midwayfair

Mic output transformer with center tap acting weird
« on: November 03, 2020, 12:53:56 AM »
I've got a Cinemag 24110 (https://cinemag.biz/mic_output/PDF/CM-24110.pdf) that's behaving strangely.

I measure ~350R between each of the primary leads and the center tap, but I'm not getting any voltage on the center tap when I apply phantom power to the secondary. If I make a virtual center tap (with 2k2 resistors) and pull the center tap, I get phantom power at the virtual center tap as expected, and mic sounds right, so I know that the transformer is working otherwise, but I still get 0v on the transformer's center tap.

I've got another mic with a very similar transformer (same wiring, different ratio: https://cinemag.biz/mic_output/PDF/CM-24110.pdf) sitting here that is working as expected. I've noticed that I get thousands of ohms of resistance between the leads and the center tap, though, which is (a) not what I would expect but (b) definitely different from the other xfo.

I should mention that I did do one bad thing: I had the transformer flipped around backwards at first, meaning I had the primary connected to the XLR pins, which means at least hypothetically I put DC across the transformer, but I don't know how I could damage only the center tapping and not the entire transformer.

Does anything about what I've described sound like I borked the transformer such that replacing it would help? Even though the mic is working, philosophically I'd prefer to use the actual center tap.
I'm Jon. Myself's music and things I make: jonpattonmusic.com. My band: www.midwayfair.org. [Disclaimer: PCBs of guitar pedals I've designed are sold by Madbean, 1776 Effects, and JMK PCBs.]


abbey road d enfer

Re: Mic output transformer with center tap acting weird
« Reply #1 on: November 03, 2020, 01:14:22 AM »
I've got a Cinemag 24110 (https://cinemag.biz/mic_output/PDF/CM-24110.pdf) that's behaving strangely.

I measure ~350R between each of the primary leads and the center tap, but I'm not getting any voltage on the center tap when I apply phantom power to the secondary.
Seems perfectly normal to me. Xfmrs offer galvanic isolation.

Quote
I've got another mic with a very similar transformer (same wiring, different ratio: https://cinemag.biz/mic_output/PDF/CM-24110.pdf) sitting here that is working as expected. I've noticed that I get thousands of ohms of resistance between the leads and the center tap, though, which is (a) not what I would expect but (b) definitely different from the other xfo.
Looks like you've reversed primary and secondary.

Quote
Does anything about what I've described sound like I borked the transformer such that replacing it would help?
No; it just looks like you've made wrong connections.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

midwayfair

Re: Mic output transformer with center tap acting weird
« Reply #2 on: November 03, 2020, 09:11:39 AM »
Seems perfectly normal to me. Xfmrs offer galvanic isolation.
 Looks like you've reversed primary and secondary.
 No; it just looks like you've made wrong connections.

Huh. I mean, what you say makes complete sense, if it weren't for the fact that when I had the transformer connected in the other direction, the mic was crazy loud, like the xfo was acting as a 1:4 instead (which is why I was pretty sure that I had the leads swapped). I'm really confused here.
I'm Jon. Myself's music and things I make: jonpattonmusic.com. My band: www.midwayfair.org. [Disclaimer: PCBs of guitar pedals I've designed are sold by Madbean, 1776 Effects, and JMK PCBs.]

midwayfair

Re: Mic output transformer with center tap acting weird
« Reply #3 on: November 03, 2020, 09:50:43 AM »
Grabbed an AC power supply and a couple other transformers to make sure I wasn't going crazy.

Cinemag 2510 (8:1): ~15V on the primary side, ~1.8 on the secondary side. So that's working fine.

Cinemag 11021: 15V on primary ... 3.42 V on the secondary. This is supposed to be 2:1! Now I wonder if they were mislabeled ... ? Time to pull the transformer out of the mic and test it with the AC supply.

EDIT: OKay pulled the wires on the 24110 from the mic.

15VAC on brown and red (primary according to the datasheet): 3.2V on yellow and orange, 7.5V to the center tap (relative to brown or red)
15VAC on yellow and orange: 13.6V on the red and brown wires, 6.8V on the center tap. (It's probably multiplying the voltage but my multimeter is dragging it down?)

So it looks like primary -> secondary gets me the right step down ratio, but then the center tap is on the high voltage side. (On the 2:1 that's behaving properly, the center tap is definitely on the low voltage side.) In other words, the center tap is on the wrong side to both provide phantom power and act as a step-down. Am I interpreting this right?
« Last Edit: November 03, 2020, 10:22:55 AM by midwayfair »
I'm Jon. Myself's music and things I make: jonpattonmusic.com. My band: www.midwayfair.org. [Disclaimer: PCBs of guitar pedals I've designed are sold by Madbean, 1776 Effects, and JMK PCBs.]

midwayfair

Re: Mic output transformer with center tap acting weird
« Reply #4 on: November 03, 2020, 11:36:56 AM »
okay, mystery solved: I called Dave and confirmed that the ones they're winding in my current order have the center tap on the secondary, whereas this one the center tap is on the primary. I'll have to try to find some other use for this little guy. Should work as a mic input if I stick it in a bell or perhaps as an interstage in something.
I'm Jon. Myself's music and things I make: jonpattonmusic.com. My band: www.midwayfair.org. [Disclaimer: PCBs of guitar pedals I've designed are sold by Madbean, 1776 Effects, and JMK PCBs.]


 

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