Audio transformer questions.

Maybe someone can put me out of my misery and answer these mic-transformer related questions. I've been trying to get definitive answers for several weeks with not too much luck.

#1 Is it possible to damage a mic transformer by applying DC (eg resistance meter)? I read somewhere that the DC from meters can magnetise the core (seems unlikely to me, but...). If so, can a tape head demagnetiser (wand) be used to reverse the process?

#2 What are the relative merits of applying the +48v to the centre tap as opposed to using 6k8 resistors? (there may be a link with #1 here?) The Gyraf uses resistors, some others use the centre tap. If the resistors are left in circuit, they have a bearing on the input impedance, so may affect the design of other parts of the input circuit. Isn't it better not to be hanging things off the primary if it can be avoided?

#3 The Sowter website lists transformers for mic level and line level. Is there any disadvantage in using a line level transformer for mic input, if the turns ratios are the same? Eg can the 4383 (LA-2A input type) be used as a mic transformer?

#4 Any web resourses that go into audio transformer design in some detail?

There are more, but this thread is long enough already! :grin:

Cheers,

Steve

It is possible to magnetize the core by reading the windings with an ohmmeter. One fix for this is to apply a gradually increasing (and then gradually decreasing down to zero) low-frequency AC signal to the windings.

It is preferable to apply phantom through a pair of matched resistors rather than via the centertap. Then you need not worry about DC imbalance between sections of the winding.

Bill Whitlock's chapter on audio transformers in the latest edition of Handbook for Sound Engineers is an excellent introduction to the topic. It's a big, very expensive book, but you might find it at your local library or a bookstore with a liberal "read before buying" policy like Barnes and Noble.

Interesting that you should use the LA-2A example, because the original LA-2A used a 1:10 mic input transformer for its input. There really is no practical difference between mic and line transformers in most manufacturer's product lines, except that mic input transformers typically are well-shielded, have higher step-up ratios and usually perform best when driven by a low impedance source, which is not true of all line input transformers (like the ones designed for high impedance sources).