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In Reply to: RE: Can headphone-out be routed to RCA Jacks in-Amp? posted by Rapalyea on December 20, 2013 at 09:16:58
All you would need in most cases is a simple (1/8" or 1/4" plug to dual RCA) adapter cable.However, headphone amps will often (though not always) have much higher output impedance ratings than real, dedicated preamps do. It is almost always better when the output impedance rating of your "preamp" is very much lower than the input impedance rating of your power amp. If the ratio of output impedance to input impedance is too low, dynamics and/or flat frequency response can become compromised.
So, unless the input impedance rating of your power amp is unusually high, the sound quality you will get from a typical headphone amp (used as a "preamp") will probably not be as good as the sound quality you will get from a decent dedicated preamp with it's much lower output impedance rating.
I do have a pair of ASL Wave 8 tube monoblocks that have unusually high 100 K ohm input impedance ratings. And they do sound surprisingly good when driven by my DNA Sonett tube headphone amp (again, with it's high-ish output impedance rating) - partly because the ratio of output to input impedance is acceptably high. Just remember that many (or most) power amps do not have such a high input impedance rating.
But, the basic answer is "YES". A headphone amp can most certainly be used as a makeshift "preamp" in a pinch.
Edits: 12/21/13 12/21/13 12/21/13 12/22/13 12/22/13Follow Ups:
"However, headphone amps will often have much higher output impedance ratings than real, dedicated preamps do. It is almost always better when the output impedance rating of your "preamp" is very much lower than the input impedance rating of your power amp (a ratio of 10 to 1 or higher would be good). If this ratio of output impedance to input impedance is too low, dynamics and/or flat frequency response can become compromised."
You generally have that backwards. It's incredibly rare to see a power amp with an input impedance lower than 10K, while 16 Ohm headphones are very common. (So your headphone amp "should" have a 2 Ohm output impedance and your preamp will be OK with a 1K output impedance)
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 IEC "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 much less than 30 ohms. My DNA Sonett has switchable "hi/low" output impedance (28 ohms/120 ohms) but (especially with tube headphone amps) lower output impedance ratings than that still seem to be relatively rare.
This is ironic, because very low impedance headphones definitely seem to perform better (especially for tightness of bass) when driven by a suitable amplifier with an ultra low output impedance. Some people seem to prefer the looser, muddier bass (quantity over quality) that sometimes occurs when a headphone amps output impedance is too high for the headphones, but what can you say?
I believe 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 low(er) impedance standards. But dedicated preamps still tend to have lower output impedance ratings than many headphone amps do, and this is one possible reason why they might not perform as well as real preamps for the task of pre-amplification.
The TRS output jacks used on most headphone amps might also create some sonic degradation when the amp is used as a "preamp". Dedicated RCA or XLR output jacks, asd used on real preamps, are superior connection devices.
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.
It has been claimed that, for maximum EFI/RMI rejection the higher the aforementioned input/output impedance ratio, the better. Others claim that this entire impedance matching issue matters but little. I, of course, tend to think that a higher ratio is best.
Edits: 12/22/13 12/22/13 12/22/13 12/22/13
There are a whole ton of preamps with output impedances in the 600-1000 Ohm range. I do not know of any headphone amps with output impedances this high. There are also plenty of headphone amps with low output impedances that will run well with the low impedance loads some modern headphones provide. (The IHF standard doesn't seem to matter anymore)
Beyond output impedance, there is also the amount of current that the output stage can deliver, and this is typically a bit higher with a headphone amp compared to a preamp (hence your solid state example with 0 Ohms output, it's a pure current source!).
"The TRS output jacks used on most headphone amps might also create some sonic degradation when the amp is used as a "preamp". Dedicated RCA or XLR output jacks, asd used on real preamps, are superior connection devices."
I'm not sure that a Nuetrik locking 1/4" jack is going to sound all that different compared to a pair of Neutrik XLR's.
(Sorry for the delay in responding, I have been away for Xmas vacation...)
The main reason why I believe that a low(er) output impedance preamp is best is because I have actually tried this thing out for myself.
Toggling my headphone amp's selector switch on both "high" and "low" output impedance settings, the low impedance setting always seems to sound best (when it is used as a "preamp") - at least with the amps I have on hand. But, many headphone amps do not have an output impedance selector switch so not everyone will have the option of experimenting with such a thing, as I have done.
While I cannot say for sure that identical results would be produced in every system, I feel fairly confident in saying that the preamp's output impedance might be something to consider when setting up a system.
It really depends a lot on what that switch is doing.
In a solid state headphone amp, it's likely that the output impedance switch would be altering the amount of feedback in the amplifier in order to bring the output impedance down.
In a tube headphone amp, you could be toggling output transformer windings, or in fact also toggling feedback settings (or both).
It isn't surprising that you are hearing differences between these settings, but it is unlikely that it is the actual output impedance itself that is causing your preferences (unless you're driving a very low impedance load, like less than 600 Ohms).
The amp in question (DNA Sonett) is a zero feedback design so I would assume that true, dual output impedance switching is one of it's features. So, for now, I will continue to assume that a low(er) output impedance setting (below 120 ohm "IEC standard") is better when a headphone amp is used as a "preamp" - at least in some cases. Thanks for the input.
Edits: 12/29/13
This is an additional interesting observation. When you switch the impedance setting on that amp from high to low, you are actually using less of the output transformer secondary (less copper), and increasing the step-down ratio.
I have a similar headphone amp in my armada with the same brand of output transformers, and I have actually made observations similar to yours - that I tend to prefer that the amp is set on the lowest impedance setting regardless of what I'm driving.
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