Transformer coupling pros and cons (in a nutshell)

The shortest answer to questions of this kind (which I regularly receive) is: purely sonically, transformer-coupled output has a more calm presentation, with a more quiet background, and a somewhat more precise soundstage. There is one important thing that may explain this: transformer coupling provides a 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 with 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 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 the 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 the 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 the 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 does low line output impedance matter: everywhere, nowhere, or does it matter only to make interconnect cable sonically unimportant?)

12 thoughts on “Transformer coupling pros and cons (in a nutshell)”

  1. Mike Agsteribbe

    Your comment is little too technical for me.
    My system:
    Aurender N 100h
    Exogal dac, will be replaced
    Sugden integrated class A amplifier
    Trenner and Friedl Art spekers
    Is it possible to give my some advise concerning the output transformers with this info?

    Regards,
    Mike Agsteribbe, the Netherlands

  2. Mike,

    Unfortunately it is hard to really predict the outcome, especially not getting into technical matters.

    But generally, this “equation” mostly includes the DAC itself, the interconnect cable, and amplifier input impedance, and system grounding. Loudspeakers are out of it, anyhow but contributing to the system by their own sonic properties.

    Also, I plan to add some, and hopefully most practical information on this topic very soon. And there is one very special reason for this: we are just moving to toroidal output transformers, and toroids are real unique in this domain. Also, with them, the DAC output impedance remains quite low, 30 Ohm, thus making such a transformer coupled output more universal than it was before.

    Regards

  3. Ciao.
    La domanda che voglio porti é se non hai immaginato un dac che implementi la tecnologia r2r magari con i trasformatori in uscita.
    Sul fronte ingressi secondo me sarebbe utile un ingresso i2s su standard hdmi.
    Grazie 1000.
    Ps
    Posseggo un d09 e suona bene.

  4. Hello Luciano,

    All the Audial DACs released up to now were real multibit DACs. Model S Mk3 has I2S input, and optional output coupling transformers too.

    If you are however referring literally to discrete resistors R2R, yes, such a DAC is on the way too. It will also have some sort of direct PCM input, probably I2S, and output coupling transformers.

    Ciao

  5. Low end performance will not be affected using coupling opt. What’s needed is the reshuffling of of PS & decoupling cap types & values as different brand of caps performs differently. Looking at cap specs alone will not tell you how it sounds.

  6. Thank you Pedja for designing such a great dac that allowed me to muck around with it for 3yrs plus. Lol.

    Cheers

  7. You are welcome, Jaffrie.

    I have given up trying to associate capacitors sound to any technical parameter, a long time ago.

    With coupling transformers, this can be easier, since a lot of it, and things get more critical towards the bottom end, depends on its inductance. But then again, there is more about transformers than it is their inductance, and you can not increase it by not changing the other parameters at that. And even the inductance itself is not that plain simple.

    Regards

  8. Well as the saying goes, there’s many ways to skin a cat Pedja. Think me & Eldam is the craziest of the AyA bunch as we both experiment alot. Re coupling opt yes inductance will slow down the sound some what & bass as you say will not go as deep hence for myself I choose a relative large opt that could handle 50 ma of dc but that’s not the main reason, the crux is the size of the EI core, this in itself will help retain bass. Even with coupling opt, caps will still greatly affect the sound of AyA & this is what I love about AyA cause every adjustment or tweak that I make, I can hear the changes be it good or bad. I have 1 one thing more to experiement & if it works will share it with fellow AyA members

    Cheers & takecare

  9. Salve, mi sono incuriosito a questi vostro dac, visto che sto pensando di comprare uno ma con le uscite Bilanciate, il dac devi collegarlo al mio notebook sul quale ho installato Foobar2000, tieni conto che adesso uso il dac Aune S16 con soddisfazione, ma penso che con il vostro DAC il suono possa ancora migliorare. Ora… vi chiedo.. cosa ne pensate e a quale prezzo potrei averlo qui da ma in Italia. Grazie – Ciao

  10. Hello Gianfranco,

    There is no Audial representative in Italy, but you can order everything directly from this site. We ship worldwide.

    Regards.

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