Stepdown 240-120V transformer.. Kev, any help?

Discussion in 'Mixing & Song Critique' started by composition4, Sep 28, 2005.

  1. composition4

    composition4 Guest


    Kev sorry I'm turning to you in specific again but you're a fountain of knowledge when it comes to things electrical... and I don't know much about the electrical/electronic side of things..

    I've just bought myself an OzAudio HM-6 headphone amp from US; it uses an internal transformer, I have contacted OzAudio (nice guys!) and they say it wants 120v.

    I know I need a step-down transformer, and am looking at a 240-110v unit on eBay;

    I know 500W is overkill, but my main question is will this unit be okay, considering it's 110v output, and the headphone amp needs 120v?

    Thanks in advance anyone!

  2. iznogood

    iznogood Member

    Feb 9, 2004
    the transformer most likely is 2:1...

    this means 220V in = 110V out


    240V in = 120V out

    voila :cool:
  3. Pre Amp

    Pre Amp Guest

    Yes, that transformer will work nicely!

    (FYI----I work in an electrical panel shop in PA)
    15 years exp.

  4. Kev

    Kev Well-Known Member

    Nov 13, 2001
    basically the guys are correct

    your specific question to,
    " ... considering it's 110v output, and the headphone amp needs 120v? "
    is sometimes valid and that subtle difference in voltages of
    240, 230 and 220 for Aust / EU / British
    10, 115 and 120 for the Americas
    can cause unexpected issues
    ... I can explain if you want.

    Guessing what might be inside this box I suspect a typical step down POWER transformer will probably satisfy.

    This unit could also be handy to have for many things and yes 500 is strong. It will be usful if someone brings an American Guitar Amp to you.

    just for comparison,
    Jaycar CAT. NO. MF1084 500W
    RRP $99.95


    Jaycar CAT. NO. MF1086 1000W
    RRP $129.00

    Jaycar CAT. NO. MF1091 50VA
    RRP $219.00

    A second thought could be to change it internally.
    All things are possible but I would have to see inside.
    There may be a possible solder mod
    A simple replacment of the power torriod (if it has one ) for a 240V unit.
    A complete guess ... but something like this
    Jaycar CAT. NO. MT2086
    RRP $21.95

    shut up Kev !!
    would need to see inside to know
  5. composition4

    composition4 Guest

    Thanks for the responses guys!

    And don't shut up Kev, I appreciate your help whenever you offer it!

    Funny you mention, I was thinking about internally modifying it, that way will save the need for any external transformers etc.
    Should get the HP amp within a few days, I'll take a peek inside when I get it.
  6. composition4

    composition4 Guest

    Hey Kev, got the unit, it just uses one of those laminated mains transformers... like this sort of thing, as far as I can tell anyway...

    Is it safe if I measure the voltage from each of the wires (5 coming out of it leading to the PCB) and get a 240v multitap transformer with enough current, and just replace it?

    Do I have to worry about anything else, or is this a safe swap?

  7. Kev

    Kev Well-Known Member

    Nov 13, 2001
    but it's a 110 input ?

    you are going to use the step down transformer and then measure the outputs of the internal.

    Remember they are AC volts and you have 5 wires so it is likely you have two for lighting and three as the main Centre Tapped dual power supply.
    This is only a guess and I would like to see inside.

    Be careful
    draw a picture and clearly write down your findings. Show where you had the probes ... cos you will forget tomorrow.

    Don't touch things inside with the power on IF you don't know what you are doing.

    Are you in Melbourne ?
  8. composition4

    composition4 Guest

    Nah I'm in Tasmania :(

    My old man is good with electronics, used to repair TVs/amps etc at one stage, but I only see him once every 2/3 weeks; I can solder (had practice on cables etc) and know some basic electronics, but if it requires more than just a drop-in replacement I'll get him to help me out.

    Turns out I can't get hold of a stepdown transformer, so not sure what I can do. I may email OzAudio (they're very helpful!) and ask them what voltages come off the transformer... Actually I'll do that now.

    I'll take some pictures tonight when I get home and post them, maybe you'd be able to tell me something!

  9. composition4

    composition4 Guest

    I've opened up the unit and it's using a whole bunch of NJM4560 op amps. I downloaded the datasheet and it says they run from 4V to 18V.

    Doesn't look like there's any other active components in there... do you think it'd be okay to run a 12V (secondary) CT transformer to replace the existing one? Would it matter if the secondary voltage was different than what the original transformer was, as long as it's within the specs for the opamps?
  10. composition4

    composition4 Guest

    (oh by the way there's only 3 wires coming out of transformer, not 5 like I thought)
  11. Kev

    Kev Well-Known Member

    Nov 13, 2001
    yep, three wires

    AC with a centre tap.

    Typically op-amps will run on + and - 15 volts DC regulated.

    An AC transformer of 12 volts may rectify to a DC level high enough for regulation back down to 15

    I suggest that the AC is probably 15V 0V 15V
    30V with a Centre Tap ... (CT)

    does that make sense ?

    Pleae use a meter to check for AC volts with respect to the centre tap.
    12 V is possible but it would be call ed a 24Volt Centre Tap in a catalog.

    :roll: picture please
  12. composition4

    composition4 Guest

    Yup, makes sense!

    Only problem is, I now can't get hold of a stepdown transformer to measure the voltage from the original transformer

    I've emailed OzAudio for the original specs, but still waiting on a reply..
    Yup I will post pics, I'll have to dust off my camera! :)

    If it helps, the centre wire from the transformer leads to earth rail... does this mean that it is a CT transformer?
  13. Kev

    Kev Well-Known Member

    Nov 13, 2001

    as for the rails ... look for the voltage regulators

    a three legged IC
    might be labelled

    7815 (pos 15volt reg)
    7915 (neg 15volt reg)
  14. composition4

    composition4 Guest

    Ahh yep.. so I could find out that way..

    See why I turn to you for help? :) I'll keep you posted.

    By the way for anyone considering a headphone amp, this unit is great... 4 injects (mono) to assign over 6 channels, can control level of injects differently for each channel. Effects send and return, stereo main input.

    And I'll even tell you what it *sounds* like once I get it running ;)

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