Audio Transformers Theory

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Audio Transformers Theory

Post by Tony Wells on Mon Jun 21, 2010 11:42 pm

I would like someone to explain the specifications/parameters of a typical audio transformer. Unless I am misinformed, I believe the impedance is given for a 1 Khz signal. I am having a little trouble getting a few things straight in my mind regarding the relationship between the DC Resistance, the Impedance, Reflected Impedance, Turns Ratio, and Power Rating.
For instance, given the DC Resistance of a given winding, can the impedance be calculated knowing at most 1 other factor? What does it take to actually measure the impedance @ 1Khz?How is the impedance of a primary related to the specific load resistance of an output tube? Can a mismatch between the two be closed with a resistor to make an acceptable match? Can a choke be used instead of a resistor?
I guess I could get to reading all the theory in the books I have, but maybe someone could answer in the context of selecting an appropriate output transformer for a 42 tube.

Tony

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Re: Audio Transformers Theory

Post by Bill Cahill on Tue Jul 13, 2010 12:01 pm

Let me try to help this way....
The only things that matter are impedence, dc resistance, and, wattage.

First, say you need a transformer for a 50C5 audio output.
The correct impedence would be 2,500 ohms primary. Not readable with ohm meter. Secondary: 4 ohms.
Wattage would be say 2 watts.

Now, I don't remember exactly, but, in dc resistance, not as important as impedence, but, gives you a pretty good idea where you are. That would be approx. 200 ohms.
Secondary would measure something like 2 ohms, or, maybe less.
Wattage would still have to be figured by what the transformer mannufacturer wound it for.
The wattage on a 50C5, depending on voltage applied by power supply, could be up to 2 watts. In that case, a min. of 2 watt transformer is needed, up to 4 watts, but, generally not that high, unless the circuit is PP with two output tubes. In that case, 35C5 is likely. Then the wattage would beapprox. 4 watts of audio. In that case the total impedence would also be higher.
I think around 4500 ohms impedence.

Now, remember, you cannot measure impedence with aqn ohmmeter, but, an ohmmeter will give you a rough idea of where you are.
The higher the impedence, the higher the dc resistance.
Bill Cahill

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