Alternative to tapped inductor in 3110a or 3115a?

[GrooveControl]

EDIT: I used the wrong value. Looks like it should be .02mh. Ignore blue line and question. thx.


With the exception of the coils, I found the exact value components of the high pass filter in my spare parts drawer (for the -10db pad). I also found a .24mh coil that I was able to wind down to .20mh (measured it in speakerworkshop).

Then assembled the network and took freq resp measurements, without the big coil, here are the results. 2445J on 2380A.

Green = no hf boost
Red = 2uf bipass
Blue = .20mh + 3uf bipass

Something unexpected is happening with that coil I unwound. It's a Solen S18.24 that I unwound from the outside.

I'd like to get the response above 7khz up by about 3db. Any thoughts?

[Baron030]

Hi GrooveControl

Yes, the correct value is 0.02mH. By using a 0.20mH coil instead, you can clearly see how the 0.2mH coil and the 0.3uf cap is resonating at about 6,499Hz. Call it one of those D’Oh moments . We have all had them at one time or another. When you finish un-wounding that coil down to 0.02mH, you will see just how tiny that thing really is.

Baron030

[Baron030]

Well, as you all know 4313B has already come up with a non-tapped autotransformer version of the 3110A network. But, I thought I would do some tests and see if I could come up with some suggestions for constructing autotransformers for use in other crossover network designs for which a non-tapped autotransformer version has yet to be created. And also possibly bust a few autotransformer myths in the process. Pictured below is the high pass section of 3110A network made from some stuff that I just had lying around. And since, I did not want to destroy a perfectly good Jantzen 15 gauge Air Core inductor for these tests, I decided to nick the insulation in just 2 places and then solder some leads to create taps. There are 3 things that I wanted to determine in my tests.

1. When using air-core coils so the voltage drives come close to the mathematical formulas or not?
2. And which formula appears to be more accurate? Turn Ration or Percentage of Inductance?
3. And does it make a difference where the secondary is? Should it be near the core or can it be near the outside of the coil?

[Baron030]

The first big surprise was measuring the tapped coil’s inductance. It would appear that when it comes to inductance the total is much greater than the sum of its parts. So, any myth of stringing together a bunch of inductors together to create an autotransformer is totally “BUSTED”. The values just don’t add up right.
Since I wanted to determine if there were any differences in secondary winding locations, I purposely tried to created two separate tap points with as close to same turn rations as I could.
Starting with the secondary windings closest to the core, that is to say that pin #1 is the primary input and pin #5 is the secondary output and pins #2 & #6 are the common leads. The turn ratio formula calculates the output to be -6.7db. And the percentage of inductance formula calculates the output to be approximately -8.2db.
Alternatively, with the secondary windings being closest to the outside of the coil, that is to say that pin #2 is the primary input and pin #4 is the secondary output and pins #1 & #3 are the common leads. The turn ratio formula, it calculates the output to be -7.5db. And the percentage of inductance formula calculates the output to be approximately -6.5db.

[Baron030]

Now we come to some test results. I should point out that in the 3110A network, when the HF Boost is set to its minimum setting that a 3.9 ohms resistor (R4) is bypassed. And so it would appear that the output level is actually higher than when the HF Boost is set its other positions. And that I used a 0.025mH coil instead of the required 0.020mH value. So, the “MAX” HF Boost comes in at a little lower frequency then it should be.
Inside Secondary:

Outside Secondary:

[Baron030]

Admittedly, looking at the test results, it does have me scratching my head a lot. There are a few things that I kind of expected like the measured output voltage drives being lower than their calculated values. And if you think about it, when was the last time you ever saw an air-core power transformer? Or a tube amp with an air-core output transformer? So, it’s not surprising that measured values are lower because the mathematical formulas are not taking into account the greater core losses associated with air-core coils.
Since, the curves are fairly flat at 2511Hz, I will use that frequency for direct comparisons between measured and calculated values.

Inside Secondary:
Calculated: Turn Ratio Formula: -6.7db, % of Inductance Formula: -8.2db
Measured: No Boost: -9.34dbV, Mid Boost: - 12.26dbV Max Boost: -12.29dbV

Outside Secondary:
Calculated: Turn Ratio Formula: -7.5db, % of Inductance Formula: -6.5db
Measured: No Boost: -7.4dbV, Mid Boost: - 10.24dbV Max Boost: -10.46dbV

It would appear to me that the “Percentage of Inductance” formula might be far more accurate than a “Turn Ration” formula when it comes to constructing coils. And in the case of an air-core coil, the location of the secondary does not appear to make much of a difference. And yet it is strange, how 146 turns near the outside can generate more inductance and voltage drive then 160 turns wound near the center. So, I am thinking of creating another tap point near the core with hopefully something close to the outer tap’s 0.726mH of inductance and I will run some additional tests.

Baron030

[Baron030]

Since, the results of my first tests just did not seem to me as being very conclusive as to which secondary location came closest to fitting the mathematical model. I decided to create another tap and rerun my tests. And then I soon realized that I did count the total windings correctly. The correct total count is 386 turns and this changes all of the “Turn Ratio” formula calculated values in my previous test.
Pictured below are the correct values and the new Clio Results:





Since, the curves are fairly flat at 2511Hz, I will use that frequency again for direct comparisons between measured and calculated values.

0.785mH - Inside Secondary: Calculated Turn Ratio: -5.7db, % of Inductance: -6.8db
Measured: Min Boost (R4 shorted): -7.12dbV, Min Boost (with R4): -8.90dbV, Mid Boost: - 9.90dbV Max Boost: -10.12dbV

0.726mH - Outside Secondary: Calculated Turn Ratio: -8.4db, % of Inductance: -6.5db
Measured: Min Boost (R4 shorted): -7.37dbV, Min Boost (with R4): -9.16dbV, Mid Boost: - 10.20dbV Max Boost: -10.43dbV

Well, I think the differences between the two formulas and the measured results appear closer together in the case where the secondary winding is wound closest to the core.

So, my recommendations for anyone wanting to make an autotransformer would be to use an iron core from an Erse I-Bar coil and follow the percentage of Inductance formula, and wind the secondary windings (taps) closest to the core.
Baron030

[4313B]

So, my recommendations for anyone wanting to make an autotransformer...

Mine would be to try and live life a little, travel a bit, see the world, enjoy a cold beer, volunteer in your community... something, anything, besides sitting around making tapped autotransformers. Hell, worst case, fire up a game of World of Warcraft for fuck's sake.

Of course, if someone has already done all that other shit and the only thing left to do on the bucket list is wind an autotransformer then by all means...

[Baron030]

But, what about the people that want that real “Autotransformer” sound?

Whatever that is?
I am kidding of course…

No, I only did this research just to prove or disprove a few ideas and to kill a few myths.

Baron030

[4313B]

I think it's great.

Thank you for taking the time.