Audio Transformers 101

[MisterCMRR]

When I still worked for a radio station, they used in some cases consumer CD players equiped with a 1:1 output transformer, to obtain a balanced output. (The output of a CD player is already pretty high.)
To make the output 'short circuit proof', they added a 22 ohm resistor between the output and the primary winding of the transformer. Together with the output impedance of the driving circuit, this relative high source impedance resulted in increased distortion.
My tests showed that a higher source impedance driving a transformer significantly increases transformer distortion.
So to make things 'short circuit proof', it is better to add a resistor after the secundary winding!
(And drive the transformer from a low impedance.)

While you're correct about driving the transformer primary from a low source impedance (less than 10% of the transformer's primary DC resistance is the point of diminishing returns), there's a caveat. An output transformer may have considerable distributed capacitance (effectively in parallel with primary) and it can destabilize the driving output stage. Using the series resistor on the primary prevents the driver from experiencing the effect at near-Mhz frequencies. A better technique is to use a ferrite bead or parallel L and R on the primary side (3 uH and 39 ohms is common). A ferrite bead with an impedance around 50 ohms at 1 MHz usually works well ... and the impedance of either method at audio frequencies is near-zero! Jensen may still sell the JT- OLI output isolator, which winds the inductor on the body of the resistor.

 

[RuudNL]

In this specific case (a CD player) the output impedance wasn't even near zero, so an extra resistor in series with the primary winding only could have made things worse.

 

[soapF]

thank you for posting this, as well as the paper by bill. tied a lot of concepts together for me.