DC isolated

[oran.outan]

Hi All

I've see some thread talking about but I'm still a bit confused.
I'm building an 2 channel Line to Mic circuit (globally send line output from soundcard into Mic pre) and I like to DC protect the signal chain against +48v. (some of the stupid things that can happen so easily). So I like to put a transformer for isolate the circuit, but my skills are a bit limited and all of the transformer I have used on the past are noted "Do not pass DC through windings". So if I understand +48v will 'burn' them !
Do you have any recommendation about which or type of transformer to use ! I will need a 1:1 - 150/150
Is that situation need gapped transformer ?

thanks
0-0

 

[abbey road d enfer]

. So if I understand +48v will 'burn' them !

No, because 48V is applied equally to both sides of the winding, not through. Current may go through if one leg is shorted to ground, though.
Anyway it is not likely to "burn" the xfmr, just magnetize it, which is not a big issue. **
If you're anxious about it, you may want to add DC-blocking capacitors between the secondary and the mic input. For that you would need film capacitors of rather large value (about 50uF), that are quite expensive and bulky.
https://fr.farnell.com/epcos/b32526t1476k000/condensateur-47-f-100v-10-pet/dp/2797797
**Demagnetizing a xfmr requires applying progressively a low-frequency signal, with a max amplitude of about 1Vrms and decreasing it also progressively. Tell tales of a magnetized xfmr are loss of bass and distortion.

Do you have any recommendation about which or type of transformer to use ! I will need a 1:1 - 150/150

For this application, I would think a step-down xfmr would be more appropriate, that would allow to drop down the signal from line to mic level, as well as decreasing the impedance.
Typically a 8:1 or 10:1 xfmr is used for that task. That would be a 200:20k or 200:12k where you swap primary and secondary.

Is that situation need gapped transformer ?

No, since there is no DC current through.

 

[warpie]

It's called "phantom" power for a reason

 

[oran.outan]

Ooooohhhh, well !!! I see my misunderstanding...

Do you have any recommendation about which or type of transformer to use ! I will need a 1:1 - 150/150

The circuit i've made as an input impedance of 8.1k and output impedance of 150ohm. I should be fine with a 1:1 - 150/150 no !? as the point of the xfmr is 'mostly' for DC protect...


thanks @abbey road d enfer !!!