Katz's Corner Episode 20: How Insensitive!

How Insensitive! Part 1 of a Series

The Four Questions
Every prospective headphone and amplifier buyer needs the answers to these four questions:

  1. Is my new headphone as sensitive as my previous headphone—what level will it deliver in comparison?
  2. Can my amplifier drive this headphone to an adequate level without introducing audible distortion?
  3. Will I hear amplifier noise through my new headphone?
  4. Does my amplifier have enough gain to drive an insensitive headphone?

These are basic questions, but headphone and amplifier manufacturers aren't doing much to help people answer them! Not without trying to turn us consumers into engineers. Sure, they publish specs. There's a standard that's supposed to help us, IEC standard 60268-7, which specifies how to report a headphone's SPL* under laboratory conditions. This is known as the headphone's sensitivity. For example, "97 dB/mW" means that the headphone will produce 97 dB SPL when it draws 1 mW of power. Sennheiser, on the other hand, has chosen to display the SPL the headphone will produce when fed 1 Volt at 1 kHz. It is possible to convert between the voltage and the power spec using the headphone's impedance and Ohms law.

Sennheiser's approach allows us to answer question 1, but only for Sennheiser headphones. So if you want to compare Sennheisers with AKGs, you'll have to dig out your calculator. Is it too much for us to ask for a spec that will allow consumers to easily answer all four questions, without having to become an engineer? Seriously, both milliwatts and volts are pretty lame because by themselves they do not let us answer all four questions. The idea of using headphone power consumption (in milliwatts) is silly anyway, how did that get started? A manufacturer has to calculate the power draw only to satisfy the requirements of IEC 60268-7. Using power to specify sensitivity is totally indirect because we have to convert to volts anyway. It's like trying to specify a car's gas mileage as "miles per cubic foot of gas tank volume". Yeah, if you also know the volume of the gas tank, and can calculate the math, you might be able to convert that to miles per gallon. But why not specify performance in a human way that we can use? The IEC spec can only answer question #1, and only if the headphones have the same impedance. But headphone impedances vary all over the map, so that makes the IEC approach pretty lame. The Sennheiser approach is just a bit more useful.

The impedance determines the power drawn, but the voltage determines the SPL!


A dB is a dB is a dB!
It is totally possible to specify headphone sensitivity and amplifier output so that consumers can easily answer all four critical questions just by reading a spec sheet and sometimes doing simple addition or subtraction. Let's compare some headphone models which have wildly different sensitivity and impedance. Look at the yellow section of Fig. 1, which displays the manufacturer's specifications. Since Sennheiser defines the HD 650 sensitivity at 1 volt, I converted that to watts just to fill a hole in the chart.


Fig. 1 Headphone Sensitivity Chart

Our goal is to evaluate amplifiers and headphones together, so we have to marry them with the same units of measurement. Later we will have to convert amplifier specs from watts to volts. Since manufacturers have not standardized on a single SPL, I converted all the headphone specs to the voltage necessary to produce a common SPL, 97 dB (column 6).

I recommend that an amplifier should be able to deliver at least 110 dB SPL at or just below clipping (nominally 1% THD). Of course that would be 110 dB on momentary peaks, not sustained level, as we do not want to go deaf! I would not advise doing continuous sine wave testing at 110 dB SPL, it could damage some headphones. Notice that only 0.020 watts (20 mW) is necessary to drive an LCD-4 to 110 dB (column 8). We can see that power is the red herring: there no relationship between the wattage and the SPL of headphones with different impedances. Instead, direct your attention to column 6, "Volts @ 110 dB SPL." We can scan the column and observe which headphones require the most and the least voltage to deliver the same SPL. For example, the LCD-4 requires 2 volts to deliver 110 dB. But the one which requires the most voltage (the most insensitive phone) is the Sennheiser HD650, which requires 2.2 volts to deliver 110 dB.

Using voltage and a common SPL is a step forward, but not enough. Voltage is a linear measurement, as opposed to logarithmic decibels. Decibels make our life far more easy. Take a look at the green area of the chart. In the penultimate column I convert voltage to dBu** for 110 dB SPL. We can read dBu directly on any standard audio voltmeter. dBu is so much more useful than volts, because there is a simple relationship between dBu and dB SPL. If we raise the dBu level at the headphone or at the amplifier's input by 1 dB, the headphone SPL will also rise by 1 dB. We can directly (and meaningfully) compare the sensitivities of any headphones when they are expressed in dBu. For example, we see that the LCD-4 requires +8.2 dBu to drive it to 110 dB, and the HD650 requires +9.2 dBu. So the HD650 is 1 dB less sensitive than the LCD-4. The HD650 and the LCD-4 are poor matches for cheap headphone amplifiers, which often overload as they cannot deliver the required level.

(Editor's Note: It's important to note that Bob's article is based on published specs and not measurements taken on the same system. My measurements show the LCD-4 as being less sensitive than the HD 650. In my opinion, this discrepancy is yet more evidence that the IEC spec needs a tune-up along the lines Bob is drawing. Even though his numbers may not reflect reality, his logic is sound and his point is on target.)

Without using dBu, these facts would not be obvious from the manufacturer's published specs without rolling out your logarithms. Our chart does this for you. Manufacturers may not have anything to hide, but by using ineffective units, they are hiding the obvious facts from us. For example, in the last column, by using the LCD-4 as the reference, we can immediately see that the Oppo PM-3 is a whopping 14 dB more sensitive than the LCD-4 and the new Audeze LCD-MX4 is 18 dB more sensitive! 18 dB is the difference between a shout and a whisper. This clearly reveals an imminent danger: If you plug an LCD-MX4 into your headphone amplifier after listening to the LCD-4 without changing your volume control, you will damage your ears! It will be 18 dB higher! With any headphone amplifier, turn down the headphone volume when switching headphones.

In summary, we need to know that an amplifier can deliver a particular voltage into the load of the headphone, and we need to express that voltage in dBu. If manufacturers and reviewers specify headphone sensitivity and amplifier capability in dB SPL and dBu, then we can directly compare amplifier and headphone performance by simple addition or subtraction. So I urge every manufacturer to provide dBu specifications. This should not be hard. Then eventually I hope that the IEC will revise its standards to use dBu for the reasons I describe here.

For example, let's say that Audeze and Pass employ or include dBu in each of their specs:

Audeze LCD-4 Sensitivity: 97 dB SPL at -4.8 dBu

Pass HPA-1: Maximum output: 22.2 dBu

Decibels make our life easier! We can simply add 97 + 5 + 22 to learn that the Pass can drive the LCD-4 up to 124 dB SPL. A ridiculously loud value, but worth knowing nevertheless. It means the Pass has tremendous headroom—which is a big deal for headroom junkies like yours truly. I hope that this idea will spread among headphone enthusiasts and researchers around the world. In the meantime, I will make my spreadsheets available to you after I finish writing this article series. You want the mysteries solved easily, so feel free to post the specs of your favorite headphones and amplifiers in the comments below, which I will add to the spreadsheet. Inquiring minds want to know dBu, and I'll be glad to help. For amps, post their rated wattage, load, and if available, voltage gain and noise. For headphones, post their IEC or Sennheiser specs. Hint: One volt is 2.2 dBu, so Sennheiser specs are made at +2.2 dBu. If you're already given dBu, the rest is simple addition and subtraction!

Let's look at the Amplifier and DAC chart.

KatzCorner_Ep20_Table_Fig2 Fig. 2. Amplifiers, DACs and Headphone SPL (work in Progress)

For the Pass HPA-1 I used John Atkinson's measurements from his Stereophile Magazine review. Let's see if this headphone amplifier can drive the most insensitive headphone to a satisfactory listening level. In other words, does it have adequate voltage gain? I feel that -12 dBu (0.2 v) is a reasonable "typical" unbalanced analog output level of a DAC at -20 dBFS. I recommend that an amplifier must have enough gain to drive an insensitive headphone to at least 85 dB SPL with -12 dBu input. I recommend that a DAC with built-in headphone amplifier be able to drive an insensitive headphone to at least 85 dB SPL at -20 dBFS. This ensures that the quietest, most dynamic music (such as a well-recorded classical symphony) can be reproduced adequately loud.

For example, as seen in green on the above chart, the Pass HPA-1 can deliver 97 dB SPL from the Sennheiser HD650 with a -12 dBu input, with volume control at maximum. So it has about 12 dB more gain than my minimum recommendation, which is probably a good thing since it accommodates occasional lower level sources, and situations where a listener needs to bring up soft passages while editing or recording. The extra gain also helps accommodate EQ plugins which may include some digital attenuation.

The Prism Callia's headphone jack, shown in the green section of the chart, can produce 99 dB SPL at -20 dBFS from an HD650 at maximum headphone volume. That's about 14 dB more output than my minimum recommendation. So both amplifiers can produce more than adequate level with the most insensitive headphones using any reasonable musical source.

The Callia DAC has a DIP switch in the back that sets the headphone amp gain. The manual attempts to simplify the process by describing the switch settings in terms of headphone impedance, low, medium, and high. But this misleads, since this is just a gain setting: Its "high impedance" (actually "high gain") setting is specified as +18 dBu at 0 dBFS. I then subtracted 20 dB to arrive at the level the Callia would produce at -20 dBFS (column 4). The output impedance of the Prism remains at a constant 4 ohms. The user may leave the switch in the so-called high impedance setting, even when using low impedance headphones, just adjust the headphone volume control accordingly. Change the dip switch only if the adjustment range of the volume control becomes crowded or you hear amplifier hiss when the headphone volume is turned all the way down.

Another useful paradigm is to express an amplifier's noise floor in dB SPL, shown in yellow in Fig. 2. I calculated the SPL of the amplifier's noise with a very sensitive and a very insensitive headphone. You can see that the Pass would produce only 18 dB SPL (wide band) with the LCD-4 and 32 dB with the Oppo PM-3. Both noise floors are probably inaudible except in an extremely quiet room. Especially, 18 dB SPL (measured wideband) would be insignificant. When I get a Pass for review I'll listen to its noise with the sensitive Oppo cans and verify if it is truly inaudible. These values were easy to calculate with simple subtraction: For example: +8.2 dBu is required to drive the LCD-4 to 110 dB SPL. The amp's THD+Noise floor is -83.8 dBu. Thus, 110 - 8.2 - 83.8 = 18 dB SPL. See how easy decibels are to work with!

Distortion Measurement in Decibels
Just like linear volts, linear % distortion really doesn't tell us the magnitude. That's why it's helpful to express THD and noise specs in decibels, even better in dB SPL of each partial for a particular headphone. We can estimate whether a hum component will be audible or the audible importance of harmonics. A further refinement would be to weight the spectral graph according to the equal loudness curves, which I may attempt to do sometime in the future. Even more powerful would be to calculate psychoacoustic masking of fundamentals, harmonics, and noise. But masking calculations are beyond my current knowledge. Worldwide, I think there are only a handful of psychoacousticians capable of intelligently quantifying masking perception.

The Final Proof is in the Listening
In part 2 of this series, I'll refine these spreadsheets and present more amplifier measurements. We're also going to verify these recommendations by critical listening. I'm a self-confessed headroom junkie: personally I've found that, in general, the more powerful amplifiers sound best, even if we never play them loud. To my ears they seem to have more impact, perhaps because they are not compressing musical peaks. Is a half watt amp enough? If so, then why does Audeze recommend between 2 and 4 watts to drive the LCD-4, even though the charts show that only 20 mW will drive the cans to 110 dB SPL? I intend to find out the reasons for this discrepancy, and present recommendations that we can quantify and help you make educated choices.

Here's a photo of "Amplifier Jungle" chez Katz. I hope to have two more representative contenders on loan for review so we can compare amps from the smallest to the biggest, with the least and most sensitive headphones. Stay tuned!


Fig. 3. Amplifier Jungle. From left to right, top to bottom. JDS Labs O2, Audeze Deckard, Mjolnir KGSS Carbon HV, AMB M3 (Katz custom build), Prism Callia DAC/Amp, Mjolnir Pure Bipolar

Footnote: The Meaning of Sensitivity
Keep in mind that headphone sensitivity is an approximation. It's usually measured using a sine wave tone. 500 or 1 kHz. Of course sensitivity will vary according to the headphone's response at other frequencies unless the headphone is perfectly flat (and none of them are). But 1 kHz is a useful place to compare one headphone's sensitivity with another's. Impedance is also a nominal value: Planar-magnetic headphones usually exhibit a very even impedance across the spectrum, so if the manufacturer says "200 ohms", it's likely to be maintained across the audible range. But moving coil headphones' impedance varies across the spectrum, often significantly.

As long as an amplifier has a reasonably low output impedance, headphone impedance variations should not cause a problem. Many say 1 ohm is acceptable. Headphone amplifiers built into gear like receivers and integrated amplifiers often employ buildout resistors, as much as 80 Ohms or more in some cases. This is an outdated philosophy, it ruins damping factor and makes insensitive headphones harder to drive. Even when the headphone is direct coupled to the output stage, manufacturer's philosophies differ. Some advocate having super low output impedance, which requires large amounts of negative feedback, but I feel that extreme negative feedback can lower the total distortion so much that auditory unmasking begins to take place: bad harmonics might become audible when they are no longer masked by the good ones.

Both Mjolnir and Pass's philosophy for their discrete amplifiers is that the amp should perform with reasonable distortion (say, under 0.1 % THD at normal listening levels, assuming the major harmonic contributions are 2nd and possibly 3rd). Both manufacturers point out that super low distortion (say, 0.003%) can produce harsh or bright sounding results because of masking and unmasking. I personally prefer an amplifier which sounds just a little on the warm side versus one that's subjectively cold, dry, bright, or harsh. As long as the amp doesn't sound too warm or artificially "tubey". Loose, unregulated power supplies can make an amplifier sound "flubby" and thick, like some tube amp designs of the early 50s. We have a lot to learn, and in this series I will try to sonically compare the sound of amplifiers which have vanishingly low distortion and high negative feedback versus those with more reasonable distortion.

* SPL, Sound Pressure Level is not loudness! It is a measurable physical quantity, but loudness is a perceived quantity. SPL is related to loudness, as a higher SPL is usually perceived as louder. But to quantify the human perception of loudness requires knowledge of frequency content (among many other things). Here's a chart of SPLs of typical sound sources.


** dBu stands for "dB unterminated." dBu is voltage expressed in dB with a reference of 0.7746 volts, usually rounded to 0.775. I believe the term was coined by Neve Corporation when matched impedances began going out of style. Most audio-rated voltmeters can read dBu directly. The scale may read "dBm reference 600 ohms", but it's actually reading voltage in dB with a 0.775 volt reference. dBm is a measurement of power that once was very relevant but not for many years.

Vinhcomputer's picture

"Instead, direct your attention to column 6, "Volts @ 110 dB SPL.""
Do you means column 7, since in the line "Notice that only 0.020 watts (20 mW) is necessary to drive an LCD-4 to 110 dB (column 8)" before that, column 8 was about Watts @ 110 dB.

Bob Katz's picture

Yes, you're right. I knew about the error in the text, but I figured since the columns aren't numbered but they are labeled someone would easily see the correct column. Let me know if it bothers you terribly and I'll ask Tyll to correct the typo.

tony's picture

It sounds good and I like the looks of the darn thing, plus it's well liked by reviewers and peers, I guess I'll buy it, ca-ching. It goes to the Closet if it doesn't work out. Then, read more reviews and buy another Amp. This keeps Audio Dealers in business ( and makes 'em crazy ). Welcome to Audiophila, Analog Planet and a house full of gear or eBay reselling.

Isn't this how we selected Amps for our own ownership?

I got an Asgard recommendation from Tyll and Steve G., back in 2011, I still like the little Amp., mine is an Asgard 2 ( I no longer will say it's a Shit Asgard 2 because I'm not happy with the Company's shitty name ).

I've felt that Dan D'Augostino has some secret that has his designs winning widespread "love" and high pricing levels. Dan seems to hint of his "headroom" and build quality. Hmm. I've never owned one, I'm a wanna-be Krell owner.

Every reviewer seems to own something from N.Pass, is there a 'why' to this?

My frustrated answer to Amp buying is to buy Active Loudspeakers and "live" with their levels of Professional performance. The Meridian stuff is wonderful.

Are we about to learn about amplifier performance relative to transducers?, bring it on, "I'm all Ears".

Thanks Bob

Tony in Michigan

castleofargh's picture

"why does Audeze recommend between 2 and 4 watts to drive the LCD-4"
my guess is that they're lazy and have kept the same stuff since the first models, when the headphones were both low sensi and low impedance.

I find the "more power is better" quite annoying because it's so simple to agree with it and forget about the rest like most audiophiles already do. of course in isolation it seems right. need more, have more. don't need more, that's ok. but in that great nonsense, why bother with headphone amps? let's go for a 200W speaker amp it must be so good. more is better! and the guy ends up with background noise, and the most impractical volume knob for his needs. yay!!!

if you give me the same noise floor in volt(or dBu or dBV^_^) and same low distortions, then sure, why not have the more powerful amp. but discussing power without accounting for other variables, bad idea. I'm not talking about you obviously, I'm talking about people who don't have a clue and will take big power numbers as their lead variable thanks to misunderstanding what you said.

as for your dBu translation of sensi, while I have nothing against some change in impractical half baked standards, I'm not sure you're idea would solve much.
those who never bothered learning about the use of impedance and sensi are unlikely to make more efforts to learn about dBu(and probably didn't read your post until the end).
as the amp output is still dependent on the load's impedance anyway, in the end we just replace one unit by another and now have 3 units instead of 2 for sensi that only ever required one ^_^. if it became a universal standard, fine, but otherwise I don't see this helping in practice.

Bob Katz's picture

Maybe you'll be more enlightened about my points and change your mind after you read parts 2 and 3 of this series. +22 dBu max level seems to work for me... It's got the headroom and preserves the transients. If it doesn't work for you, then you can keep your low powered amps. No skin off my back.

jagwap's picture

I have worked in pro audio. Having spent some time on the headphone forums the general public, well the headphone obsessed general public, have a some confusion on this. I completely agree that the current sensitivity standard is unhelpful to the people looking on the side of the box. But I think Tyll's measurement is more useful: Volts rms to give 90dB. This is particularly useful for checking if you phone or DAP can drive a headphone. The other way around such as dB SPL at 0.1V rms is also more intuitive.

Great that you are addressing this, and I look forward to the next parts. I wish someone would do the opposite for speakers, as the sensitivity there is generally given in dB SPL per Watt, but 1 watt is for 8 ohms, regardless of the actual impedance. So really 2.828V rms. This means speaker manufacturers have been lowering impedances for years to get better sensitivity numbers. (I've seen budget speakers with 1.2 ohm minimum impedances from big name manufacturers)

miko64's picture

If you look at Pass PHA-1 spect you see power 20 Ohm 3.5W and at 300 Ohm 200 mW, which means roughly 300 mW at 200 Ohm. This might explain why Audeze might suggest 2-4 W... At the same time a 2W at 200 Ohm Amp would yield (if not current limited) to a 100 W amp at 4 Ohm - Such as Pass XA30.8

Bob Katz's picture

The only reason I mentioned "why is Audeze recommending 2 to 4 watts" is that we're still living in what I would call a transitional world and I needed to quote the manufacturer's exact words. However, the point of my article, which at that point in the text should have already been made.... is that the so-called "3.5 watt" amplifier is really an amplifier capable of producing +22 dBu into your load of choice. It's the volts, guys, logarithmic volts to be exact. I don't care if amplifier manufacturers continue to specify the wattage, but I do want to seem them publish the dBu level at maximum. It makes too much sense. Headphone amp manufacturers should have been specifying dBu from day one.... watts was the red herring and if they never used it you never would have missed it. It's truly irrelevant to start computing that at 200 ohm it's 300 mw or whatever. IRRELEVANT... it's the volts, guys! That's what drives the SPL, not the watts.

Bob Katz's picture

...Even loudspeakers and power amplifiers will benefit from using a dBu spec. Then we can convert the amplifier's output to the loudspeaker in dB SPL without even using a calculator, if you can add 2 columns of numbers in your head :-)

detlev24's picture

Actually, there are manufacturers who do specify in dBu - plus provide comprehensible measurements; so one can objectively predict the amplifier's performance on a specific headphone very accurately.

Tho whom that are interested: just have a look at the technical specifications and measurements [see manual] of the RME ADI-2 Pro.


I really do not understand why such a great device is being ignored by the vast 'audiophile' majority - as it represents one of the best and certainly the most complete source money can buy, especially for headphones. It drives probably 99% of all headphones to high volume with plenty of headroom left for peaks! [One exception I can think of is the AKG K1000, which would need an even higher output >level< than provided by the ADI-2 Pro in balanced mode.]


Bob Katz's picture

RME has always been an excellent company from the technical point of view. I have three of
RME's professional MADI interfaces and have had other RME products in the past. These are digital in and out interfaces. But I've leaned away from their ADCs and DACs, preferring the sound quality of other professional and consumer products.

Another excellent company whose products I've always admired is Benchmark. I purchased their first DAC 1 and gave it a rave sonic review in Pro Audio Review Magazine many years ago. I imagine its successor is an even better sounding and performing DAC. Benchmark has always had excellent technical specifications and they also specify their headphone output level in dBu. Maybe we'll start a movement!

castleofargh's picture

I'm not telling you anything new when saying that pro gears often provide dBu. even my cheap focusrite 2I2 gives dBu values.
and I use it to feed cheap active JBL speakers where the input gain let me chose between +4dBu and -10dBV ^_^. which annoys me more than it should.
even then, the information is related to the integrated amp. it's not for the drivers.
IMO, headphones mostly give values related to watts because amps usually provide max power output into a load. change the amp specs and the headphones will probably follow. but I'm not sure the average consumer would know better that way.
or if it would just as I said give a third way to express the same thing that some would use on purpose. you know those manufacturers who systematically pick a different or incomplete nomenclature so that people will have a hard time making direct comparison with the competition.
even expressing max power without clearly saying if it's per channel and @1%THD or 0.1% or just plain max output, should IMO be illegal. on DAPs they often don't even bother specifying the load... anyway, I'm starting to whine again^_^. don't take my suspicions as me saying everything is fine and you're wrong, it's really not, I hate how unreliable/unclear specs are in consumer audio. I'm only afraid it would end up creating more confusion in the wrong hands(marketing!!! I mean marketing!!!!).

coastman25's picture

Here is a person trying to tell you what is and instead of honouring that and appreciating it you instead castigate, criticise for mere typos, and fail to appreciate the value this guy is telling you.

Tyll & Katz will not be around forever yet they represent what matters in the audio world. No wonder you people have elected a stupid moron as your president because it’s exactly what you deserve.

You guys would not know genuine sincerity & honesty if you met it in your soup!

tony's picture

Dear Sir,

We did NOT elect a moron. The "moron" was elected by "the Swamp". The moron was 3,000,000 Votes Short of Winning. The Good and Honest Citizens of America are ( rightfully ) in Shock over all this!

Otherwise, I agree with your valuable observations.

Tony in Michigan

ps. Can you share your experiences with SET amplifiers?

coastman25's picture

Hi Tony
When you ask about SET amplifiers do you mean single-ended triode vacuum tube electronic amplifiers that use a single triode to produce an output, in contrast to a push-pull amplifier which uses a pair of devices with antiphase inputs to generate an output with the wanted signals added and the distortion components subtracted? On the other hand, is this your attempt at humour?
Perhaps you could share with us your thoughts and experience about why there is too much lead in the water in many parts of Michigan and seemingly throughout the USA?
Chris in Thailand

tony's picture

I'm serious about SET amps.

I was a Dealer/Retailer for Audible Illusion PreAmps and loved their musical performance, I just learned that they were SET types. I'm told that even Solid State stuff can be SET types. Hence the Question

Lead in Water is a Govt. infrastructure issue.
Americans feel themselves to be Capitalists, wish to be free from Govt. Regulations and burdens of taxation.
Americans with sufficient funds will purchase bottled water & water purification equipment. Americans don't desire equality with the suffering poor. Being poor is the important curse/distinction defining the lower 50% of our population. If the poor had money they would get priority from decision makers, insuring things like clean water.

Iowa is in central United States, it is Corporate Farm Country, it has the most polluted water in the country, their pollution comes from fertilizers, their solution comes from treatment plants. Iowa's citizens buy their water privately.

As I visit Europe I get the distinct impression that Europe is far more civilized than the gun toting, Wild West United States. Of course, that could simply be my Auto Industry & Corporate point of view. I do admire the Socialist Systems of Europe but Europeans don't seem to own large houses in Michigan's finest neighborhoods or nice Florida get-a-ways on the Gulf Coast.

The United States is an exclusive Country Club with yearly Dues and up-front Membership Fees. We are in the Deferred-Maintance mode and we are in debt for about 20 Trillion Dollars but we can still afford a Moron as our Spiritual Leader. Go-figure

Tony in Michigan

coastman25's picture

At first, I thought you were being sarcastic. Then I Goggled ‘Audible Illusion PreAmps’ and there is such a company! Surely, people would prefer genuine audio bliss rather than an Illusion of it or perhaps it does not matter.
I do not have a PreAmp and I am more of a solid-state type of person. Perhaps it is something in the water.
Thank you for your very informative, insightful and illuminating take on the USA.

Chris in Thailand

tony's picture

Hell yes, I'll buy that.

How do y'all find "Bliss" in Thailand?

I hope that you're a full line Linn guy orrrrrrr maybe a full-on Meridian guy ( I imported both, back in the 1980s ).

I'll be disappointed if you are satiated by a measly Bose Wave Radio or a JVC Rack System, for gods sake.

But then,

You might be a Recording Studio guy with Pro-level stuff. Hmm. ( suggesting Nagra )

Tony in Michigan

DanWiggins's picture

In general, I agree that sensitivity/efficiency ratings can be confusing. But there are a few things to consider:

- A transducer is a current drive device. The Lorentz force is what causes the diaphragm to move (for dynamic, balanced armature, and planar magnetic transducers). That is based upon current flowing in the voice coil (or trace, in the case of planars). So to a certain extent it makes sense to consider power, as it is really current that dictates your SPL (SPL is proportional to acceleration, and acceleration is proportional to current).

- Voltage sensitivity can be gamed, and in the headphone world it can be very deceptive. The easiest way to raise your voltage sensitivity is to cut your impedance. V=IR, SPL is proportional to current, hey I just got free SPL by adjusting R - nothing to do with V or the other transducer/system parameters at all!

In the pro/home world, most amplifiers have output impedances measured in tenths of an Ohm or less. Lowering the impedance of your speaker from 8 Ohms to 4 really won't cause much of an issue - as long as you have a fairly low output impedance. But when the load starts to approach the output impedance level (the load is just a couple of multiples higher than the output impedance) then you start to get frequency response modulation NOT from the amp or transducer, but the complex impedance-based voltage divider of the load to the output impedance.

Take a very complex, low impedance speaker (like the Infinity Primus P363: http://www.audioholics.com/tower-speaker-reviews/primus-p363/primus-p363...) and hook it to the 16 Ohm taps of a tube amp. Measure the frequency response. Now hook it to a quality solid state amp with a 0.1 Ohm output impedance and measure the frequency response again. BIG differences!

Most cell phones have 4+ Ohms output impedance, and many headphone amps are still well over 2 Ohms. If you have a nice 30+ Ohm impedance for your headphone, this isn't an issue; the modulation will be minimal. But what about those headphones (typically those with multiple transducers inside) where the impedance not only is highly complex and full of peaks and dips, but very low? Some IEMs reach down to the 10 Ohm or lower level. Sure, that gets you lots of voltage sensitivity - but you also have a LOT of frequency response shenanigans going on with just about every amplifier you may use, because the odds are the output impedance of the amp is a significant portion of the load impedance.

So all that said, yes, voltage can be a great way to spec (and is also the way that EN50332 sets sensitivity maximums), but it does come at the cost of being easily "gamed" by cutting impedance, and such game-playing will almost always result in a lowered sonic experience because of the output impedance of most of your amps. Specifying sensitivity with power tends to reduce that, because whilst it is still dependent upon voltage, that is only half the equation; the other half being current. So the impact is lowered.

Much like specifying pro/home/car audio transducers. You may see 98 dB SPL @ 2.83Vrms for one woofer, and 96 dB SPL @ 1W for another. Which would you choose? Well, if the former was a 4 Ohm speaker, and the latter was an 8 Ohm speaker, I'd take the latter. Sure the "number" is lower (96 versus 98) but you can hang two of the latter off a single 4 Ohm stable amp, and gain a full 4 dB in output relative to the former.

Just some things to think about! In fact, Tyll covered this very thing: https://www.innerfidelity.com/content/musings-headphone-amplifier-output...

jagwap's picture

speaker sensitivity states dB/W, but it is always dB/W @ 8 ohms, meaning 2.828V rms. So while you are correct if the industry actually used watts as the reference, but they don't.

You make so good points, and lowering from 8 to 4 ohms should not be an issue. But when big name speaker manufactures state their impedances as 8 ohms, minimum 3, then it is obvious there is a lot of spec-manship in their marketing numbers. Once you go south of 3 ohms and add a large phase angle, you start nudging the safe operating area of a lot of amps, and you get the limiters cutting in without the listener's knowledge (if that is how the protection is set up).

DanWiggins's picture

I've been designing transducers and audio products for 25 years, and I am not aware of any standard that says speakers rated at dB SPL/W are always referenced to an 8 Ohm load. In fact, it is quite common in the transducer world to list both sensitivity (dB SPL @ 2.83 Vrms) and efficiency (dB SPL @ 1W, 1m) specifically to address this issue of sensitivity changing with impedance.

Bob Katz's picture

It would be nice if every manufacturer displays the amplifier's gain at 2.83 volts out, or even 1 volt, whatever floats their boat, for the moment. As well, of course as its output impedance and whether it may change its output voltage whether loaded by 4 or 8 ohms. It is the nitty gritty that gets us every time!

jagwap's picture

Let me put it this way: There are a large number of loudspeaker manufactures who specify sensitivity in dB/1W, but use 2.828V rms, while having a 4 ohm or less product. These are big name famous brands.

Bob Katz's picture

But I don't care whether manufacturers "game the system" because ultimately, it's the voltage capability (into the prospective load) that will determine the headphone's output level. Power specs for headphones were not invented to try to make it fairer.... well, honestly, I'm not sure why they came about, but it is an IEC standard. I just glanced at the standard the other day and I see that an alternate is permitted: the number of volts required to drive the headphone to 94 dB SPL. I'm not sure why manufacturers didn't adopt that portion of the standard instead of the silly milliwatts. Dan, you obviously know how that's computed, but I'll summarize it for our readers:

1) The manufacturer measure's the headphone's impedance at 1 kHz (although the IEC specifies 500 Hz I'm not sure how everyone disobeyed that and started doing 1 kHz, but I digress)
2) He calculates the voltage which would produce 1 mW across that impedance
3) He measures the SPL that the headphone produces with that source voltage --- notice, not current, not power.... he uses a voltmeter for this and hopefully a full official manikin test head like Tyll has
4) He publishes the SPL with 1 mW, and does not publish the voltage, even though he measured the voltage first.

Crazy, silly, indirect procedure. Time to see a psychiatrist. :-)


amartignano's picture

Hi Bob, very interesting article, I've tried to play a bit with calculation with other headphones and some amplifiers. See




For headphone sensitivity comparison I choose to show SPL @ 0 dBu (= @ 0.7746 V), then I've calculated the max theoretical SPL for the given headphone/amp combo.

I ask you a question: give the results, am I wrong if I say that the best headphones for low powered amp are what they are because they have the higher dB/V sensitivity, and that the low impedance has nothing to do with their easyness of amplification? Also because even the most underpowered amp saturates at higher voltage for higher impedance.


Bob Katz's picture

You're on the right page with those spreadsheets. In Episode 22 (two episodes down in this series) I measure the clipping level of various amplifiers into both a 20 ohm and 110 ohm load. If you're anxious to see that spreadsheet before it's published here at Innerfidelity, drop me a line through my website, digido.com.

Yes, I am saying that the low impedance has little or nothing to do with the high sensitivity. It's the fact that the amplifier does not have to deliver a very high voltage, given the VOLTAGE sensitivity of the headphone. The amp simply has to be able to feed that voltage to the phones. Still, it's important to measure the distortion produced by that amp when it delivers that voltage into a low impedance load, in case it runs out of steam, as some might do.

Regarding the low impedance cans: Basically it is the voltage that drives the headphone, but lower impedance headphones are more sensitive because more current flows through the moving coils, delivering more output SPL. But even though it is more current... which could starve some amplifiers, because the headphones deliver more SPL, the amplifier doesn't have to produce a very high voltage, and so (usually) doesn't get starved.... and so even battery-operated mobile devices, which don't deliver much voltage, can produce enough level from typical low impedance phones.

The problem usually arises in the other direction, with the high and medium impedance phones, which are very insensitive, they require very high voltage to drive them to a given SPL. Battery and under-powered amplifiers can't deliver that voltage. It doesn't require much wattage, because watts will be lower, which is V squared/Z, and Z is much higher. And the wattage and the impedance become irrelevant from that point on. All we need to know is the voltage and we can compute the output level of the phone with that amp.

Does that help?

amartignano's picture

Thank you, this was really helpful to understand how the things work.
I try to repeat to see if I've understood correctly:
headphones are "easier" to make them reach high SPL if they have higher dB/V sensitivity. It happens that usually low impedance headphones have also high sensitivity because of the higher current running through their coils. But this is not a general rule: for example the AKG K701 is a low impedance headphone that has fairly low sensitivity (97.7 dB @ 0 dBu calculating from Innerfidelity's measurements), while the Sennheiser HD660S is an higher impedance headphone with a quite mid-high sensitivity (101.8 dB @ 0 dBu from declared data, 104.6 dB (!) from Innerfidelity's mesurements). Given that amps saturates at least at the same Voltage for higher impedance (generally at higher voltage), is the higher impedance HD660S the "easier to amp" between the two.
I'm very interested in your amp clipping measurements!

Bob Katz's picture

Your summary is good. Let me add this see if it makes things clearer for any of you: If you start with sensitivity watts (milliwatts) for headphone A and for headphone B, and they have different impedances, then you cannot directly compare sensitivity A against B. It will be completely meaningless.

If you start with volts you're a bit better but you still have to go through logarithmic calculations. But if you start with dBu you can directly compare one headphone with another just by subtraction. If every headphone manufacturer standardized on, say, 1 volt and supplied the SPL, then again you'd have a direct comparison by addition or subtraction. As soon as we bring ampliifers into the equation, you'd still have to jump through hoops unless we made dBu the common language for both headphones and amplifiers. Then it's just simple addition and subtration. Does this help?

Coming soon! This wednesday, the 6th, we'll release the next episode in this series, my subjective article on the sound of the amps. Then the week after, the measurements (including clipping points). If you can't wait two weeks for my spreadsheet, send a comment in to me at my website, digido.com and I'll email it to you.

amartignano's picture

Thanks Bob! Very clear. I'll write to you through the website!


mariscosyketchup's picture

I've see you listed the LCD-4 with a 97db sensitivity, however, Tyll measurements show a 90db sensitivity. Due to Audeze's unit to unit variation track record, what's the real sensitivity of LCD-4, 90 or 97db?

Thanks for the article Bob

Bob Katz's picture

Thanks for your kind words. That's a big difference, 90 to 97!

The problem you bring up is exactly the same as Tyll brings up in an editors note in my column. Audeze would have to tell us the exact method by which they make their measurements as well as whether the sensitivity of the headphone has been a moving target. It would be very useful if Tyll, who has an official head, and someone else (Sean Olive?) who has the same head, would measure the same physical Audeze headphone so we can see if at the least those two measurement systems agree. In the meantime, this discrepancy dilutes the strength of my article, which saddens me.

amartignano's picture

I think it's more a problem of the Audeze errating specs: for example in many "old" reviews the LcdX is rated 95 dB/mW, while now on the officail site is rated 103 dB/mW, we don't know when and where they created the 8 more dB. Headphone.com even reports them 103 dB/Vrms (clearly an error). We really need a common and understandable way to report sensitivity.

amartignano's picture

I think that it's also interesting to see the SPL values per current flowing in the coils, in order to see which headphones are more "inherently" efficient, or better which headphones are able to more efficiently convert the current into membrane acceleration, wich should be proportional to the magnetic field (B) and inversely proportional to the driver mass. Data in dB/mA column (the last in this figure).


No surprise to see Beyerdynamic T1 tesla (high B) drivers shows to be the more capable to produce acceleration for a given current, and no surprise also the see LCD4 more capable than LCDX, given the claim of an high B in the promos.
It's maybe surprising that some high efficiency headphones are highly efficient because of the low impedance and subsequent high current flowin, but NOT because they are inherently capable of convert current into acceleration = SPL: indeed they are very "inefficient" in doing this.

Quite interesting I think :)

Max Eliaser's picture

You need power to produce deep bass. Your fire alarm can play at extremely high volumes using a minuscule amount of power, but to achieve the same SPL at 40hz requires some very specialized and high-powered equipment. I believe the same applies even to a headphone with a ruler-flat frequency response-- lower frequencies require more power. Hence, a headphone might sound fine on an underpowered amp at the standard 1khz test frequency, but it'll clip in the bass range.

Therefore, I don't think it's fair to call power output a red herring.