Mic Input Transformer Inductance

[PRR]

> Less winding capacitance means more leakage inductance and vice versa.

Not really. Inductance goes by number of turns. Capacitance goes by the bulk of the winding.

> a few nF. ...in the range of a few pF.  ...a rise at high frequency.....

Vague. How much is "few"? How high is "high"?

Get good numbers. Plot on Reactance Chart. Also plot nominal mike impedance.

The "few nF" job seems to resonate near 200KHz and 300 Ohms. This is a decade above the audio band, and not far off from mike impedance, so I would not expect trouble.

The "few pF" job *appears* to ring at >1MHz(!) and 25K. I feel sure this is a measurement error. C is more likely "few hundred pFd". This plots 200KHz and 2.5K, higher than mike, so not well damped.

 

[ricardo]

The thing is, the inductance varies with signal amplitude... at around 500mV I get the highest inductance. At 1V it's a bit less but at 25mV it's alot less (about 1,5H less).

I read that transformers with Iron core behave that way but I know that the core is made out of Mumetal.

Yes.  Transformers with ferrous cores (incl. Mumetal) behave like that.  The pic shows a small output transformer for a stick mike fed by an emitter follower running 1.23mA & 4u7.  So Rs is 10R and the load is the usual 2k of a mike preamp.

This is about worst case for the bumps.  A higher Rs and/or smaller load would show 'smaller' & 'shallower' bumps cos the resonance with the 4u7 is lower Q.  I used to do fancy stuff with Zobels in my Calrec transformer mike days to build in sub & supersonic filtering.  No 4u7 means no bumps.

I then also traced the signal with a scope and found that the frequency response from 31,5Hz to 15Khz is the flattest when applying about 500mV.

If this stuff is important to you, its best to do frequency responses with your transformers at various levels with your circuit rather than agonise over the Inductance specs.

The makers are reluctant to quote Inductance cos it varies so much.  The better makers will show a Response Curve and will specify the level and also source & load resistances.

At 25mV I get a big bump at the high frequencies...

The rise above 100kHz in my red curve is simply HF noise cos the level is so low.

 

[abbey road d enfer]

The better makers will show a Response Curve and will specify the level and also source & load resistances.

The very best include a sticker that says: "For use at -10dBu only"!  ;D