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Amplifier power requirement

Discussion in 'Sound Science' started by Bhargu, May 13, 2018.
  1. Bhargu
    TL;DR, trying to understand if the amp should be able to push the cans to a max 110dB, 115dB or 120dB, or more.

    Hi all, first, sorry for the long post.

    I have been looking for a portable headphone amplifier for a HD650 and an upcoming HE4XX. I got a bit intrested in a few sub $250 units, iFi iDSD Nano BL in particular (one of the few with single USB for data and charging). Most of them are rated at around 300mW at 32 ohms. I asked around in many threads, but kept getting the response that most of them barely drives HE4XX. I found this a bit confusing, which could be due to my lack of proper understanding of the topic. Hence this post. I am trying to fill in the holes in my knowledge of the topic, so as to decide on what to get.

    I keep hearing that modern recordings have a dynamic range which will push the ceiling about 20 dB above average listening levels. Assuming one listens at 90 dB (I don't think I do at that level), that should be 110 dB. So, is it enough to select an amp which will push the headphones to 110 dB. Or, do we need it to be pushed to 115 dB, 120 dB or more? Also, what would be the requirement of old classical/rock music (as in how many dB above normal listening volume)?

    As I understand, most people feel that there is some distortion in the sound above 3 o'clock position, even in relatively good amps, which approximates to about 70% of the total power of the amp at that load for a log/commercial pot. In amplifiers one obeserves that the actual measured output of an amplifier is sometimes only around 60% to 80% of the company rated power output. I don't know if it is applicable to headphone amplifiers. Still, if I assume that the actual output will be only 70% of the rated output, that gives us about 50% of the total rated power, at 3 o'clock position.

    As an example, let us take iFi Nano BL and HE4XX (stuff I am considering). iFi rates the amp at 285mW at 30 ohms. HE4XX is rated at 93 dB/mW SPL at 35 ohms, but innerfidelity measurements showed it at 49 ohms, which puts the Nano's power at around 174 mW. If we take a safe 50% of it, it is about 87 mW. Now HE4XX can achieve 112 dB at that power. So, will this power be enough for someone who listens normally at 90dB? How will it change with old classicals vs modern recordings?

    Also, I keep reading that planars need more current that voltage. But my understanding was that current and voltage are related to load. So, if there is not enough current, the voltage will also be lower. So I am having a hard time understanding how there can be enough volume but the cans sound weak. One reason I could think of was that when the current reaches peak achievable values, the amps characteristics starts to change or audible distortions starts to appear, changing the sound signature. This could be the cause of the weak sound even though the volume is satisfactory. Am I right about this? If so, at what percentage of the peak current can we expect an audio amplifier to start behaving erratically?
  2. castleofargh Contributor
    seems to me like you have a fairly good understanding of the issue.

    first: audiophiles will be audiophiles. a great many will blame amp power for stuff often unrelated to power. it's a typical result of people who rely fully on their own impressions to draw objective conclusions about electricity needs. more often than not, they don't have a clue what they're talking about. so having people tell you that you need 1.21gigawatts to properly drive the HE4XX doesn't necessarily mean anything. some may talk about issues with low impedance load on that amp, which is always possible. others may just be in the "more is better" movement, always advocating for crazy numbers because they don't know what power is but it looks like it would be cool to have a lot.

    second: as you've guessed, a significant aspect is how loud you're going to listen to music. if you're never going to have peaks above 90dB, then your power requirements will be that much smaller. it can be tricky to estimate that properly so as for anything we don't know, we usually get a little paranoid ^_^.
    I use replay gain(which takes down most modern records by 10 to 12dB usually). I also use EQ on everything so that's often up to 6dB of gain lost in the battle. and for various reasons(mostly laziness), I often remove another 10 to 15dB with digital volume in foobar, and go up when the sound is really quiet(like with classical music that doesn't fit within replaygain settings without clipping so it's a good deal quieter than the rest).

    the dB pile up rapidly so it's good that I have some headroom. with all that I personally never came close to reaching a 115dB limit with my use but we can argue that I almost never listen to music loudly. I'm sure some people will push their music to the max. to have the experience like we're at the live show. I'd advise against that, but people will do whatever they want. only you can tell if you go crazy or if your version of crazy is when you push to 90dB. I'm kind of like that, my typical listening level at home is in the 60 to 70dB, when I go crazy over some guitar solo, I rarely get past 90dB. now on the street with my DAP, I'm already around 80dB when calm, and louder if I don't have very isolating IEMs. it's a matter of circumstances and ambient noise.
    still IMO 115dB is very safe for almost anybody, and 110dB should already allow for pretty loud listening under most circumstance. every extra 10dB is about double the perceived loudness, so it rapidly becomes overwhelming.

    now if you're tempted by something with specs really close to the maximum you've decided upon, it might be a good idea to contact the brand and ask for the specific specs, maybe even straight ask them what they think about driving your headphones with it. most amp guys I've talked to over the years have been very honest. I even remember one telling me that I didn't need their amp. I'm suggesting this because sometimes specs aren't as clear as they seem. sometimes the max power isn't at 1%THD, sometimes the value isn't for one channel... so it can be worth it to make sure if something like -6dB can be decisive in your choice.

    now that I've gone down the rabbit hole, there is also the matter of your source. most outputs will be measured with about 2volt coming from the DAC. if instead you use a portable device the output might more often range from 0.5v to 1V. so again that could sometimes lead to 6 to 12dB lost on a misunderstanding.

    and if you're a huge classical listener, you will indeed need a little more headroom than if you listen to rap and pop, simply because at same average subjective level, classical will probably peak higher, being typically more dynamic.

    I believe all those uncertainties are the reason why we're used to aim as high as 115 or even 120dB for some people. if you know all your specs and they're not fantasy specs, IMO 110dB max is already versatile enough for most uses. short of being live rock concert kind of loud, those do reach 120dB or even more if your country doesn't have strict regulations on the matter.
  3. RRod
    120dB peak would be what I look for, given my most dynamic classical stuff. That's 20dB crest on top of 100dB SPL RMS, which is darn friggin loud. I don't get these people who listen to 0 crest stuff at 120dB SPL; seems almost impossible.
  4. Bhargu
    @castleofargh Thanks for the in-depth reply. I haven't thought about replay-gain correction. I have tracks with around -15 dB. I don't use eq much other than for spike-taming. I typically listen in 75-90 dB range for music. I was wondering about 115/120 dB mainly because I listen to a lot of classical stuff and they tend to have a higher dynamic range.

    I was looking mainly for a portable dac/amp combo, but the pickings are hard under $250. Most devices just have enough power to reach 115 dB at full power. Any higher will be limited either by voltage or current characteristics (limits). My main can being 300ohm and new one being 35 ohm, I will be forced to select one or the other with most portable dac/amp combos.
  5. Bhargu
    That is also my biggest concern. I listen at 90 dB max and keep hearing that I might need more than 20 dB headroom for classical stuff. The portable units that I am looking at just about achieves 115 dB.
  6. RRod
    It's definitely a valid concern. My Magni definitely has enough juice for my PM-3 for all my material, but that's an estimated 130dB about the combo can push out total. If I shaved off 10dB I think the amp would still cut it, hence he 120dB. At 110, I'd start to miss something with my lowest-level stuff. But if I were a metal head, it would be no issue. This is one reason I haven't gotten myself wireless cans yet: everyone I've tried seems to have an amp specifically designed NOT to go that loud.

    Note: As CoA mentioned above, DSP can start requiring some headroom as well. I save 6dB for my EQ + crossfeed, which can make for a significant change in my edge-case power requirements.
    Last edited: May 13, 2018
  7. ev13wt
    wow. 120dB.

    Are you guys looking for tinnitus?
  8. RRod
    That's safe room for max peak. For my classical listening, RMS is usually about 80dB(C). For low-mastered albums (-30dBFS RMS), that means about a 110dB peak, but that happens once in the album. It's not like Death Magnetic.
    ev13wt likes this.
  9. bigshot
    With classical music, it's more effective to try to lower your noise floor than it is to raise the volume. If you get too loud, it doesn't sound natural. Better to listen in a very quiet room at a normal listening level.
  10. RRod
    I find the volume perfectly natural, with the lowest material just a smidge above the noise floor of the room, which is 35dBA if i get things turned off, which is pretty good for an untreated suburban home. I have maybe a two-handful of albums that require this much juice, but there they are.
  11. bigshot
    Stuff above 80dB gets uncomfortable for me. I can’t remember experiencing anything louder than that in a concert hall.
  12. RRod
    My 80dBC above would go a bit lower A-weighted. I would be surprised if Mahler at front orchestra center didn't push out >80dBA at some point, though. My kids love this piece by Gubaidulina that at one point has 5 soloists beating bass drums hard right at the front of the stage; I can't see that staying at vacuum loudness.
    Last edited: May 14, 2018
  13. Bhargu
    I got a friend's SPL meter (which hasn't been caliberated in a long time) and measured my music again. I only had IEMs in hand, but they were pushing out 70-75 dB and peaking at 80dB at the maximum volume that was comfortable for me. I won't be able to listen at 80dB peaks for long sessions. How will that translate to with headphones?
  14. RRod
    I'll let someone like CoA handle that as they have more experience measuring IEMs. But would it be possible to get an example track? 75dB RMS with a 80dB peak isn't the kind of material that should require an external amp.
  15. Bhargu
    Well, I am not sure the peak was really 80dB. This SPL meter had some issues with transient peak measurement. I was listening to some classic rock music from 70's. I was mostly listening at 70 dB (could have been lower) volume average.

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