Loud enough volume vs. Fully driven

There is no difference. Though normally the RMS listening volume isn't "adequate" because you need extra voltage to ensure no peaks are clipped in quiet songs. I tend to see 110-115 dB recommended for adequate volume, but most people can survive with less with most music. There's also the issue of the finding an amp's sweet spot, where noise and THD is at its minimum, but well-designed amps (say the Objective2 for example) will be able to approach their maximum voltage with little increase in THD.

 

Headphone sensitivity is normally given in dB/mW, though sometimes dB/V. Every 10 dB is a doubling of volume, sqrt(10) times more voltage, and 10x the power (mW). To find the amount of power needed to reach 110 dB, for example, you take the headphone's sensitivity, subtract it from 110, then divide it by either 10 (if sensitivity is in dB/mW) or 20 (if in dB/V). Then raise 10 to the power of whatever you get, and that's the amount of voltage or power needed depending on what the sensitivity was in. Example:

 

LCD-2 has a sensitivity of 91 dB/mW. To get to 110 dB, you need 10^((110-91)/10) = 10^1.9 = about 79.4 mW.

HD800 has a sensitivity of 102 dB/V. To get to 110 dB, you need 10^((110-102)/20) = 10^0.4 = 2.5 Vrms.

 

Now the question is, how do you convert between Vrms and mW? This is where impedance comes in. The relationship is defined as:

 

P (power in Watts) = V^2/R (resistance in ohms)

 

So how much voltage does the above LCD-2 need? We plug in 79.4 mW and its 50ish ohm resistance:

 

0.0794 = V^2 / 50

V = sqrt(0.0794 * 50) = sqrt(3.97) = about 1.99 Vrms.

 

How much power does the HD800 need?

 

P = 2.5^2 / 300 = 6.25/300 = 0.02 W = about 20 mW.

 

So because of its low sensitivity, the LCD-2 needs four times as much power, but because of the HD800's impedance, it takes more voltage to get a quarter of that power.

 

Manufacturers rarely publish power into various loads, and often just quote a maximum Vrms. You can use these equations to estimate how much power various impedances will get. Remember though that since low impedances need power but not much voltage, they need a lot of current (P = I * V, I is current in amperes). Since manufacturers rarely publish maximum current, power will often be limited into low impedances. Then there's also the output impedance of the amp, which reduces the voltage available to low impedances. I can describe that if you want, but I don't feel like it.

^How do you explain headphones like the HE6? Most normal HP-amps (after my impression) drives them loud enough, but they are still not fully driven.

Placebo, or the amps distort too much at that voltage. Regardless, it has nothing to do with the amp's voltage itself, as long as you have enough for peaks. Point me to a valid physical reason you need more voltage than you ever use, and/or a blind test demonstrating the ability to tell one adequately powered and measured amp from one equally well measuring but more powerful amp. I'd be interested in seeing it.

 

You have to make sure you're volume matching closely, too. The more powerful the amp, the more volume you'll get from a little turn of the knob. Even a broadband 0.5 or 1 dB volume change may be audible, and will swing your vote in favor of the louder amp.

 

Keep in mind that the HE-6 is harder to drive than Hifiman claims. For whatever reason, their sensitivity spec is about 6 dB/mW higher than Tyll Hertsens measured. They're closer to about 77 dB/mW, so they need four times more power and 2 times more voltage than the manufacturer suggests.

Edited by Head Injury - 1/19/12 at 7:26am

 

So, the guy who made the headphone have a placebo and measured it wrong?

 

I always thought that with more power that amplifiers drove the speakers/headphones with more ease.

 

Makes me also wonder what guys are doing with monoblocks with too much power to drive their speakers...

 

Are you saying that they are all wrong in these aspects?

 

 

 

 

There are many things that audiophiles unnecessarily spend money on, despite not resulting in an improvement that can be proven with measurements to be significant, or heard in double-blind tests. Overpowered amplifiers are just one of them. As usual, there is a grain of truth behind the myth, since - due to poor design or implementation - some amplifiers do start to distort significantly well before reaching the clipping level, but this is not universally true, nor even typical with well designed and implemented newer solid state amplifiers.

 

Edited by stv014 - 1/19/12 at 7:56am

So if I personally get great volume out of my 600Ω beyers with an iPod, does that mean I'm driving them well?  Providing no clipping on recordings with 15+ db of dr of course.

Well, I do not know how good (or bad) the audio output of the iPod is, so it can only be decided for sure by doing an extensive set of measurements with that headphone as the load and at the intended volume setting. If there is no better equipment available, then at least by creating a set of test signals as a lossless audio file, playing it with the iPod (with the correct load and volume, of course), and recording it with a good sound card (one that does not significantly alter the results itself), and analyzing the recorded file.

 

Edited by stv014 - 1/19/12 at 8:52am

That's not placebo 

 

They measured it differently, or interpreted the results differently. That doesn't necessarily mean wrong, just not typical. Seeing as Tyll's measurements are in line with most other manufacturers, it stands to reason that for normal use and in comparison to other headphones Hifiman's measurements for the HE-6 aren't trustworthy.

 

"Ease" depends on your meaning. More power means higher peak volume without clipping, that's it. A 15 Vrms won't necessarily reach 7 Vrms any "easier" than a 7 Vrms amp. I'd say that depends on distortion. And if the 7 Vrms amp can reach 7 Vrms cleanly and then clips only after, but the 15 Vrms starts ramping up distortion from 3 Vrms onward, there's a good chance the 7 Vrms amp will have an easier time.

 

Like stv014 says, audiophiles are wrong about a lot of things. You have to consider the fact that they might be wrong about power, if they do stupid stuff like buy magic rocks to put on their amps. Most audiophiles aren't at all familiar with the physics of what they buy. They just see a bigger number, or a glowing review in a magazine, and they decide it must make a difference.

Another aspect you have to consider is electrical damping. A high output impedance coupled with a low-impedance headphone would result in a sound that could be categorized as poorly driven. iPods in particular tend to have a higher output impedance so they affect the frequency response of headphones specced at or under 32ohms. Although it's possible that the listener might prefer the sound of an underdamped headphone.

 

The HE-6 is an extreme example, most headphones are more sensitive by 20db+.

Also consider that  headphone impedance is just an average, it will vary significantly over the full frequency range, so an amp with 'headroom' will be able to fully drive the phones over the full range.

For "underpowering", it is only an issue when the impedance is significantly lower than the nominal value, because of the increased current requirement. This does not happen with full size headphones in most cases, but for example some 16Ω IEMs may drop to only a few Ω at certain frequencies. For an amplifier with high output impedance, any reactance is a problem because it changes the frequency response, but it has nothing to do with maximum power or headroom. A 120Ω output impedance with a 32Ω headphone is poor damping factor even if the amplifier can output 1000W of power. It is a common, but incorrect belief that impedance peaks at certain frequencies make a headphone (like the HD598) hard to drive because "high impedance requires more power"; in reality, the problem with such peaky impedance is making sure that the amplifier can still act as close to an ideal voltage source as possible, and that mainly needs low output impedance and insensitivity to reactive loads.

 

Reviving this thread because I think my question is related and there's no need to start a new thread then.

 

I recently got a LCD-3 and because I just like tube amps, I got what I thought to be the nicest pure (OTL, not transformer-coupled) one for my budget, a Woo Audio WA2. I love the combo, it's an audible step up for me and at that budget pretty much my end-of-game. But apparently this amp is not capable of delivering enough juice to my LCD-3, i.e. they are not "fully driven" (?)

 

Going by Woo's numbers (310mW@32ohm, 400mW@60ohm), at about 50ohm of the LCD-3, the amp would output an estimated 380-390 (didn't do the math), maybe some more with the upgraded tubes I use. I guess the exact number wouldn't matter because even Aude'ze themselves recommend something considerably larger: about 2000mW, and apparently the LCD-3 can handle up to 15W (15000mW?!) Transformer-coupled tube amps like the Lyr can obviously deliver that kind of power, but as mentioned above I just like tubes all the way.

 

Back in the real world, the loudest I listen to is a little past 12 o'clock, and during the burn-in (mostly due to the failing driver issue in the past, rather than any expected sound quality changes) I turn it up to the max and use them as quasi-desktop speakers. With no distortion that I can hear, after hours of varied music. I would never in my life put that on my ears, not even for the shortest peak at any frequency! So there should be more than enough power headroom left to ensure nothing gets clipped or altered at the volume I normally listen to...

 

Now, in an objective, electronic engineering (?) perspective, which part am I supposedly missing out on, and why? The impedance graphs I have seen for the LCD-3 are almost ruler flat, so there shouldn't any notable changes in treble/mids/bass etc relative to each other? As for "soundstage", how would a varying amount of power affect that, isn't it more a function of the actual recording (mic setup)?

 

Enlighten me with some sound science :)

Edited by TheGrumpyOldMan - 3/15/12 at 1:45pm

Those are the maximum power ratings (i.e. significantly more power than that for a longer period of time may damage the drivers), however, it does not necessarily mean that you need that much power. For the LCD-2 (rev 2), the few hundred mW of power is more than enough, assuming that the amplifier can actually get anywhere near that with low distortion. OTL tube amplifiers may not like power hungry low impedance headphones, though, but it is hard to tell without measurements.

 

Edited by stv014 - 3/15/12 at 3:25pm

Well, I have a new pair of HE-6s and have been driving them with my big Kenwood amp and an EF-5. Both sound great, the Kenwood requires less gain, not surprising. The EF-5 sounds more "pleasant" but the Kenwood completely lacks all background noise no matter where I set the gain, so to me it sounds more sterile. Then I got curious about the Little Dot I+. First, I put on my magical hat of bias removal, so that I could look at the whole thing objectively, and then plugged it in. I had to almost max out the volume for a typical listening level, and there was an even louder background hiss (though still not enough to be distracting while music is playing), but they sounded fine. Eagles, Monster Magnet, Cee-Lo, Dvorak, all good.

So, $130 hybrid amp drives $1300 cans without causing harm to equipment or listener. Shocking, I know.