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Alexander

Measuring autotransformer values....

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Has anyone taken the time to have measured say a 3636 autotransformer for mH & DCR at each tap combination?
I mean with a lab like level of accuracy and not an eBay $20 type meter.

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Maybe @BEC,  ALK Engineering or how about the manufacturer?

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20 minutes ago, babadono said:

Maybe @BEC,  ALK Engineering or how about the manufacturer?

All I have been able to find from either supplier  has been the product of those combos in db's . But when I want to make up a circuit say in LTSplice I would like to “recreate” a T2A,T5A or T7A as needed.

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I should add: I do have basic rudimentary means to measure this stuff but feel the actual accuracy would be very much in question

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2 hours ago, Alexander said:

 

 

 

Has anyone taken the time to have measured say a 3636 autotransformer for mH & DCR at each tap combination?
I mean with a lab like level of accuracy and not an eBay $20 type meter.

 

I tried, but I seemed to get weird results on my DE-5000.  Seems like the numbers would fluctuate each time I measured.  More importantly, It seems that the measurement between taps 0 and 3 would not always equal 1/4 the value between taps 0 and 5.

 

The issue, however, is that the output voltage of an autoformer is based on the winding ratio, not the inductance ratio.  Inductance is based on several variables, and windings is only one of them.  LTSpice let's you specify the inductance and DCR only.  All other variables being equal, a 40 mH inductor should have twice the windings as a 10 mH inductor.

 

My T2A LTSpice autoformer equals twenty 10 mH inductors in series and coupled.  Tap 4 equals 14 of the inductors.  Tap 3 equals 10 of the inductors.  Tap 2 equals 7 of the inductors.  Taps 1 equals 5 of the inductors.  

 

It comes out pretty close. My reasoning was that each of the 10 mH inductors would have the same number of windings.

 

I can try to post my Type AA if you would like.

 

Mike

 

  

 

 

 

 

 

 

 

 

 

 

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4 hours ago, mboxler said:

 

 

 

I can try to post my Type AA if you would like.

 

Mike

Hey Mike, I would love to see your example.

 

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I don't think it's all that complicated.  Sure, a transformer coil is going to have some inductance when measured singly for such, but I don't believe it's acting like an inductor (i.e. a filter element) while being used as a transformer.

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1 hour ago, Alexander said:

Hey Mike, I would love to see your example.

 

This may be hard to see.  I'd be happy to send you the .asc file if you want.

 

Mike

Screenshot (3).png

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10 hours ago, glens said:

I don't think it's all that complicated.  Sure, a transformer coil is going to have some inductance when measured singly for such, but I don't believe it's acting like an inductor (i.e. a filter element) while being used as a transformer.

Yes they are over thinking it for audio frequencies.

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If I was trying to guess and wanted real numbers, I would put a good 8ohm inductive load resistor on the taps and measure with something like woofer tester or DATS.  You can extrapolate the numbers at each tap based on the change in impedance.

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Here's the response curve of the 20 X 10 mH inductor (mutually coupled) T2A, using tap 4 (14 inductors).  Green is voltage across taps 0 and 5, red across 0 and 4.

Not sure how close it is the the voltage across a real K-77.

 

1853984283_Screenshot(5).thumb.png.b266adf03b41adb6d4e844d9d09ca930.png

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18 hours ago, mboxler said:

This may be hard to see.  I'd be happy to send you the .asc file if you want.

 

Mike

Screenshot (3).png

 

.

OK. I see what you have done here, not sure how you got there but if it works I will follow your example, thank you for sharing this.

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glens and pzannucci, the problem is in the LTSplice program there are no elements (that I can find anyway) to represent an autotransformer when making even a simple Klipsch OEM crossover.  To just have a cap in series with the mid drive makes a  mess of things so I am looking for a way to find a work around.

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1 hour ago, Alexander said:

glens and pzannucci, the problem is in the LTSplice program there are no elements (that I can find anyway) to represent an autotransformer when making even a simple Klipsch OEM crossover.  To just have a cap in series with the mid drive makes a  mess of things so I am looking for a way to find a work around.

 Coupling series inductors using the K command is the only way.

 

While having a few beers with my brothers, I got an idea.  I disconnected my 3619-ET from ALK crossover and measured the inductance from 0 - 5, 76 mH.  Again I tried to measure the other taps, but the reading would always change each time I tried.  I decided to calculate the inductances based on the turns ratios involved.  I used 76 mH as my baseline inductance.  I think it works.

 

Maybe you can also try this and see if it's close.  It's sure a lot cleaner.  The response curve is similar, but the phase curve changed.

 

The DCR values for inductors L1 thru L5 are .108 .076 .054 .038 and .093 ohms respectfully.  Around .37 ohms total.

 

Mike

Screenshot (6).png

Screenshot (7).png

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Why not simply derive the impedance as seen from the high side of the transformer and use that in your calculations, treating the transformer and driver as a single entity?

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This thread may be relevant - http://www.northreadingeng.com/Audio/group/viewtopic.php?f=12&t=10&p=38#p38

 

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1 hour ago, Wirrunna said:

 

Crap!  I've seen that thread but never scrolled down far enough to see Mr. Warren's schematic of a T2A.  Since the inductances of the various taps are listed on the T2A datasheet, he made it easier by replacing one four tap autoformer with four one tap autoformers.  I, on the other hand, have been struggling with the one four tap math.

The inductance of the T2A taps 0 and 5 must be 45.6 mH...4 times the 11.4 mH inductance between taps 0 and 3.

 

 

 

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On 2/21/2020 at 8:38 PM, glens said:

...I don't believe it's acting like an inductor (i.e. a filter element) while being used as a transformer.


Right. The inductance is so high it’s not doing anything. Well, it is introducing phase shift. 

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On 2/22/2020 at 12:38 PM, glens said:

I don't think it's all that complicated.  Sure, a transformer coil is going to have some inductance when measured singly for such, but I don't believe it's acting like an inductor (i.e. a filter element) while being used as a transformer.

I agree. I have always ignored the autotransformer in LTspice x-over models

Here is the LTspice run plot of my home brew ES networks and the REW measurement of the network at the speaker terminals with the autotransformer in circuit with the mid. I may add a T2A autotransformer and the resistor attenuator to the LTspice model and compare the run prediction but don't hold your breath.

 

Capture of ES outputs at speaker terminals.jpg

LTspice ES400 + ES4500 white background.jpg

Edited by Wirrunna
Changed LTspice plot to white background
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