[SanjiWatsuki]

Hey Head-Fi!
 
I've been seeing this myth floating around a lot and I feel like I'm constantly correcting posts about how hard it is to drive a headphone on this. The worst part is that new users read this generalization and then start telling other people that impedance is how hard it is to drive a headphone, and then more new users read those posts...
 
Like most myths, this one is rooted in some truth. Generally speaking, high impedance headphones will be more difficult to drive, especially those with 300 to 600 ohm impedance. This is not an absolute!
 
The second part of the equation is the sensitivity rating of the headphones. This tells you how many dB the headphone outputs at either 1 mW of power or 1V. This is equally important!
 
For a good example, compare the Sony MDR-V6 and the AKG K701. The Sony MDR-V6 has 63 ohm impedance, whereas the AKG K701 has 62 ohm impedance. By the myth, the K701 should be easier to drive. Wrong! The Sony MDR-V6 has a sensitivity rating of 106dB/mW and the AKG K701 has a sensitivity rating of ~92 dB/mW. In reality, a portable device may take the MDR-V6 to 110dB, but an amp may need 5 times as much voltage to make the AKG K701 reach the same loudness.
 
From a purely technical perspective, it is harder to drive a low impedance source -- it's just that low impedance headphones nowadays typically are paired with high sensitivity ratings because they are designed to be played from portable devices.  If a headphone has a low impedance and a high sensitivity rating, it will be very easy to drive. If a headphone has a high impedance rating and a high sensitivity rating, it should be easy to drive. If a headphone has a high impedance and a low sensitivity rating, it will be hard to drive. If a headphone has a low sensitivity rating and a low impedance rating, it will be very hard to drive.

 

[NinjaSquirt]

Good point. Good warning. To be fair, given how most people talk about it, its easy to see how one would come to the original false conclusion. I know I certainly believed that when I first started getting into the headphone world.

 

[SanjiWatsuki]

Same here. I thought impedance was the end-all at first, but it's just wrong. I think what bugs me more is when I see the same person talking about difficulty to drive in terms of impedance only over and over and over again. 
 
Quote:

Good point. Good warning. To be fair, given how most people talk about it, its easy to see how one would come to the original false conclusion. I know I certainly believed that when I first started getting into the headphone world.

 
 

 

[Head Injury]

I've been fighting this rumor for a while. Maybe you'll get somewhere 

 
BTW, an amp will need over 10 times the power to drive the K701 to the same volume as the V6, going by the specs in your post. 10 dB = 10x power. Specifically it would need around 25x as much power and 5x as much voltage.

 

[SanjiWatsuki]

Oh, wow. I was thinking in my head "okay, calculate it in voltage and put voltage" and thought I wrote "voltage." Whoops. Edited and fixed.
 
Quote:

I've been fighting this rumor for a while. Maybe you'll get somewhere 

 
BTW, an amp will need over 10 times the power to drive the K701 to the same volume as the V6, going by the specs in your post. 10 dB = 10x power. Specifically it would need around 25x as much power and 5x as much voltage.

 

 

[johnman1116]

Wow this is great stuff! I actually didnt know the stuff about about dB/mW.
So if sensitivity rating takes precedence over impedance (if im getting that right), then what does impedance really do..? 

 

[SanjiWatsuki]

Well, it actually comes down to a formula.
 
Power in Watts = ( Voltage * Voltage ) / Impedance
dB SPL = Sensitivity in dB/mW + 10 * LOG ( Power in mW)
 
As impedance goes up, the voltage squared is divided by a bigger number, but sensitivity gives a higher starting point for that dB SPL calculation. They both work together to determine how hard it is to drive a headphone, but the sensitivity is probably the more important component.  
Quote:

Wow this is great stuff! I actually didnt know the stuff about about dB/mW.
So if sensitivity rating takes precedence over impedance (if im getting that right), then what does impedance really do..? 

 
 

 

[ObeyurMaster]

Thanks for the important info. Needless to say, I was of the opinion that lower the impedance of the headphones, easier it is to drive. I did not even know about the sensitivity ratings.

 

[stv014]

I also find this myth to be very annoying. What is even worse, is that even considering the sensitivity, the suggestion that any high impedance headphone could be easy to drive is usually countered with the argument that "yes, it will be loud but not driven to its full potential in terms of sound quality", citing unmeasurable and unquantifiable aspects like "control", "sound stage", "dynamics", and others. According to the myth, there is some magical property of high impedance that will always make it "hard to drive" no matter what, and require some very expensive (usually tube) amplifier. These suggestions discourage many people from buying headphones that would in reality work fine with their setup.
 
However, just as harmful is the belief that low impedance is always easy to drive. For example, the Sennheiser HD5xx series is frequently recommended to newbies on the ground that the ~50 Ohm impedance must be "easy to drive", yet in fact these have an impedance peak of nearly 300 Ohm at about 90 Hz, making them more sensitive to the output impedance of the source and require a higher damping factor than most other headphones. Low impedance is also often recommended for sources that have known high output impedance (that includes many sound cards that do not have a dedicated headphone amplifier); in reality, from a 120 Ohm output impedance source a 250 Ohm headphone will get more power than a 32 Ohm one, not to mention the really poor electrical damping with the latter. The source may also feed the output through a capacitor (to remove DC), this again works in the favor of high impedance loads (the lower the impedance of the load, the worse the bass extension gets). For a given output voltage, amplifiers usually also perform worse with lower impedance loads in terms of distortion.
 
Even a combination of high impedance and low sensitivity can have its advantages, as long as sufficient volume can be achieved without clipping or noticeable distortion, as it will make any volume independent noise or hum from the source quieter. With an analog volume control that is implemented using a simple stereo potentiometer, there may also be some channel imbalance when the volume has to be turned down very low because of the sensitive headphone (this affects the FiiO E9, for example).
 

 

[dirkgently]

Very good write up!  I have to admit, being new to this pursuit, I wasn't really aware of the relationship between resistance and sensitivity.

 

[bbophead]

Quote:

I also find this myth to be very annoying. What is even worse, is that even considering the sensitivity, the suggestion that any high impedance headphone could be easy to drive is usually countered with the argument that "yes, it will be loud but not driven to its full potential in terms of sound quality", citing unmeasurable and unquantifiable aspects like "control", "sound stage", "dynamics", and others. According to the myth, there is some magical property of high impedance that will always make it "hard to drive" no matter what, and require some very expensive (usually tube) amplifier. These suggestions discourage many people from buying headphones that would in reality work fine with their setup.
 
However, just as harmful is the belief that low impedance is always easy to drive. For example, the Sennheiser HD5xx series is frequently recommended to newbies on the ground that the ~50 Ohm impedance must be "easy to drive", yet in fact these have an impedance peak of nearly 300 Ohm at about 90 Hz, making them more sensitive to the output impedance of the source and require a higher damping factor than most other headphones. Low impedance is also often recommended for sources that have known high output impedance (that includes many sound cards that do not have a dedicated headphone amplifier); in reality, from a 120 Ohm output impedance source a 250 Ohm headphone will get more power than a 32 Ohm one, not to mention the really poor electrical damping with the latter. The source may also feed the output through a capacitor (to remove DC), this again works in the favor of high impedance loads (the lower the impedance of the load, the worse the bass extension gets). For a given output voltage, amplifiers usually also perform worse with lower impedance loads in terms of distortion.
 
Even a combination of high impedance and low sensitivity can have its advantages, as long as sufficient volume can be achieved without clipping or noticeable distortion, as it will make any volume independent noise or hum from the source quieter. With an analog volume control that is implemented using a simple stereo potentiometer, there may also be some channel imbalance when the volume has to be turned down very low because of the sensitive headphone (this affects the FiiO E9, for example).
 

Please be patient and help me out here.

 
I have Grados, 32 ohm and efficient as you no doubt know, and I want to buy an all tube headphone amp.  What output impedance figure should I be looking for?  TIA.
 
 

 

[Head Injury]

Quote:

Please be patient and help me out here.

 
I have Grados, 32 ohm and efficient as you no doubt know, and I want to buy an all tube headphone amp.  What output impedance figure should I be looking for?  TIA.

Close to 0 ohms if you want neutrality (though the amp itself may be colored), whatever else if you want coloration (mid-bass at 100 Hz in particular).
 
The effects of up to 10 ohms will be subtle, even inaudible.

 

[bbophead]

Quote:

Close to 0 ohms if you want neutrality (though the amp itself may be colored), whatever else if you want coloration (mid-bass at 100 Hz in particular).
 
The effects of up to 10 ohms will be subtle, even inaudible.

Hey, if I wanted neutrality I wouldn't have Grados, now would I?

 
But, seriously, thanks for the info.
 
 

 

[AVU]

It's not so much a myth as a misconception borne of lack of understanding.  And even for people who do understand, there's something to be said for simplicity.  We could have started with the sensitivity rating and argued the same thing re: impedance.
 
New people coming to the hobby generally want answers to specific questions (see above) and many not be interested in having to calculate dB SPL = Sensitivity in dB/mW + 10 * LOG ( ( Voltage * Voltage ) / Impedance).
 

 

[ShooTa]

ahh thankyou for correcting my misconceptions ... ive been one of those that took on the impedance = usually harder to drive idea, i did realise that it wasnt always the case but i didnt know what the casue was -thankyou OP/.