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In Reply to: RE: Disagree, for the most part... posted by genungo on December 22, 2013 at 14:13:12
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.
Follow Ups:
(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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