Jul 13, 2016

How to use a headphone amp

I hear plenty of talk of there being "sound benefits" to be had in adding a headphone amplifier in between my device (phone/music player) and my headphones.
Unfortunately, the "why" seems to be a bit mystical and the "how" tends to remain unexplained.
There are cases where the need is obvious; with a set of Beyerdynamic cans of very high Ohmage, the circuitry in a mobile phone will be insufficient to drive it properly, and an amplifier is undoubtedly necessary.
But I'm running IEMs (and sometimes some Grado SR80s), where a high power amp isn't ludicrously necessary. I gather (and have the Fireye Minis that are supposedly a help) that some improvements in sound quality can be had in this range by using a headphone amp. To the straight questions, applicable to this sort of case...
  • What are the settings supposed to look like that would give me "better sound" when hooking my NuForce HEM2s to my OnePlus One with a headphone amp (let's say, FiiO E18?) in between?
  • How can I verify that it's working, and improving things?
  • What should I be listening for as verifications of improvement?

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You don't have to ask the internet if your sound is better or not with an amp. It's usually pretty obvious. If you don't hear a difference, you don't need one! Don't get sucked into the audiophile world of chasing tiny incremental improvements.
Most headphones released in the past five years are designed to work with mobile devices and so will not strictly require an amp. But I find an amp usually adds some body and depth to your music. With sensitive IEMs, a good amp will have a quieter backdrop as well - it's amazing how a silent background improves the sound of your music.
Many of the most famous and expensive headphones were released before digital mobile devices became ubiquitous and have a higher impedance to match up better with expensive home stereos or professional studio gear. To use them with mobile you'll probably want an amp. Because so many high end headphones are high impedance, many people have the impression that high impedance headphones are inherently better, or that a headphone isn't serious if it doesn't require a lot of amplification.
Not true! In fact, the newest high end headphones hitting the market are almost all low impedance to better match up with how people listen to their music today.
So, trust your ears. I use headphone amps at home because I think it sounds better, but when I am on the move I just plug right into my phone and that sounds great too.
Armaegis summed it up pretty well but I would like to add that the E18 is not only an amp but also a DAC. This will move the conversion of the audio, from 1's and 0's into an analogue waveform, from the internal chip in the phone to the E18. Be aware that with the way Android audio works you may need additional apps to get this to function at all and it is unlikely that you will get to use the E18 with every app. The amp only counterpart(s) would be the E12/E12A. To verify that it is "working" you will simply need to plug in your IEMS to the amp output and see if you've got sound. I don't use an amp with my 6 Plus or my HTC one but I do with my X3ii. I primarily use single-driver dynamic IEM's with my phones. Single drivers are not near as sensitive to impedance mismatches as those that employ a crossover, in particular low impedance multi BA designs. That HEM2 is a single driver BA with a 26 ohm nominal impedance, it *shouldn't be too sensitivity to the output impedance. If it were my money and I had no plans to change to a multi-driver IEM, I would skip the amp and just use the phone. The OPO has a pretty good reputation as far the HO goes.
I don't have an E18; was using it as an example. Yep, aware of difference between Amp and DAP; I use a FiiO E10 to get (one imagines nicer) sound out of my media computer.
For my particular situation, it seems like my phone (OPO) and my DAC (iBasso DX50) both have enough power to drive IEMs decently well, so that an amp doesn't seem likely to help much there. But it is pretty much "the question" what benefit there would be to adding in an E18 or such, and how to determine that it's doing some good.
It's nice to verify some cases where "Amp isn't hugely helpful." But I'll bet many would benefit from knowing how to measure that "Amp is helping", so thanks for the comments!
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.
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