Good morning everyone!
I would like to know if by chance someone has measured the impedance vs frequency of the secondaries of a power transformer
I read a statement that left me perplexed
And that is that the power transformers should all be with separate coils while the output ones of a tube amplifier toroidal with coaxial windings
In the sense that the former should have a narrow impedance curve while the latter have a wide one
In short, I'm a bit confused
To be honest i have just purchased a Dayton Audio Dats v3 impedance meter that could be useful
In short i would like to replace mains transformers with better ones But i have no clue about the best option
That should be one passing only 50Hz ? 😊
Maybe there is an industrial grade one out there that could be fantastic for an amplifier
Who knows ?
I would like to know if by chance someone has measured the impedance vs frequency of the secondaries of a power transformer
I read a statement that left me perplexed
And that is that the power transformers should all be with separate coils while the output ones of a tube amplifier toroidal with coaxial windings
In the sense that the former should have a narrow impedance curve while the latter have a wide one
In short, I'm a bit confused
To be honest i have just purchased a Dayton Audio Dats v3 impedance meter that could be useful
In short i would like to replace mains transformers with better ones But i have no clue about the best option
That should be one passing only 50Hz ? 😊
Maybe there is an industrial grade one out there that could be fantastic for an amplifier
Who knows ?
It's not unusual for power transformers to be wound on compartmentalised bobbins. Before you use one for audio, measure to assess it's impedance and identify self-resonance.
Hi thank you sincerely for your kind and valuable advice
I guess asap i will be able to get the impedance curve of the secondaries and see for myself
I like the split bobbins type I do not know why
Anyway ... how could i measure the self-resonance ? to be honest i have not clue about it What is it ?
I guess asap i will be able to get the impedance curve of the secondaries and see for myself
I like the split bobbins type I do not know why
Anyway ... how could i measure the self-resonance ? to be honest i have not clue about it What is it ?
Hi it is me again with a impedance sweep of a mains transformer secondary winding measured with the Dayton Audio DATS V3
can this be correct ?
what really shocks me is how different is the DC resistance (2.5 ohm) vs the impedance measured at 50Hz (around 20 ohm)
am I doing something stupid?
from what I understand the lower the output impedance at 50Hz the better the transformer
am I wrong?
And this measured on the primary
can this be correct ?
what really shocks me is how different is the DC resistance (2.5 ohm) vs the impedance measured at 50Hz (around 20 ohm)
am I doing something stupid?
from what I understand the lower the output impedance at 50Hz the better the transformer
am I wrong?
And this measured on the primary
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Hi ! No I have connected the Dats leads to one secondary and in the second graph to the primary and run an impedance sweep
Measuring a transformer impedance on itself is not always useful. A transformer 'transforms' the load at the secondary to the primary.
If you measure at the primary you measure the load impedance at the secondary transformed up or down with the square of the winding ratio.
Since the transformer is not ideal there is what is called 'leakage inductance' which determines the transformer bandwidth, among other things.
But generally you wouldn't measure a transormer without any load.
Jan
If you measure at the primary you measure the load impedance at the secondary transformed up or down with the square of the winding ratio.
Since the transformer is not ideal there is what is called 'leakage inductance' which determines the transformer bandwidth, among other things.
But generally you wouldn't measure a transormer without any load.
Jan
Hi thank you sincerely All for the very valuable input
My point is the following
How do you select a mains transformer to be used in a power amp?
Keeping same voltages and VA are they all fine the same?
My understanding is that high impedance is not a good thing in a mains transformer
Maybe I am wrong
I am here to learn
It would be nice to have a set of measurements that can tell how good a transformer is for powering an audio amp
For instance I am quite surprise by the variation of impedance from DC to 50Hz
Usually the datasheets provide the DC resistance of the secondaries only
My point is the following
How do you select a mains transformer to be used in a power amp?
Keeping same voltages and VA are they all fine the same?
My understanding is that high impedance is not a good thing in a mains transformer
Maybe I am wrong
I am here to learn
It would be nice to have a set of measurements that can tell how good a transformer is for powering an audio amp
For instance I am quite surprise by the variation of impedance from DC to 50Hz
Usually the datasheets provide the DC resistance of the secondaries only
The transformer doesn't know there is an audio amplifier down the line.
The amplifier is designed to work best on a supply voltage of x volt and max y amps.
So you need a power transformer that can provide the x V at max y amps before it starts to drop.
So calculate the VA and add a factor of 2 for safety, done.
Your measurements of the unloaded transformer have pretty much no relationship to how it will work powering an amp.
Jan
maybe i have been reading too much about to keep low impedance path for current in power supplies
I understand that power supply caps will provide the needed energy during music peaks
but they must be recharged fast before the next peak
And a low impedance circuit can do that i.e. recharging power supply caps fast
I would place a voltmeter right on the output devices pins to check for any voltage fluctuation
or at least on power supply caps
I understand that power supply caps will provide the needed energy during music peaks
but they must be recharged fast before the next peak
And a low impedance circuit can do that i.e. recharging power supply caps fast
I would place a voltmeter right on the output devices pins to check for any voltage fluctuation
or at least on power supply caps
I can tell you exactly what the variation on the power supply caps will be.
They will be charged every 10 ms (for 50Hz mains) to almost the top value of the secondary.
They will discharged by the current drawn by the amp until the next charge-up.
The formula is that the voltage drops 1V per sec with 1A current draw per Farad capacitance: C.V = I.T (actually C*deltaV = I* deltaT).
Example: You have 10.000 uF (0.01F) charged up to 50V. Your amp draws 5A during 0.01sec (a transient). After the 0.01sec the voltage on the C has dropped by: deltaV = I*T/C = 5*10^-2/10^-2 = 5V.
Recharging with a good power transformer will probably take about 20% of the 10ms is 2ms. So on that cap you will see a sawtooth going up a bit less than 5V in 2ms and then dropping back a bit less than 5V in the next 8msec.
Jan
They will be charged every 10 ms (for 50Hz mains) to almost the top value of the secondary.
They will discharged by the current drawn by the amp until the next charge-up.
The formula is that the voltage drops 1V per sec with 1A current draw per Farad capacitance: C.V = I.T (actually C*deltaV = I* deltaT).
Example: You have 10.000 uF (0.01F) charged up to 50V. Your amp draws 5A during 0.01sec (a transient). After the 0.01sec the voltage on the C has dropped by: deltaV = I*T/C = 5*10^-2/10^-2 = 5V.
Recharging with a good power transformer will probably take about 20% of the 10ms is 2ms. So on that cap you will see a sawtooth going up a bit less than 5V in 2ms and then dropping back a bit less than 5V in the next 8msec.
Jan
Here is a small, indicative compendium of transformers parameters according to the construction type:
You simply need to divide the normalized parameter by the VA rating to obtain the approximate value. The secondary values can be obtained by multiplying the result by (n2/n1)²
You simply need to divide the normalized parameter by the VA rating to obtain the approximate value. The secondary values can be obtained by multiplying the result by (n2/n1)²
A transformer has an inductance (plus wire resistance) and that inductance needs to be high enough to limit the idle current (unloaded) to a small part of the rated load current. LV's compendium is interesting and asserts that the larger the transformer, the larger the appropriate idle current. An ideal transformer has infinite inductance and zero idle current.
The benefit of separate bobbins per winding is manufacturing flexibility, the ability to use the same primary bobbin for different transformers. As we see from LV's compendium, this results in higher leakage inductance. However, leakage inductance is not as much an issue for a 50/60Hz transformer as it is for a broad-band ~audio transformer. It may be an advantage in limiting inrush current, which is a problem with toroidals. The inrush problem is a result of pushing the rated operating voltage to the point where a single half cycle without the benefit of preceding opposite half cycle, can saturate the core.
Assigning an "impedance" to a transformer winding is a judgment call, not an electrical parameter. A transformer is useful where the inductive reactance is the impedance at the lowest operating frequency, and many times the parasitic DC resistance. So, a transformer rated down to 40Hz can be used down to 20Hz at half the rated impedance if you can tolerate twice the losses. A winding with a DC resistance "R" is not useful below a frequency where the inducive reactance is less than "10R".
Large transformers are twice better because they have less wire resistance and more inductance.
The benefit of separate bobbins per winding is manufacturing flexibility, the ability to use the same primary bobbin for different transformers. As we see from LV's compendium, this results in higher leakage inductance. However, leakage inductance is not as much an issue for a 50/60Hz transformer as it is for a broad-band ~audio transformer. It may be an advantage in limiting inrush current, which is a problem with toroidals. The inrush problem is a result of pushing the rated operating voltage to the point where a single half cycle without the benefit of preceding opposite half cycle, can saturate the core.
Assigning an "impedance" to a transformer winding is a judgment call, not an electrical parameter. A transformer is useful where the inductive reactance is the impedance at the lowest operating frequency, and many times the parasitic DC resistance. So, a transformer rated down to 40Hz can be used down to 20Hz at half the rated impedance if you can tolerate twice the losses. A winding with a DC resistance "R" is not useful below a frequency where the inducive reactance is less than "10R".
Large transformers are twice better because they have less wire resistance and more inductance.
In the compendium above, there are some unit inconsistencies:
The first (Lm) is the formula to compute the magnetizing inductance based on the VA rating, the others are the constants to use in a formula; sorry for the confusion, but I think that the way to use them is pretty obvious
The first (Lm) is the formula to compute the magnetizing inductance based on the VA rating, the others are the constants to use in a formula; sorry for the confusion, but I think that the way to use them is pretty obvious
Thank you very much for your kind explanation This is difficult for me to understand
I understand graphs numbers but equations are too difficult
I guess that you mean that it is all in the transformer selection ?
I have a question
How far a mains transformer can be from the diodes bridge in a class AB amp ?
or how long can be an AC umbilical connecting transformer and bridge without creating issues ?
this is an old curiosity of mine as I like transformers outside the amp chassis
i would like to extract the transformer from an amp and made a connection with power speakon plugs