understanding transformer distortion

[12afael]

I`m trying to understand distortion parameters on transformers. A transformer manufacturer gave me this specification.

With a source impedance of 600 Ohms driving it and loaded with 2.43K, it does +21dBu at 20Hz.  At 50 Ohms source impedance, it does +16dBu. 

when he say "it does xx dBu at 20Hz" how I should interpret that? there is a standard THD for that measure? 1%? or it mean that before that dBu it will burn???  ???

other thing that I don't understand is why it is worst at a lower impedance at audio transformer chapter by Bill Whitlock http://jensen-transformers.com/wp-content/uploads/2014/09/Audio-Transformers-Chapter.pdf he states that lower source impedance result in a lower distortion.

thanks in advance

Rafael

 

[JohnRoberts]

Transformer distortion generally rises with increasing level and falling frequency. So a specification at a certain level at 20Hz tells you that the distortion will be lower for all higher frequencies and lower levels.

The distortion vs frequency is related the the magnetic flux capability which generally tracks with size/weight (bigger are usually lower distortion all else equal).

Driving transformers from a low source impedance is a different mechanism and related to the ability of the source voltage to ignore non-linearities related to the magnetic media.

JR

 

[mjrippe]

Yes, most often the max rated output is quoted at 1% THD.

 

[abbey road d enfer]

I`m trying to understand distortion parameters on transformers. A transformer manufacturer gave me this specification.

With a source impedance of 600 Ohms driving it and loaded with 2.43K, it does +21dBu at 20Hz.  At 50 Ohms source impedance, it does +16dBu. 

when he say "it does xx dBu at 20Hz" how I should interpret that? there is a standard THD for that measure? 1%? or it mean that before that dBu it will burn???  ???

other thing that I don't understand is why it is worst at a lower impedance at audio transformer chapter by Bill Whitlock http://jensen-transformers.com/wp-content/uploads/2014/09/Audio-Transformers-Chapter.pdf he states that lower source impedance result in a lower distortion.

thanks in advance

Rafael

Most of the distortion in a transformer comes from partial saturation of the core material. The higher the magnetic flux, the higher the distortion. For a given voltage, the necessary flux is inversely dependant on frequency, so lower frfequencies mean more flux, hence more saturation.
In fact, a transformer rated at say +20dBu at 50Hz is quite capable of +40dBu at 1kHz, but only +10 at 20Hz.
Saturation of the core tends to reduce the actual impedance of the windings, which creates a voltage divider between the source + internal resistance.
The higher this series resistance, the higher the voltage loss; since this is a non-linear process, this voltage loss comes out as distortion.

 

[PRR]

> how I should interpret that?

 

[ricardo]

With a source impedance of 600 Ohms driving it and loaded with 2.43K, it does +21dBu at 20Hz.  At 50 Ohms source impedance, it does +16dBu.
...
other thing that I don't understand is why it is worst at a lower impedance at audio transformer chapter by Bill Whitlock http://jensen-transformers.com/wp-content/uploads/2014/09/Audio-Transformers-Chapter.pdf he states that lower source impedance result in a lower distortion.

Which manufacturer is that?

Whitlock is correct.

At 50R source, you should be able to go 600/50 =12x (another 20.6dB) higher for the same THD & frequency.  But a caveat is the Equivalent Series resistance of the windings need to be taken into account.  A good 600R transformer will have about 60R as seen by that winding.

3% THD is another common THD point for rating transformers.

The relationship is 6dB/8ve for resistance, level, frequency & THD.

Assuming the 600R 21dBu 20Hz is 3% THD, you can expect

• 300R 21dBu 20Hz 1.5%
• 600R 15dBu 20Hz 1.5%
• 600R 15dBu 10Hz 3%
• 300R 27dBu 10Hz 3% etc

When THD gets down to about 0.1%, (and above about 5%) other factors will be dominant and this breaks down.

BTW, this is an accurate and reliable measure of the LF "goodness" of a transformer.  Unlike measuring 'inductance' which will vary with level and frequency.

I'd recommend using 3% THD as this, mostly 3rd, is the amount first clearly seen on a scope.  You can do this without $$$ gear.

 

[PRR]

> there is a standard THD for that measure?

No. You or they could ass-ume various numbers. But if they don't say, then ass-ume the worst. 10%? 30%?

> it mean that before that dBu it will burn???  ???

Probably not. If it does, get pictures!!

The different ratings for different impedance "could" make sense if these are different winding connections. However 600+50 is a very odd combination. 600+150 is very common, the 150 has half the turns and will take half the voltage. dBu is a voltage, 0.775V. But 600:150 should be a 6dB difference and you are quoted a 5dB difference.

Ask for a technical person.

 

[CJ]

here is a pic showing different hookups and what they do to max level,

500 ohms is used as the example,

600 is more common nowadays,
but today we are rolling with an old school UTC chart,

1000  gauss and 16 dBm  are just for a starting ref point,

turns are from a UTC A-20, 600 to 600 xfmr,

 

[CJ]

distortion in transformers will depend on many things, including what the transformer is hooked up to, level, core size and material, turns ratio, wire resistance,

since the B-H curve is non-linear, flux vs input current is non-linear,
and this B-H curve changes with level,

here is something to help you fall asleep>

 

[CJ]

let's back up a few chapters>

 

[CJ]

this guy sounds like a genius,

 

[CJ]

now you are enlightened individuals, be sure to show others the way,  ???

 

[gyraf]

Very, very good info. Thanks.

 

[Samuel Groner]

distortion in transformers will depend on may things, including what the transformer is hooked up to, level, core size and material, turns ratio, wire resistance,

since the B-H curve is non-linear, flux vs input current is non-linear,
and this B-H curve changes with level,

here is something to help you fall asleep>

Thanks--where's that from?

Samuel

 

[CJ]

Transformers for Electronic Circuits - Second Edition,
Nathan Grossner

yes i need a scanner,

you can skip the math by winding a coil and then listening  to it,  ;D

 

[ruffrecords]

I have always found magnetics rather arcane with its terminology that I find hard to visualise. Ohms law is no problem. I can imagine current (electrons) flowing through a resistor and how much gets through depends on the constriction of the resistance and the force you push it with (voltage). What are the magnetic equivalents?

Cheers

Ian

 

[emrr]

600:150 should be a 6dB difference and you are quoted a 5dB difference.

Don't forget insertion losses.  I rarely ever measure 6dB actual for a 150:600 transformation, usually 4.5-5dB range. 

 

[abbey road d enfer]

I have always found magnetics rather arcane with its terminology that I find hard to visualise. Ohms law is no problem. I can imagine current (electrons) flowing through a resistor and how much gets through depends on the constriction of the resistance and the force you push it with (voltage). What are the magnetic equivalents?

Cheers

Ian

Reluctance is the magnetic equivalent of resistance. Flux is the equivalent of current; the word for the equivalent of voltage is "magnetomotive force".
The equivalent of Ohm's law is Hopkinson's.
Things get more complicated when ferromagnetic materials are involved.

 

[emrr]

Tangent: Considerations regarding DC resistance, AC impedance, and level.  I noticed an audio transmitter modulation transformer with the following specs, which are quite far from what we expect for small signal line level audio.

5500 : 2800 ohms
68 : 23.5 DCR

With an unknown small (er than a brick) transformer, we'd assume it to be 600 : 250 roughly. 

 

[JohnRoberts]

600:150 should be a 6dB difference and you are quoted a 5dB difference.

Don't forget insertion losses.  I rarely ever measure 6dB actual for a 150:600 transformation, usually 4.5-5dB range.

Just to be pedantic because I don't have anything useful to contribute, it has been argued that transformer ratios should not be specified as dB because decibels are a measure of power levels or ratios and transformers generally have the same input and output power give or take a little loss. If a transformer steps up the voltage it steps down the current so power remains relatively constant.

So all transformer ratios are 0 dB.

Sorry...

JR