# Audio Transformers Theory

## Audio Transformers Theory

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

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

**Tony Wells**- Member
- Number of posts : 20

Age : 58

Registration date : 2010-06-07

## Re: Audio Transformers Theory

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

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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