Inductance vs Frequency-A-10

I rigged up an external signal generator so I could check inductance of a transformer at different frequencies.
That is, I can null out the General Radio impedance bridge at different freequencies to see how the inductance and Q changes.
This was the missing link as far as blueprinting transformers, as now I have a better illustration of what is happenning at the low end.
Before, all I had was a straight line that went down to about 1 hertz.
Well, if every transformer I test has the same line, it really does not tell me much.
But now, I have a "fingerprint of the low end response.
This, combined with the high end freq response provides a more complete picture of the transformers response.

Here is a graph showing a UTC Black A-10 vs a light gray A-10.
Inductance is on the left axis.
Frequency is on the bottom axis.
I added a few "Q" figures also. Enjoy!
cj

What does it all mean Mr. Holmes?????

The low end response is mainly determined by the core.
The high end response is mainly determined by the coil structure.
Before, all I had was a staight line going down to dc for most of the transformers, which means I didn't really have much to go on as far as differentiating transfomers by their low end response.
But you can see from the graph, there is a lot going on from around 100 hz to 500 hz.
Interestingly enough, the impedance bridge has an internal oscillator set at 1 k hz, which is in a very non linear part of the inductance curve.
Anything past 1 khz is suspect, as the x-former was very hard to null out.
cj

:?

I applaud your efforts to test more thoroughly. You should pick up a book that includes transformer testing. There are several other tests you could make with your setup. For example, you can short the secondary and test the leakage inductance.

The common models of transformers have several inductors, capacitors and resistances in the model. Until you get a full set of values, you are still playing with half a deck. Even then, the RLC equivalent circuit won't tell you much about the magnetic characteristics of the core.

It seems a good time to post this link again. Here's a great article I scanned a while back:
1.5MB PDF

Yes, I check the leakage both ways , pri shorted, and sec shorted.
I did read the Crowehusrt article that PRR came up with, which was very imformative.
And of course, everybody knows that the real test is plugging it in to the circuit and having a good listen.
If you have any of the book titles, I would be interested.
Thanks,
cj :guinness:

So how does the A-10 compare to the HA-100X??

I'd love to know as I have both a real A-10 and a reissue HA-100X and would love to know which you recommend putting in my LA-2A.

:green:

JC

I don't understand all the mystique about LA2A input transformers. Even though the original design used a stepup transformer, I think that any decent 1:1 should work well.

Yes, I am not all that impressed with the winding structure.
The triple shield is nice, but then again, that would seem to help more at mic level than line level, as once you get off the noise floor, shielding is not as critical.
I am going to unwind the A10 to see how it matches up with the 100X. Probably tonite.
As most LA2 builders complain about overdriving the line amp into distortion quite easily, the Jensen mod, 10k:10k seems like a logical choice, or Lundahl, Sowter bridging iron would work well also.
:guinness:

good stuff cj, can't wait to see more.

I did the jensen mod to my LA2 and I think it sounds great.
a BIG improvement.

Lance

so if your thinking 1-1 transformers would you consider using a utc a20?
I know that its designed for lowerer impeadence but the ratios would be right?

thoughts Wilebee

Remember, only one of the transformer windings should be terminated in its nominal impedance. So, if you're driving from a 600-ohm source, the A-20 would work fine. If your source impedance is lower, it can be built out with series resistors. In practice, this might not even be necessary. That's why it's handy to have a basic complement of test equipment to check for ringing, etc. in a transformer that's being run outside of its nominal impedance range.

But I thought the point here may be that UTC transformers might not be the best choice?

BUT

perhaps thats why the la2a has the sound it does. like I've said before, I have an ADL opto compressor that was designed around the la2a. It has custom wound transformers and it is very clean and transparent. more so than the la2a. I often think it lacks a bit in personalty.

2 cents.