We shall first establish some simple electronics rules.
Transformers do not have an intrinsic input impedance; transformers have a reflective impedance. This is an important point often misunderstood in the pro-audio world.
A transformer’s input impedance is dependent on the load on its secondary, whether resistive or reactive.
Also, a transformer’s impedance is related to the square of its turns ratio.
For example, if we set up the TransDrive at PRL/SERIES using a 1:2 ratio transformer loaded, then we can expect an audio gain of +6dB. To find out what the input impedance is, we square the ratio numbers. 1 squared is still 1, and 2 squared is 4. So if there’s a load of 600 ohms on the secondary, the input impedance looking into the primary would look like 150 ohms. This would be a heavy load for the average pro-audio devices output.
In another example, the Apogee Symphony I/O MkII specifications say the line input impedance is 10k ohms, which is the same for the line input of older Neve console modules. If the TransDrive contains transformers with a 1:2 ratio, with the secondary loaded at 10k ohms going into these units, then the primary input impedance would be 2500 ohms, or 2.5k ohms. This is easier to drive for the average pro audio devices output.
The 600 ohms was established back in the days of analog telephones as the nominal impedance between transmission lines. Because communication across the country was far more important than recording or reproducing music, the audio world borrowed transformers from the telephone industry up until the 1970s. Today, the idea of pro-audio gear having 600 ohms has stuck; even though it has nothing to do with how pro-audio must work.