Transformers Loads and Phase.

Design and use of the various types.
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dave slagle
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Transformers Loads and Phase.

Post by dave slagle »

For some time now I have been against the resistive loading of transformers and here are some measurments of a 4:1 transformer designed to load a 5687 @ 15ma for a linestage.

Many people would call this a 10K:600 transformer (actually 9600:600) and some would even go as far as to terminate the secondary with a 600R resistor.


Here is the transformer with only the test gear as the load. Pretty cool... 4:1 is = -12dB.

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Now look what happens when you terminate it with a 600 ohm resistor. 2.5dB of gain is lost. (1dB in the transformer and 1.5dB from the source) The high frequency rolls off and the HF phase starts shifting in the audio band.

Image

Now lets look at the load presented to the source with no secondary termination.

Image

Now some would argue that this is a nonlinear load and that the secondary needs to be terminated. Other circumstances (say we were using a pentode) require a well defined load so here it is with 600R on the secondary.


Image

Looks pretty good... just over 10K which is 9600 plus copper DCR. This all comes at the cost of the hit to the frequency response and phase seen in the frequency response plots.

The solution? Just load the primary. Here is the impedance plot of a 9600 ohm resistor in parallel with the primary.

Image

and to round things up the phase and frequency of the transformer with 9600 ohms across the primary.

Image

dave
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