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In Reply to: RE: Sure, headphone output jacks can be used as a "preamp out"... posted by Caucasian Blackplate on December 21, 2013 at 21:37:12
What manufacturers "should" do and what they actually do are often two different things, in part because industry standards have a way of changing more slowly than marketplace innovations do. I know it sounds strange, but many (or even most) headphone amps have output impedance ratings much higher than 2 ohms, even at their lowest impedance setting. There are exceptions, of course, but headphone amps still tend to have higher output impedance ratings than dedicated preamps do. I think it has something to do with the fact that very low-impedance headphones were once more rare in the "high-end" and/or professional fields than they are today.
High impedance headphones tend to sound best when amplified with high output impedance (120 ohms was once stipulated as the industry "standard"), while low impedance headphones tend to like a low(er) impedance output. But even today, it can be hard to find full-sized headphone amps with output impedance ratings of less than 30 ohms.
I belive that, with the rise in popularity of of micro-sized portable players and the portable low impedance "easy-to-drive" headphones designed for them, manufacturers started to adopt a low(er) impedance standard. But dedicated preamps still tend to have lower output impedance ratings than headphone amps do and this is one reason they do not perform as well as real preamps do for the task of pre-amplification.
That said, I once owned a solid-state Meier Corda headphone amp that was rated at "0 ohms output impedance", but such seems to be the exception rather than the rule as of yet.
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