- 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.?
- Hi Rick!
> I have been a lurker here for a while...I have a question about micBecause they are rarely used, so the market is small, and because
> transformers. Why are they so expensive?
esoteric audio stuff tends to have a price tag that has no sensible
relationship to its actual value!
> I have access to a coilNot hard at all.
> winder, how hard would it be to wind my own transformers ?
> 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
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,
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!
Visit my hobby website!