On this page we want to explain you, how and with which measuring devices we are testing and claculating our output transformers for HiFi amplification.

Here you can find the most important formulas for measuring output transformers, even if they are coming from foreign companies. There aren´t really international standards regarding these parameters, but we are hoping to give a small overview and a look inside for a better understanding.

Inductance "L"

For measuring the inductance of the transformer we are using three different rms voltages with a value of 1, 10 und 230 Volts/50cps. With the measured current, which is flowing through the complete primary winding you can calculate with the following formula the indutivity of the transformer.

We are using these three voltages to show you, that the transfomer has enough headroom to transfer low frequendies at low voltages too.

For measuring the inductance of a single ended transformer, we are using together with the 50 cps rms voltages a dc current for given you a better report of how the dc current is affecting the inductance of the transformer.

L-C-R measuring bridge HAMEG HM 8018/leakage inductance

As written before, there is no international standard, how to measure the leakage inductance of a transformer. In the sixties there were made a lot of field trials between the IRT, Telefunken and TAB, regarding this point. As soon as possible, we will publish them on our sites. The leakage inductance is a reactive coil, which is lying in series with the main inductance of the transformer. When the frequencies getting higher, the magnetic field from the primary windings is no longer able to be transformed to all secondary windings and vice versa. In this case the reactive and the main inductance are a frequency voltage devider.

For measuring the leakage inductance of our transformers, we are using this international available LCR measuring bridge, which uses the following rms voltages and frequecies

main inductivty: 0,35 Volts@160 cps

leakage inductivity: 0,35 Volts@1600 cps

We publish both values to give you a relation between both values.

Frequency Response

For shure, the frequency response of an output transformer depends on the used inner resitance on the primary side and the secondary load. In the near future we will publish the frequency-response characteric for the developed use.

Resonance Frequency/Capacitance

With the shown measurment set up it is possible to caculate the capacitance of a transformer.

After getting the resonance frequency of the transformers, the measured values can be put into the following formula:

The winding capacitance is lying with the main inductivity parallel. By increasing the frequency it´s resistance becomes smaller. At which resistance this happens can be calculated with this formula:

As measurement voltage we are using here as most other transformer manufactures too an rms voltage of 1 Volt.

Primary Resistance/ Secundary Resistance

These values are the dc-resistance of the windings.

Efficiency Factor/ I (loss)

Both values depend on each other. In working conditions the ac-current flows through the primary and secondary winding. Due to the winding resistance apart of it will be changed into heat and will no longer appear at the output. These internal losses will be shown in relation between the rms power at the output and the comlpete transfered power. This ratio will be written in Germany in "%", international in "dB". To find out these values, we have to regard the transformers from it´s secondary side. The primary resistance will betransformed with it´s squared turn ratio to the secondary. Than, the total resistance of the transformer is as follow:

Let us take for example the output transformer SOT-005 with a secondary load of 4 Ohm.

Now we can calculate the efficiency factor:

put in the values into the formula:

Written in percent are these 88% or international in dB

What does it mean? Above the table schows two different transformers with the same turn ratio of 20,91, same Ra of 3,5 kOhm, the same power of 13 Watts but with differents winding resistances. By comparing the calculated values you will see, how much power will be lost by

transforming into heat.

For shure you have now recognized how important low dc winding resistances are. Especially in Single Ended circuits with a lower output it is very important to use transformers with a low I(loss). Otherwise you will loose a lot of more power with the choice of a wrong output transformer. But also, with a higher primary resistance the supply voltage will drop down in cause of the voltage devider the primary resistance builds with the resistance of the used tube.

Wattage/Distortion

The published wattage of the transformer was calculate with an inductance ot 1,5 Tesla. Detailed data for different inductances, different wattages at different low frequencies will follow. The wattage for SE output-Transformers was calculated with an induction of 0,7 Tesla and a dc current load of 20AW/cm.

Core distortion is a very complex subject. All values were measured with a HAMEG distortion meter.

I don´t know, if all explications can be understood. Please let us know, when there are some misunderstandings or if we have used the wrong words for special things. Write us an E-Mail. Thank you very much.