Iron Core Transformers Without Static Screens

Few days ago I got samples of a bunch of different Iron Core Transformers Without Static Screens.

I have ordered the tranformers with shielding. It is far from proper mu shielding, but better than nothing. The vendor will, for about an extra buck, wrap a copper foil around the unit and bring its connection out to a pin.

Anyhow, the 1:1 has a very nice square wave response. Tiny little overshoot, no ringing and a pretty good bandwidth (a lot on the top!). I was thinking of hooking one in front of an SSM2019 or an INA103 and make a capless pre, and calling it the "Heresy".

Then I went on to try the higher ratio ones to see if it were possible to use them as mic input. The 1:8 transformer looks interesting, but I have to find good termination/zobel network values to improve the step response.

These transformers have no shield between windings. Would housing them in a mu foil/sheet make sense or is it all lost without the internal screen?
Could they be used as microphone input transformers? If not what are the shortcomings.

Thanks,
Tamas

For mic inputs, it probably wouldn't be worth doing without the electrostatic shield. Besides, the M6 core material will cause distortion at such low signal levels. (Still, I can't help but wonder if they would put a shield between the primary and secondary if you asked them. Then again, that changes the whole geometry of the primary/secondary coupling, and they'd have to recalculate the number of turns, etc, which they might not be willing to do for a measly buck...).

The PC series are fine for line inputs/outputs under +24dBM, with one caveat: their winding resistance is high. (I know this because I ordered a few samples and measured their DCR and inductance). What this means is that the insertion loss will be higher than you expect if, for example, you dare to actually run a "600 ohm" coupling transformer into 600 ohms :wink:

Remember that the DC resistance of the windings appears in series with the reflected impedances, and represents "copper loss." The primary and secondary DCR of their PC600/600 is on the order of 130-140 ohms per winding! That would cause a whopping 3dB loss when coupling an "ideal" zero-impedance source to a 600-ohm load. This is a consequence of the fine wire they have to use to fit the requisite number of turns on the small core.

Ya can't cheat the laws of physics; this is why small low-level transformers use different core materials with higher permeability.

The PC series are usable for certain things, particularly at $5 a pop, but the WSM series are better overall transformers if you can spare the room and can afford the weight and the higher price.

JensenJT-16-A,SSM2019,OP275,JT11.
No caps in the audio path.



Still working on it.....
Sounds fat.
I have some edcors I'm going to try out soon.

Sweet!!!

> no shield between windings. Would housing them in a mu foil/sheet make sense or is it all lost without the internal screen?

Two different things. (Mu)Iron around the outside reduces external magnetic fields. Copper around the outside reduces external electric field (and may reduce magnetic field). Copper between windings reduces capacitively coupled crap leaking from winding to winding.

None of these are needed if you have a clean mike out in a pasture. In fact I've run mikes through no-shield transformers just sitting in a mostly-closed steel box, laying on lamp dimmer lines. Going the whole nine yards with Mu and foil is a worst-case fix: 98% of gigs can get away with a tin tuna can or nothing at all, and some care in placement (not on top of the power-amp's transformer).

a while back i had edcor make me a pcb mount, ~1:7 transformer with a split primary, foil tape shield on the outside, and an electrostatic shield between the primary and secondary. it cost about $12. the windings each had a DCR of over 100ohms each, i think.

i only tried it briefly in an api preamp config- new api opamp and the profile output. the edcor did work, was actually quite flat from about 15hz-25khz with a gentle roll off on the top end. unfortunately, below about 1khz the THD+N figure on the whole circuit rose to just under 0.1%. compare this to a cinemag 1:8, which stays around 0.01% down to 100hz. this is all sweeping with a -20db test signal.

anyway, it does work, and you can definitely HEAR it vs. a cleaner trafo. i'd bet that with some work edcor could probably make a low ratio mic input with the external tape and internal screen shielding that would sound good for not too much money.

ed

Two different things. (Mu)Iron around the outside reduces external magnetic fields. Copper around the outside reduces external electric field (and may reduce magnetic field). Copper between windings reduces capacitively coupled crap leaking from winding to winding.

Click to expand...

Thank You PRR. I will have to read up on capacitive coupling in transformers.

[quote author="edanderson"]a while back i had edcor make me a pcb mount, ~1:7 transformer with a split primary, foil tape shield on the outside, and an electrostatic shield between the primary and secondary. it cost about $12. the windings each had a DCR of over 100ohms each, i think.

i only tried it briefly in an api preamp config- new api opamp and the profile output. the edcor did work, was actually quite flat from about 15hz-25khz with a gentle roll off on the top end. unfortunately, below about 1khz the THD+N figure on the whole circuit rose to just under 0.1%. compare this to a cinemag 1:8, which stays around 0.01% down to 100hz. this is all sweeping with a -20db test signal.
ed[/quote]

Oh yeah, this sounds very interesting. Looking at mic transformer specifications the DCR is usually half of that value, around 50 ohms or less. As I understand qualities of the M6 iron makes it necessary to pile up more windings to get enough inductance.
I need to call them to see if they can recalculate the windings for a higher permeability core.
Having a decent mic input transformer for less than $20 would be pretty amazing.
It would be interesting to see a thread on the characteristics and tricks that make these transformers perform well.

Tamas

the limitation with edcor is that they only work with m6 cores. they don't stock any nickel lams, so unless you want to have them try to make the coil for you and put the lams for the core in yourself, you can't get a nickel core transformer from them.

at some point i'd like to have edcor take another stab at an iron core mic input, but it might be a while before i get back to that.

ed

Ed,

For your 1:7 transformer experiment, were you using their PC format?

Thanks,
Tamas

[quote author="tk@halmi"]

Two different things. (Mu)Iron around the outside reduces external magnetic fields. Copper around the outside reduces external electric field (and may reduce magnetic field). Copper between windings reduces capacitively coupled crap leaking from winding to winding.

Click to expand...

Thank You PRR. I will have to read up on capacitive coupling in transformers.[/quote]

Twice I tried to wind the same ribbon transformer--one with electrostatic screen, and another without. Both times, ones with the screen had noticably better top end.

Currently there are two transformers I am planning to recore with SUPER Q80 "magic" alloy:
1. PC 150/150
2. PC 150/600

They have large lamination, 1.625x1.3 inches, so there is a lot of iron in there, but it is all M6 type. I have been making some measurements that I will repeat after redoing the core.

Tamas

tamas-

no, the mic input trafos they made up for me were done on a smaller pcb mount bobbin. i will measure them and post the specs, either before or after this weekend.

ed

here's some pix of the mic input transformer i got from edcor (left), next to an edcor WSM 600/600 (right). there's a little packing styrofoam stuck to the varnish on the mic input.




it is about an inch and a half wide and 1 1/8" tall. the bobbin is about an inch wide and 3/4" tall. the hole through the bobbin is 3/8" square.

ed

A BIG Thanks, Ed!