Remember this?
Of course you do!
Anyway, back at it after a couple of false starts, a trip to the Hole which lead to other calamities we won't discuss, and a bobbin mixup, I finally recieved the non square bobbins for the 75 EI core (Neve output type lams), so I am getting this going again. We were after more inductance, thus, the larger non sq core.
Here are the 5 bobbins, 3/4 square, 3/4 by 7/8, 3/4 by 1. by 1 1/4, by 1 1/2.
Right to left, doh!
I wound 1000 t each on the five bobbins to do a test study on inductance.
Stripped the final candidate already.
3/4 by 1 1/2 > Here is what the xfmr will approximate after winding:
Here are the inductances from the test:
75 EI - 29ga M6 Measured on B-K Precision 875 B
1000 t # 30
3/4 Sq: 11.9 Henries
7/8 stack: 12.43 H
1 inch stack: 13.29 H
1 1/4 stack: 15.68 H
1 1/2 stack: 18.69 H
Some considerations to ponder:
1) the xfmr sits inside a feedback loop. This means two things:
a) the impedance of the transformerand therefor the turns ratio and turns, might be able to be lowered. This will help lower lumped C's.
b) we have to watch for instability. If the circuit and xfmr has phase shift, the transformer will become unstable when the feedback reaches 180 degrees (positive feedback-yikes!). This can happen if there is too much capacitance, a real threat in a transformer of this ratio.
If this happens in the upper the upper pass band, we could be in trouble.
Some unstable xfmr's have been known to pop tweeters becuase of this.
Ersel B Harrison comes to mind. (Peerless, one of his models was a screamer!-forgot which)
I believe in Larry's circuit, since we are direct coupled and only have one stage, instability will not be a problem.
Maybe the reason why the Langevin AM 16 is direct coupled. Not caps to cause phase shift in that circuit which features xfmr feedback.
Anyway, now we have our Al numbers for the 5 cores.
Useful stuff for future projects.
The transformer seen was laced Lap 3, this is the way the final version will be.
This will cut down on any non balanced DC causing problems with the core.
So now we can get our turns and henries for pri and sec.
10 henries is acceptable for a 600 ohm winding.
Some folks use 20, which is even nicer, especially nowdays, with all this sub wolf stuff, but this is a mic pre, and Barry White has passed, so we don't need to worry too much, I guess.
Capacitance is the main obstacle here. Especially with the big core>longer wire length.
So we will use every trick in the book in order to deal with it.
One trick is to wind two pri's, side by side, ala UTC A-10 (sec) .
What this does is lower the "plate" area of the winding by two.
If you connect the two "pies" in series, you lower the C by 1/4 th, which is why UTC and others used this on ratios in the uper integers.
We will wind a bi fi primary to provide balanced DCR and capacitance.
Since this bi-filar winding is going to be used as a continous primary, and not a pri-sec bi fi, like the API, we do not need to worry about pri-sec capacitance.
Actually, the API uses this capacitance to it's bandwidthal favor.
A xfmr can be viewed as a transmission line. Calculate the L's and C's in this line and you can get an idea of what the performance will be like.
Some transformers can be modeled as a two pole filter, (like a 600:600, as all the L's and C's are the same on both sides, pri and sec)
Others can be modeled as a three pole filter network.
Pick the ripple you want in db along the pass band, and you can predict the rolloff in the stop band.
This is why the graphs I did on unterminated transformers resemble rolloff curves off some of the better known Bessel and Tolyschchenov type filters. (sp?)
Inter layering, while decreasing leakage, increases capacitance, so we do not want too many layers. Plus, the bi fi pri will greatly reduce leakage, but we have to watch voltage gradients.
Pri Sec Pri is a popular choice.
Aslo, we need to know a little about the end user's setup in order to do the proper hookup.
Larry, is one end of the secondary going to be grounded, or un grounded (balanced) ?
This will help me pick the best hokup for balanced capacitance.
Thanks!