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How to use a headphone amp
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First of all, hearing the "improvement" is a fuzzy thing. I don't know your gear, you don't know mine, so attempting to qualify (not even quantify) the differences is going to be impossible. Just relax and let your ears guide you. Maybe it's obvious. Maybe it's placebo/voodoo.
But let's dig into a couple basics. (please keep in mind I'm making some *very* generalized statements for the sake of simplicity)
- The impedance of the cans is not the primary factor that determines how loud it gets. What you want to look at is the "sensitivity" rating (sometimes erroneously called the "efficiency"). This is given as a decibel rating per milliwatt. Every doubling of power gets you +3dB. Power follow the equation P = V²/R (power equals voltage squared over load, in this case the headphone impedance... yeah yeah impedance is complex and changes; let's just ignore that). So we can see that yes high impedance does result in lower power, but it's not the end of the world since if you still have room to increase V then it makes up the difference very quickly due to the squared nature.
- We'll also want to consider Ohm's law, this is V = IR (voltage equals current times load). If we fidget with some of the algebra, we can get this *very* broad generalization that high impedance headphones need more voltage and less current, while low impedance cans want more current but less voltage.
- All devices with a headphone output do actually have amps built-in, but those are often very small built-in chip solutions and often have very limited voltage and current outputs. If you're trying to crank your music, usually they will hit their current limits before their voltage limit. Pushing out lots of current is what taxes the output devices and *generally* leads to more distortion.
- So here's what happens when you stick an amp in between: The amp has a very very high input impedance, usually in the neighbourhood of 50k ohms. From the source's viewpoint, this is a super easy load to drive because that means we can push voltage up and almost no current will flow. The amp circuitry now takes the that nice clean signal we just fed it, amplifies it as needed (probably doesn't even need to), and has substantially larger current buffers so there's no fear of overloading it.
- A potential downside to the amps is if there's noise from your source, then that noise will also be amplified. Ideally, your amp should have the lowest gain necessary for you to achieve your desired listening levels. But honestly, this is kind of a non-issue for most people. (someone may bring up the dangers of "double-amping", but ignore the fear mongering... at this level of gear again it's really a non-issue)
- To fuzzily answer your second and third question: typically I listen cleanliness. The bass region is the most power hungry, so I listen there for "kick" rather than "rumble". Into the midrange I listen for separation and clean definition between instruments/vocals/whatever else is there. Into the high end it's listening for the "air/shimmer". Basically all the audiophile approved buzzwords, just imagine them and see if things feel more immersive.
So that all said, IEMs generally do not need much in the way of amping. They have super high sensitivities and low impedances. If you do go with an amp, you do not want one with high gain. Current flow will be more important, but alas most specs don't mention current flow and only measure power.
Hmm, it occurs to me this could have gone into older Amp 101 thread. Oh well.