like that, but ... Even if I succeed in finding the place to connect my DC, I would still need 18-20V, instead of roughly 12 from the car battery. How do I do that? Just a single transformer? Where do I get one or how do I wind one for exactly the voltage spread I need?

    Yeah, understood, but a voltage regulators come in easy to use chips these days, right? Should be pretty easy to hook up at some place in the circuit to compensate for the fluctuation. Before or after the transformer?

    /Edmunds
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Unless you're keeping the car battery charged while you're using the amplifier, you will kill the battery in very short order. Automotive batteries are designed to output a LARGE amount of current (hundreds of Amperes) for a short duration (perhaps 30 seconds) and then be immediately recharged.

    If you discharge these batteries deeply (more than 10% or so) then they will quickly fall apart inside.

    If you really want to get a 14.5vac output, you'll need to make something like Rod Elliot's switchmode supply:
    http://sound.westhost.com/project89.htm

    Except yours would be a good bit different; as you need AC out instead of +/-35VDC. The basic idea would be similar though.
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Can you still hook the thing up? If so, I'd try using a voltmeter to find what appears to be the common supply to that board after it's been rectified, filtered and regulated. If not try using a 12V AC transformer at the input (should still work) and doing the same thing.

    Chances are somewhere you're going to find that the actual preamp runs off of 10 - 12V DC somewhere, I can't imagine a surround receiver using raw "rectified and filtered" power to a board as sensitive as the preamp.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    My point was, there's a good chance you don't need conversion or regulation, that you might be able to connect directly into the car battery. If you can find the pre-amp's DC power, you'll know what it expects.
     
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