Microphone preamp input transformer design questions.

[mkiijam]

Hello all.

I bought myself a CNC coil winder. I have all the core, bobbin, mu shielding, wire, etc, that I want. I have built more than a few transformers and they all sound fine. But I find myself with a few questions about how to "really" do this thing.

1. I have real, custom made, mu-metal shielding cans, but how do I use them!? I mean I dropped the TRX in there and epoxied it in... but Do I ground the can? Do I ground the TRX to the can? Power ground? Chassis ground?
     a. I think the answer is "yes, you ground them" so...
2. Screw holes in the bottom up through some PCB pads and carry it off to chassis ground?
     a. I neglected to have screw holes put into the bottom of my cans. Can I drill the MU without completely tweaking the Mu factor? (what's that called anyway?)
3. I have discovered through (cough-rev-cough-eng-neering-cough) that there are little copper bands inside the TRX that separate the PRI and SEC windings. I didn't put those in my little guys and all seems fine. Do I need them? Really? Why?
     a. I suppose the answer will be "yes, you need them" So...
4. Where do I "drain" the interwinding shielding? To the can? To chassis ground?

5. Epoxy vs hotglues vs ? for potting? Beeswax?

Thanks all!

[Terry Bites]

Mag shielding is only part of the can's function. Its not just diverting incident magnetic fields around the XFMR.
Both the can and the shield winding connect to local circuit ground.
The can is also a equipotential Faraday cage.
The shield winding extends this "cage" between the primary and secondary.
Its function is to neutralise the interwinding capacitance.
This attenuates common mode signals (50/ 60 Hz etc) trying to find their way across.
It also stops intefering signals being capacitively coupled into your XFMR.
A groundplane under the can will help complete the cage.

If you look at Jensen's mic XFMRs you'll see they have two shields. An input shield tied to dirty mains ground and an output shield tied to clean circuit 0V. A serious attack on common mode crap. www.jensen-transformers.com/transformers/mic-input/  If your nice linear power source doesn't have an interwinding shield, the spec of your high performance mic XFMR will be undermined to some extent.

[jonpaul]

special high u tapewound cores, no gap, no cut core
Classic Beyer, Shure, UTC, MIcrotran mic transformers are easily available on epay and elsewhere $ 15..100 each.

Typical ratios 600:50k Ohms, 150 CT:50 k Ohms

Copper bands may be Faraday shields or shading bands, post pix what you are doing.

Jon

[floobydust]

There is a lot of art and science to winding professional audio transformers, especially for mic input. People write books on the subject, it's not trivial.
I recommend reading Handbook for Sound Engineers, 3rd Ed. Audio Transformers chapter by Bill Whitlock, Jensen Transformers Inc.
More wisdom in the Jensen Transformer App Notes.

[jonpaul]

Agree and Deane Jensesn and Bill Whitlock "wrote the book"

Especiallyt the choice of wire, interlayerinsualtion, shields, and windingbuild (no layers, type of wdg etc) all are critical.

The art is to get both low THD at max level , meet the LF and HF rollofs (eg 20 Hz..20 kHz) and have grest shielding/CMRR.

We easly ahcueved a 10 uV sensitivutuy in a valve (tube) preamp with 1..2 x 6AL7 and Altec plugin octal mic trsf or UTC, 1:30 or 1:12 ratios.

The best are still from the mic mfg Beyers, Senneheiser, Shure,  or high end trsf firms Jensen, Lundhahl, etc.

Expect hard to get core materials.

Yes any metalwork on Perfection Mica Netic and CoNetic high u shields  degrades the H shield effectiveness.


Bon Chance,

Jon