[AMRadio] Resistive load on interstage audio transformers in

Jim Tonne tonne at comcast.net
Wed Mar 12 13:07:37 EST 2008


You have brought to the surface something that I learned
(not the easy way) a few decades ago.

Let us suppose we have an audio transformer that says
"600 ohms (pri) to 5kohms (sec.)"

If you put a 5 kohm resistor across the secondary, then
when  you look into the input of the transformer with an
"AC ohmmeter" you will see 600 ohms.

If you put nothing (open circuit) across the secondary,
then when you look into the input you will see a real
high impedance.  Just how high would depend on how
the transformer is built.

If you have a 600:600 transformer (one to one winding
ratio) and put a 600 ohm resistor across the secondary
you will see 600 ohms when you look into the primary.

Another little tidbit concerns microphone input transformers
used, for example, to go from a microphone to the grid of
a preamp tube.  (Remember them?!)  Here we have the 
above information and it remains true.   But dynamic 
microphones like to be unloaded.  So we use a 150:50k 
transformer and don't put anything across the secondary.  
What do we see when we look into the primary?  A nearly 
open circuit, just what the microphone wants to see.  
The transformer is terminated correctly by the microphone.

If a microphone is rated at, say, "150 ohms", do NOT
load it with 150 ohms!  Let it run open-circuit.  If you
connect it to a "150:50k" transformer, then the transformer
is terminated by the microphone.  We are terminating the
transformer by what is connected to the primary rather 
than the secondary as outlined in the first couple of 
paragraphs above.

When you load or overload or underload the transformer
the frequency response will change and just how it changes
is dependent on the way the thing is built.

- Jim Tonne

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