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Lundahl LL1545A LTSpice Simulation Issues
Lundahl LL1545A LTSpice Simulation Issues
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Old 2nd November 2017, 07:38 PM   #11
pieter t is offline pieter t  Netherlands
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Please confirm that you plan to use the transformer with 1:1 ratio; that's what I make out of your sim.
When you measure 8.3H in each primary, the total inductance of the combined primary (the 2+2) is 8.3H x 4 = 33.2H.
Your combined secondary (1+1+1+1) also has a total inductance of 33.2H as the number of windings of combined primary and secondary is the same (4 coils in series with 2H inductance each make 32H total inductance; the difference with 33.2H is some measurement inaccuracy).
In your sim I guess it is better to draw only one primary and secondary coil, with the same DCR and the same inductance.

Last edited by pieter t; 2nd November 2017 at 07:44 PM.
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Old 3rd November 2017, 12:31 AM   #12
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by lemon View Post
Abraxalito, can you attach me the file that you described like -1dB at 400K?
Here you go though looks like you've got it all sorted out now.

Ooops, I noticed I reduced the loading shunt cap at the output to 47pF while I was playing.
Attached Files
File Type: asc lemonAK lundahl.asc (3.8 KB, 5 views)
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Last edited by abraxalito; 3rd November 2017 at 12:33 AM.
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Old 3rd November 2017, 02:16 PM   #13
lemon is offline lemon  Greece
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Quote:
Originally Posted by pieter t View Post
Please confirm that you plan to use the transformer with 1:1 ratio; that's what I make out of your sim.
When you measure 8.3H in each primary, the total inductance of the combined primary (the 2+2) is 8.3H x 4 = 33.2H.
Your combined secondary (1+1+1+1) also has a total inductance of 33.2H as the number of windings of combined primary and secondary is the same (4 coils in series with 2H inductance each make 32H total inductance; the difference with 33.2H is some measurement inaccuracy).
In your sim I guess it is better to draw only one primary and secondary coil, with the same DCR and the same inductance.
Peter t, thnx for all, it is more clear to me now the right setup of this.

Quote:
Originally Posted by abraxalito View Post
Here you go though looks like you've got it all sorted out now.

Ooops, I noticed I reduced the loading shunt cap at the output to 47pF while I was playing.
Thnx, I had make a similar sim yesterday with almost identical results.
The major problem has dissolve, it was a sim mistake with R//C at the secondary but our messages are very informative to me about how transformers work.
Thanks guys!
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Old 3rd November 2017, 02:21 PM   #14
abraxalito is offline abraxalito  United Kingdom
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There's one nagging question I had about your sim - how did you come up with the coupling coefficient (0.999) ? Have you made a measurement of the leakage inductance? If not, its not a hard one to do for a reality check - the HF bandwidth will be determined by the leakage inductance.
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Old 4th November 2017, 09:19 AM   #15
lemon is offline lemon  Greece
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No, I haven't done any measurement of the leakage inductance.
I have read from the internet that the 0.999 is the good practice for transformers like lundahl.
Can you explain it with more details, how the measurement can be done?

For me the Lundahl on AK4490 was a project. I would like to known how this "super v/i" implementation vs normal opamp stage or discrete.
From what I saw, the acoustic result is very good, the only problem is the output impedance, it is almost 1.2K and a buffer stage is must.

Additionally, I compared this "super v/i" stage with the simple direct method, like each AK differential output goes to each Lundahl winding at the primaries. From what I saw the super v/i implementation gives more low frequencies linearity than the direct method and at the LTSpice Simulation there is a little better phase linearity.

Last edited by lemon; 4th November 2017 at 09:30 AM.
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Old 4th November 2017, 03:26 PM   #16
jan.didden is offline jan.didden  Europe
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Just out of curiosity, why do you call it a V/I stage? I think it is a normal V to V stage? Or am I missing something?

Jan
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Old 4th November 2017, 10:58 PM   #17
lemon is offline lemon  Greece
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You haven't missing something, sure.

It is the name that has, from the designers.
Super V/i converted (LL1684 )
In reality, the author of this schematic is Stefano Perugini, he has published this article at 1998.
A 24 Bit DAC, the Full Glass Audio Article
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Old 4th November 2017, 11:30 PM   #18
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by lemon View Post
Can you explain it with more details, how the measurement can be done?
It isn't too difficult - the leakage inductance is the inductance of the primary when all secondaries are short-circuited. The test frequency should be mid-band (say 1kHz) and the amplitude kept as low as possible. The ratio of the leakage inductance to the primary inductance (measured with the same stimulus) gives a small fraction, call it T. Then your LTspice K value is set to (1-T).

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Additionally, I compared this "super v/i" stage with the simple direct method, like each AK differential output goes to each Lundahl winding at the primaries.
In my experience with DACs which have opamp output stages the load impedance in the bass has a significant impact on the bass dynamics. Since trafos present a low impedance at LF I'd put a buffer between DAC and transformer.
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Old 5th November 2017, 11:28 AM   #19
lemon is offline lemon  Greece
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Thanks for the info.
I make some measurements with the all primaries in series like the schema that I followed. The inductance measurement was from primaries in series e.g 10-16 with 9+15 short.
From what I saw it is very difficult to make a precisely measurement.
To short all secondaries right must be soldered, I had some deviation on my measurements with no secondary soldered. The best that I had was Primary Ind. = 30.18 and Leak.Ind=29.03 with my Der-ee at 1KHz and Ls inductance measurement.
It is 0.9619 ratio, with that the FR on LTSpice has a decrease from 1KHz, that it isn't true.

Only with K=0.999 the FR is almost like real FR measurement.
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Old 5th November 2017, 12:20 PM   #20
pieter t is offline pieter t  Netherlands
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Why do you take the secondaries (9-10 and 15-16) as primaries, and the primaries as secondaries?
It makes a difference, and Lundahl is clear about this!
When you use the transformer 1:1 (but for some reason you have not been able to confirm that until now) the only thing to do is to short the two wires of your combined secondary for measuring leakage inductance, no need to "short all secondaries" as you should have created just one secondary....
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