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Mic Transformers

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  • rickbihary
    I have been a lurker here for a while...I have a question about mic transformers. Why are they so expensive? I have access to a coil winder, how hard would it
    Message 1 of 2 , Feb 1, 2006
      I have been a lurker here for a while...I have a question about mic
      transformers. Why are they so expensive? I have access to a coil
      winder, how hard would it be to wind my own transformers ? Any info on
      wire size, forms, # of turns, etc.?
    • Manfred Mornhinweg
      Hi Rick! ... Because they are rarely used, so the market is small, and because esoteric audio stuff tends to have a price tag that has no sensible relationship
      Message 2 of 2 , Feb 4, 2006
        Hi Rick!

        > I have been a lurker here for a while...I have a question about mic
        > transformers. Why are they so expensive?

        Because they are rarely used, so the market is small, and because
        esoteric audio stuff tends to have a price tag that has no sensible
        relationship to its actual value!

        > I have access to a coil
        > winder, how hard would it be to wind my own transformers ?

        Not hard at all.

        > Any info on wire size, forms, # of turns, etc.?

        Let's start with the core. While laminated iron can be used, it's not
        good in the high audio range. I prefer to wind my audio transformers on
        high permeability ferrite cores. The size can be very small. I have
        mostly used double-E cores of 20mm square, and also some pot cores of
        about 15mm diameter, and these cores were MUCH larger than what's really
        necessary.

        You need to put on enough turns so that the inductive reactance is very
        high compared to the circuit impedances. This is necessary in order to
        make magnetizing current negligible, which in turn is necessary because
        magnetic cores have a nonlinear magnetization curve, so that the
        magnetization current is nonlinear too and causes distortion. You want
        to minimize that.

        Once you have the number of turns, you select the wire size simply based
        on the available space. Half of it goes to the primary, the rest to the
        secondary. Assuming 40% copper filling tends to be reasonable. Then you
        calculate the resulting resistance, compare to circuit impedance and
        determine if the resulting loss is acceptable. If not, you need a larger
        core, or you need to wind fewer turns and accept more distortion. So a
        lot depends on the degree of linearity of the ferrite material chosen!

        While winding, I suggest to alternate layers of the primary with layers
        of the secondary. That way you get the best coupling.

        Finally, the transformer should be shielded, because even self-shielding
        cores such as pot cores do react to external magnetic fields. Mic
        transformers work at VERY low levels, and so they can be horribly
        sensitive to magnetic fields!

        All this might sound intimidating, but don't be afraid. Try it! My
        experience is that even very poorly made audio transformers work,
        somehow...! :-)

        But then, use audio transformers only when there is a very good reason
        for it. There are such reasons, indeed, but in most situations you can
        avoid a transformer, and that spares you a lot of possible problems.

        So... this was rather general. If you need more info, just ask!

        Manfred.


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