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spice model for UL output transformer?
spice model for UL output transformer?
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Old 28th November 2005, 07:30 PM   #1
docali is offline docali  Germany
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Default spice model for UL output transformer?


i would like to model an UL output XFMR and tried the following model but it yields an unsymmetric waveform. Do you have any hints how to do this?
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File Type: gif xfmr_ul.gif (7.1 KB, 2589 views)
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Old 28th November 2005, 10:36 PM   #2
Johan Potgieter is offline Johan Potgieter  South Africa
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Hi Docali,

I have never used this, but the model looks over-simplified to me. There should also be resistance and capacitance somewhere, which parameters would certainly make a difference. Especially the latter; contrary to some perceptions the intersection capacitance is often the parameter limiting high frequency response, not the leakage reactance. Leakage reactance between primary sections need also be included; in fact any nearly accurate model would be quite complex i.m.o.

The values would of course differ from unit to unit and C is mostly not given by manufacturers, thus I have chickened out in the past and settled for final response as measured in a real built-up unit.

It would thus interest me as well and I hope someone with experience would broaden our horizons here.
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Old 29th November 2005, 01:25 AM   #3
Geek is offline Geek
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Here's a 3f4 spice model for a CT transformer from Circuit Maker:

*XTRANSCT:Transformer Subcircuit Parameters
*XTRANSCT:RATIO:|Turns ratio= Secondary/Primary [1m,]|1
*XTRANSCT:RP:|Primary DC resistance [0,]|0.1
*XTRANSCT:RS:|Secondary DC resistance[0,]|0.1
*XTRANSCT:LEAK:|Leakage inductance[0,]|1u
*XTRANSCT:MAG:|Magnetizing inductance[0,]|1u
*{RATIO=1 RP=0.1 RS=0.1 LEAK=1u MAG=1u}
*Generic ct secondary type:transformer
RPRI  1 7 {RP}
VSEC1 9 4 DC 0V
FSEC1 6 2 VSEC1 {(RATIO/2)}
ESEC1 8 9 10 2 {(RATIO/2)}
RSEC1  8 3 {(RS/2)}
VSEC2 12 5 DC 0V
FSEC2 6 2 VSEC2 {(RATIO/2)}
ESEC2 11 12 10 2 {(RATIO/2)}
RSEC2 11 4 {(RS/2)}


*alias:XTRANSCT {RATIO=.5}

*alias:XTRANSCT {RATIO=.2}

*alias:XTRANSCT {RATIO=.1}

*alias:XTRANSCT {RATIO=.05}

*alias:XTRANSCT {RATIO=.04}

*alias:XTRANSCT {RATIO=.03448}
I've found this model works great with resistances specified, rather than ratios. In your netlist, just edit as follows (for 5K primary, 40% UL example):

Secondary: 0.08 (for 8 ohms)
Primary plate to CT: 4
CT to V+: 1

Adjust other params as per transformer specs.
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Old 29th November 2005, 07:02 AM   #4
docali is offline docali  Germany
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Hi, thanks so far.

I use LTSpice for simulations and it unfortunately does not work with 3f4 models. LTSpice only has a model for inductances but this model is good in my opinion, because it has model parameters for series resitance and capacitance.

To model an UL transformer it should be possible to use two inductances in series with mutual inductance. The spice directive for this is for example:
K1 L1 L2 0.999

But this did not work well in my simulations.

Best regards from germany!
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Old 29th November 2005, 09:19 AM   #5
billr is offline billr  New Zealand
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Have a look at this, it might help you, to get the UL windings, splt the primary using additional inductors, using the ratios that you need, link them, using mutual inductance, I have tried the non ul models Duncan provides and they seem to be ok.

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Old 6th January 2006, 02:43 PM   #6
jackinnj is offline jackinnj  United States
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spice model for UL output transformer?
You can use a current-controlled current-source (or voltage controlled current sourc) and add the parasitic capacitance between primary and secondary add the leakage inductances and resistances on both sides of the device. Only problem with this approach is that it works down to DC.

There are some good discussions on the theoretics of modeling non-linear RF transformers, transformers for switching supplies on the web, you just have to look.
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Old 7th January 2006, 04:39 PM   #7
Robert McLean is offline Robert McLean  Canada
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Default transformer spice models

Here are just two examples of how I do output transformers. They should work with any Spice variant as far as I know, I personally use LTSpice. I do have a vague recollection however that some spices wont allow more than 2 inductances on a K statement, so you have to put a seperate line for each pair of inductances.

Complex example, ie PP, UL, and multiple output taps.

.SUBCKT 1650R P1 Sg1 B Sg2 P2 O16 O8 O4 Com
* Push Pull transformer, with Ultralinear taps at 40%
* 5000 to 16 ohms, with 8 ohm and 4 ohm taps, 3db 15 to 60Khz
* Hammond 1650R
LP1 1 2 2.409088925H ; PRIMARY
LS1 2 B 1.070706189H ; primary, scrren grid tap portion
LS2 B 3 1.070706189H
LP2 3 4 2.409088925H
LA1 5 6 0.007348166H ; SPEAKER SECONDARY
LA2 6 7 0.003674083H
LA3 7 Com 0.021414124H
KALL LP1 LS1 LS2 LP2 LA1 LA2 LA3 0.999199994;
RP1 P1 1 30.0
RP2 Sg1 2 15.0
RP3 Sg2 3 15.0
RP4 P2 4 30.0
RS1 O16 5 0.1
RS2 O8 6 0.1
RS3 O4 7 0.1
.ENDS 1650R

P1 and P2 are the plate connections, B is the B+ connection, Sg1 and Sg2 screen grid connections, and O16, O8 and O4 the 16, 8 and 4 ohm outputs, and Com the common secondary terminal

Simplest example, ie SE, single output.

.SUBCKT 5KSE P1 P2 Sp1 Sp2
* Single ended audio transformer
* 5k to 8 ohm, 10 to 40KHz
LP1 1 P2 40.26021568H ; PRIMARY
LSA 2 Sp2 0.064416345H ; SPEAKER SECONDARY
KALL LP1 LSA 0.999499875 ;
RP1 P1 1 56
RS Sp1 2 .1

P1 and P2 are the primary ie plate connections, and Sp1 and Sp2 the secondary ie speaker connections.

When loaded by the specified input and output resistances they give the specified frequency response. They include winding resistance so you get the expected voltage drops. These are purely linear models and so do not give any distortion. So they are not realistic in that regard, but the fact that the taps "work right", ie the ultralinear acts as it should, and you can put 4 ohms on the 4 ohm tap or 8 ohms on the 8 ohm tap and get the right results make them very useful in my opinion. The frequency response may not have all the little quirks of the real transformer, but it is reasonable, not DC to infinity or anything like that.

I use the attached spreadsheet to derive the model parameters. The values given for the various transformers listed are taken from various data sheets and websites and so on. Many of the dc resistances are just pure guesses on my part. I make no claim that the models will match the real transformer, only that the model will have the frequencey response and impedance ratio that the spec sheet claims for the real transformer.

The spreadsheet is just something I whipped up for myself, so it is not particularly user friendly perhaps, but it should be fairly easy to use. Just input nominal source impedance, output impedance, winding resistances and upper and lower 3dB frequencies, and then in the rightmost columns you will get KA, LP and LS for SE types, or KA, LP1, LP2, and LS2 for PP types. Paste those values into the spice model. If you want multiple output taps then take the value for LS and paste it into cell A7 on sheet 2, set the tap impedances as required ( default is 4, 8, and 16) and then see the values for LA1, LA2 and LA3. Paste these into the spice model.
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File Type: zip transformer model parameters.zip (16.8 KB, 1180 views)
Robert McLean
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Old 8th August 2007, 02:18 PM   #8
StoneT is offline StoneT  United Kingdom
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Default Good work...

Looks like alot of thought went into that, nice one.

On a related note:
I recently bought a bargain pair of hammond 1615 output transformers (which match your table reasonably well) but when I put the measured specs (LP, LS, DCRs...) into my LTpice simulation it didn't perform as well as expected.
Normally these have a Zp-p of 5000 but I am using them with 8ohms on the 4ohm taps to get a Zp-p of 10000.
I thought that this was the problem for a while and was quite disheartened as my design didn't seem able to produce a 50Hz square wave of more than about 2W without entering class B with the 1615. (It is supposed to be a 7 - 8W pure class A amp).

However I now realise that it was the NFB that was the problem. As the output level falls on the tops of the square wave the amp tries to miantain the level through the transformer resulting in the drive signal to the output tubes increasing. As my NFB was quite substantial it tried to maintain the output exactly by a huge increase in drive signal resulting in one valve shutting off.

Seems obvious now but it vexed me for quite some time. Just thought the info might be usefull for anyone else in the same situation.

Incidentally- The differences between my measurements and your specs on the spreadsheet are mainly the primary DCR (You show 30 Ohm on each primary, I measured about 84 and 90), and the Lprimary (yours is about 6 in total, I measured about 2.4, which might account for the perfomance of your model being better at low frequencies than the hammond specs)
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Old 8th August 2007, 03:33 PM   #9
jackinnj is offline jackinnj  United States
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spice model for UL output transformer?
A really good PDF on transformer modeling:


Here's a Hammond 1608 with 8R on the secondary and 5k6 on the primary -- don't forget the interwinding capacitance in your model:

Click the image to open in full size.
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Old 17th August 2007, 02:01 PM   #10
StoneT is offline StoneT  United Kingdom
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Hi jackinnj,

What's the vertical measurement / scale of that scope? Looks terrible!

Further to my last post I have investigated further and now realise what exactly is going on.
Hammond specs show +/- 1dB at max power of 30Hz - 30KHz.
I have found this is all about how you test.
Before I bought these OPTs I had my sim running with Lp approx 50H and getting quite nice response through it.
The Lp of the 1615's i bought was worryingly low at 2.71H, but I thought oh well "+/- 1dB at max power of 30Hz - 30KHz" they must be fine. Unfortunately its a bit more complicated than that.
Yes if you feed a 200Vrms sine into the primary you get ~ 5.6Vrms out whether it's 50Hz or 10KHz, what changes is the transformers ability to accurately reflect the load.
At 10KHz the total currrent through the primary is is likely to be 200V / 10Kohm = 20mA. Fine with a class A quiescent current of 50mA. Unfortunately at lower frequencies there is not enough Lp to sustain the primary impedance and this falls drasticly.
at 50Hz that 200Vrms signal draws about 200mA through the primary, putting my amp way into class B if not B2 which it is not designed for.
Another way of looking at it would be if the OPT is fed with a 20mA sine wave and the +/- 1dB points were measured the range would probably extend down to about 500Hz or higher. Rather poorer than the 30Hz specified.
Am I right in thinking this, or am I missing something? The Hammond specs seem to assume the preceeding amp is a pure voltage source with infinate current capability. If that were the case why bother with an OPT?!
Please someone tell me I'm wrong and it will all be fine.
A little while ago I proposed using mains transformers as OPTs but was told by everyone who responded that I'd be better to go with real OPTs. The mains transformers I'm sure would have had FAR higher Lp, and while I accept the overall sound quality may not be as good it would be better than hard clipping any signal below 300Hz!
My options seem to be:
Limit the output to about 1W
Sell the Hammonds and buy some mains toroids
Redesign the entire amp + power supply as a class AB.
Sell ALL the stuff I accumulated for this amp and move on to my next project. An OTL!
To be honest I'll probably end up building this with the Hammonds to see if they do sound that bad.
Any advice? Are Hammonds just a big pile of $#!+? Should I just take up knitting?
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