Transformer coupling pros and cons (in a nutshell)

The most short answer to the questions of this kind (which I regularly receive) is: purely sonically, transformer coupled output has more calm presentation, with more quiet background, and somewhat more precise soundstage. There is one important thing that may explain this: transformer coupling provides separate return path, i.e. separate interconnection loop to each channel.

On the other hand, transformers can lack some definition at the most bottom end. Apparently this is associated to their objective (distortion) performance, but if high quality transformers are used, it takes very big loudspeakers with clean and well tuned deepest bass to notice that though.

Still, possible benefits and shortcomings depend also on the environment, so in some systems transformers can lack some dynamics. There are several reasons for their such behavior, and unfortunately it is usually not possible to predict the result in advance.

The first thing to consider is the input impedance of the next unit in chain: generally, transformers do not give their best in the low impedance environments, even if they are designed to avoid losses at 600 Ohm, such as line output transformers used in the Model S / Model S USB.

For instance, with Model A low impedance volume control (820 Ohm) they are better avoided. Some people, who are aware of my personal preferences, had an impression that during the time I silently abandoned transformer coupling. And they are correct, but only until we talk about systems with such amplifiers.

Another thing to consider is the output impedance itself. As said, the output impedance of the Model S output buffer is 3 Ohm and that is also the output impedance of directly coupled output of the Model S. Capacitors as coupling devices add the impedance of their own, which increases as frequency decreases – since the Model S uses 23 uF coupling capacitors, they add 7 Ohm at 1 kHz, and 70 Ohm at 100 Hz. On the other hand, transformers as coupling devices add about 80 Ohms across the whole audio band, because this is the sum of their windings DC resistances.

Thus, in some environments where lowest possible output impedance is needed, a direct or capacitors coupling can be preferred.

(Here we obviously have another topic, and we might discuss it once. Where and why low line output impedance does matter: everywhere, nowhere, or it does matter only to make interconnect cable sonically unimportant?)

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