Transformer voltage droop

[Preamp]

Seems like others have already bent their heads around this topic: Anyone have SPICE models for PSU Toroidal Xfmr ??.
Actually I find this to be a bit too elaborate, because it involves measuring on actual transformers. I was looking for a quicker way, not having to order and measure each possible transformer. I will run the same simulation with the model found in the linked thread and make a comparison, though, to see how big the difference is.


Charlie,
now everything works fine! Somehow the tabs for selecting the different sheets got lost and I didn't even consider there might be some more, so I didn't notice it in the first place...

[Preamp]

Here are the comparison charts. I used the simulation model from Tom Gootee with the measured data of my trafo.
The first chart shows both models with a simple resistive load on the ac, where the blue line resembles Tom's model and the red line my simple approach.
On the other charts the models were loaded with a full bridge rectifier, 4700uF (ESR=63m) and the stepped resistor. Same colour coding here.

Unfortunately I don't have a transformer with lower VA and equal voltage rating at hand. This would have made the comparison much more descriptive.

Please note that these comparison charts took more than half an hour to simulate, while the simple model is only a matter of seconds .

[Preamp]

Made a more comprehensive simulation. This time I used the complete P3A circuit with a 2 Ohm resistive load and an actual wave file as input source. The power supply consists of a single rectifier bridge for both rails, 10.000uf of capacitance for each rail, and two modeled transformers.

Simulation time was almost four hours versus a quarter; the generated raw files grew to a full 36GB and 1GB, respectively.

#1: Elaborate Model, 5 seconds of music into 2 Ohms, positive voltage rail shown
#2: Simple Model, 5 seconds of music into 2 Ohms, positive voltage rail shown
#3: Elaborate Model, section of 0.5 seconds for better resolution
#4: Simple Model, section of 0.5 seconds for better resolution
#5: Overlay of both models for direct comparison, Simple one shown in red
#6: amplifier output voltage

[PlasticIsGood]

It could be useful if flux density could be included, for which the number of turns and core area would be required. To my mind, it is important for an audio ps transformer to remain in the linear region of the BH curve, to minimise the generation of harmonics. For this reason, a transformer should be operated at its rated power. In practice, there isn't much choice: buy what a reputable manufacturer has made for your power requirement and purpose. Don't buy a higher-powered transformer just for the sake of lower winding resistance.

For the ohm's law view of the consequences of your choice, psud is good:

PSUD2

from a truly great pioneer of internet DIY audio.

[Preamp]

Originally Posted by PlasticIsGood

For this reason, a transformer should be operated at its rated power.

The datasheets I have looked at state that the transformer ratings are calculated for a temperature rise of 50 degree centigrade, which is not desirable IMO. Moreover the load of a typical class AB amp playing typical music is everything else but constant and you don't always run your amp on full power output, so the transformer won't be operating at its rated current most of the time.

[Tony]

Originally Posted by PlasticIsGood

It could be useful if flux density could be included, for which the number of turns and core area would be required. To my mind, it is important for an audio ps transformer to remain in the linear region of the BH curve, to minimise the generation of harmonics. For this reason, a transformer should be operated at its rated power. In practice, there isn't much choice: buy what a reputable manufacturer has made for your power requirement and purpose. Don't buy a higher-powered transformer just for the sake of lower winding resistance.

For the ohm's law view of the consequences of your choice, psud is good:

PSUD2

from a truly great pioneer of internet DIY audio.

+1.... "Don't buy a higher-powered transformer just for the sake of lower winding resistance." commercial transformers operate at higher flux densities so that less copper is used...if you go diy route and build your own transformers you are not constrained by this consideration, i do my traffos at less than 1T most of the time, some are in the 0.6T levels....

[PlasticIsGood]

Originally Posted by Tony

+1.... "Don't buy a higher-powered transformer just for the sake of lower winding resistance." commercial transformers operate at higher flux densities so that less copper is used...if you go diy route and build your own transformers you are not constrained by this consideration, i do my traffos at less than 1T most of the time, some are in the 0.6T levels....

Absolutely the way to go if you can do it and have the time. Most of my experience is with valves, for which it's normal to use transformers made with audio in mind, and plenty are available. For low-voltage audio, choice and availability seems more limited, perhaps because commodity-grade items are so common and cheap. Since I have only just stumbled into the 1980s, perhaps I haven't found the good stuff yet.

This wasn't the best place to raise my gripe, perhaps. It appears common in low-voltage DIY audio to use bigger transformers than necessary. Presumably this is in an effort to reduce sag during loud bits. Although this may be true, if it is done just by over-specifying a commodity-grade transformer, the chances are it will be noisy exactly when it needs to be quietest...during the quiet bits.

Perhaps everyone uses shunt regulation?

[PlasticIsGood]

Originally Posted by Preamp

The datasheets I have looked at state that the transformer ratings are calculated for a temperature rise of 50 degree centigrade, which is not desirable IMO. Moreover the load of a typical class AB amp playing typical music is everything else but constant and you don't always run your amp on full power output, so the transformer won't be operating at its rated current most of the time.

I did go on to say that it should be rated for the intended purpose. Power is by nature an average, and transformers are slow to heat, so the average can be over a long time. Hence a class AB amp won't need as big a transformer as class A. Choosing the rating is quite problematic though. If I used a commodity transformer I would consider shunt regulation, and a chimney.

Properly-specified commodity-grade transformers may well run hot, but over-specifying may result in a significant increase in electrical and mechanical noise. For most transformers, regulation via winding resistance is a necessary evil, because rated current drops the voltage across the magnetising inductance sufficiently to operate the core in its linear region.

A transformer specifically wound for audio should minimise this compromise, and several others.

I didn't mean to devalue your efforts, but rather suggest a way to make them more useful. For simulation, I made a model with a flux density output, which turned out to be quite an eye-opener.

[Preamp]

Originally Posted by PlasticIsGood

Choosing the rating is quite problematic though. If I used a commodity transformer I would consider shunt regulation, and a chimney.

I'd like to keep the trafo as cool as possible instead...

Originally Posted by PlasticIsGood

Properly-specified commodity-grade transformers may well run hot, but over-specifying may result in a significant increase in electrical and mechanical noise.

Could you specify this a bit more for me, please? I'm not that deep into transformers yet, but I guess there's no way around .
Mechanical noise should be no concern IMHO, since I haven't seen any toroidal transformer producing any, regardless of the load conditions. Don't know anything about the electrical noise you mentioned, though.

Originally Posted by PlasticIsGood

I didn't mean to devalue your efforts, but rather suggest a way to make them more useful. For simulation, I made a model with a flux density output, which turned out to be quite an eye-opener.

Since I'm not going to wind my own transformers, there seems to be no point in further doing any simulations, I guess?

[PlasticIsGood]

Originally Posted by Preamp

I'd like to keep the trafo as cool as possible instead...



Could you specify this a bit more for me, please? I'm not that deep into transformers yet, but I guess there's no way around .
Mechanical noise should be no concern IMHO, since I haven't seen any toroidal transformer producing any, regardless of the load conditions. Don't know anything about the electrical noise you mentioned, though.



Since I'm not going to wind my own transformers, there seems to be no point in further doing any simulations, I guess?

Not on my account, please. I was rather hoping you'd try including flux density. The maths is easy, up to a point, which is where I got stuck. I can model on the basis of constant inductance, and simulate the core flux. That allows me to know when the transformer would be distorting, but not to quantify the distortion. For that I need to calculate the change in inductance and feed the result back into the model. The problem is finding a way to model the BH curve of iron in SPICE.

It would be daft to value what you have by what you haven't.

The most useful thing, perhaps, would be to compare two transformers, in reality and by calculation. You seem close to being able to do that, with the same music

It may well be that, up to a point, the effects you have shown will dominate the picture.

When I use music in my spice, I get noise and distortion from a straight piece of wire