Intact Audio

The comments in the tube models thread reminded me to pass this one along.

The same that I did for the tubes can be done for transformers. Here's a couple files to get you started. Put the XFMR.TXT in the "sub" folder, like the triode_nh.txt and tetrode.txt. The XFMRASSY.TXT needs you to do a little work, since the forum doesn't recognize .ASY files. When you download it, rename XFMRASSY.TXT to XFMR.ASY and place it in the "sym" folder for LTSpice. Presto. You get a drop down menu of:

XFMR (a more or less "ideal" 1:1 transformer)
OutXFMR_5k:8 (100hy, 200 ohm) 5kohm to 8 ohm output transformer
OutXFMR_10k:8
OutXFMR_2k:8
Intrstg_1:1 (a 1:1 interstage transformer 150Hy, 400 ohm DC resistance)
Intrstg_2:1
Intrstg_2.5:1
Intrstg_3:1

Add your own by modifying the XFMR.TXT file

Enjoy,
Steve

Steve, thanks for all of these great contributions! Really, I can't say enough. I know that your help has made spice practical for many people. And fun! What's the point if it's not fun?

sbench wrote:... since the forum doesn't recognize .ASY files.

All ya gotta do is ask. You should be able to upload .ASY files now.

-- Dave

Hey Steve,
Great work! Just miss a PP-tranny: Couldn´t you please add a PP-OPT of lets say 2k:8ohm.

i'd like to (crudely) sim a circuit with a 1k5:8 push-pull OPT. the secondary is center tapped and has a DCR of 0.26 ohm; LF -3db point is 4hz according to brian sowter who wound the transformer.

what are "appropriate" values for the primary inductance? and is it correct that i have to divide the total primary inductance by 4 to get the inductance *per leg*?

thx for help!! ©

Hi Christian,
What's the resistance of the primary? To a very rough approximation the ballpark inductance can be approximated by the formula R/(2*pi*f). You *could* assume 1.5k/25 = 60Hy, but I would bet that it is lower than that. Sometimes the LF response assumes a low source resistance. If the primary resistance were 100 ohms, and zero ohm soruce, the 4Hz could be achieved with only 4Hy.

Warmest Regards,
Stephie <3

hi stephie

primary resistance is ~ 25 ohms from CT to end.

let's assume L pri = 48 H (because it's nice to divide by 4...). do i use L = 12 H per winding in spice? and what value do i use for each of the two secondary windings that together form an 8 ohm winding -- L= 0.25 H each? (the CT of these two windings is grounded).

thanks for help!!

best, ©

Hi Again. Essentially correct!

Your two primary inductors in series would be 12Hy with 25 ohms each, giving a coupled inductance of 48Hy.

The impedance ratio=the inductance ratio, so for a 1.5k impedance primary and an 8 ohm secondary, the total secondary inductance is .256Hy with .26 ohms. Since you said it was center tapped, that would be .064Hy (64mHy) and .13 ohms for each of the two inductors.

Don't forget to include the coupling coeffieient to all 4 inductors. In LTSpice that would be, for example, the Spice directive for a coupling coefficient of perhaps .99995

Your "completed" transformer would be
L1 = 12Hy + 25 ohms
L2 = 12Hy + 25 ohms
L3 = 64mHy + .13 ohms
L4 = 64mHy + .13 ohms
K1 L1 L2 L3 L4 .99995

The last is the spice directive for coupling.


Enjoy,
Stephie

P.S. You can trell you got it right if....

Connect the primary to a voltage source whose internal resistance is what youassume to be your primary impedance level (1.5k) and the secondary to your 8 ohm load. Run an AC simulation. Read the voltage on the PRIMARY. Should be -6dB.

-Stephie

got it right, thanks a lot!

what about stray (parallel) capacitance of the windings? i suppose i can't measure them directly... what are "typical" values for "normal" push-pull OPTs?

best, ©

For the capacitance, a LOT depends on how it's wound. For instance, bi-filar winding increases the coil to coil capacitance, but also provides better coupling than isolating the windings which decrease the capacitance but also the coupling.

Ballpark values? Perhaps 100pF pri to pri, 50pF sec to sec, and perhaps 25pF primary to secondary.

Usually adding a "lumped" capacitor is only a rough approximation... it's better to break the windings into smaller chunks from a sim view, but that gets tedious.

Good luck with the simulations,

Stephie <3

the secondary is center tapped and has a DCR of 0.26 ohm; LF -3db point is 4hz according to brian sowter who wound the transformer.

I found it very handy to make some models
Here are the models i use

or this one you can use it with excel

Hi nl3prc

Thanks for the exel spreadsheet. I added the files as a drop down menu. Works great..!