Preamp advice

"Actually, OTL is largely irrelevant for solid-state preamps and amplifiers, since they do not require transformers or buffer stages to regulate output impedance. What matters instead is OCL."

The main rationale for either a transformer or a capacitor at the output of a preamplifier is to block DC voltage from affecting the input stage of the downstream amplifier.  Both transformers and capacitors block DC.  This has not much to do with "regulating output impedance".  However, the term "buffer stage" is usually used to describe an active stage (tube or transistor) that is used to lower output impedance. A buffer stage can still have DC at its output, and you would need a transformer or more usually a capacitor to block it. Because SS gear operates at much lower voltages than do tubes, it is a bit easier to implement an SS circuit that does not require C or T, but in many cases an op amp or the like is used to cancel the small amount of DC offset that may be present.

@lewm. 

Not only is it incorrect to assume that a headphone amplifier cannot also function as a preamp, but there also seems to be a misunderstanding regarding the roles of transformers and capacitors in the output stage.

While both transformers and capacitors can be used to block DC, capacitors are more commonly employed for this purpose because they are cost-effective and sufficiently effective. In contrast, output transformers are primarily used for impedance matching, voltage step-up or step-down, efficient current transfer, and providing isolation between stages.

Unlike tube preamps and power amplifiers, solid-state (SS) preamps and amplifiers typically have low output impedance and therefore do not require output transformers. As a result, most modern SS preamps are already output transformer-less (OTL).

The further elimination of capacitors in the output stage—leading to output capacitor-less (OCL) designs—is driven by the goal of improving transparency. These OCL designs instead rely on alternative methods, such as servo circuits (essentially feedback control), to manage and suppress DC offset as I mentioned earlier.

@audphile1 "...Audio Research LS-28 SE..."

Excellent recommendation.

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