[sam-c]

Hi guys 
 
After a lot of research I think I about to plump for a pair of AKG K550s to replace my Monster Turbine IEMS (found them to uncomfortable but liked the noise isolation with good sound quality yet a decent amount of bass which I have found have lacked in heaphones I have owned in the past). 
 
However, I believe that these sound best with an amp and seen as though I'm willing to fork out on one I thought I would ask for some  recommendations from you guys. Budget is around £100 (approx. $158 but obviously must be available in the UK) and I dont mind going second hand. I have been looking at the Fiio E11 or E17 as well as the E7 but I dont really know the difference and if they E7 and E17 are worth the extra money? Are there any other alternatives I should consider?
 
The source will be a iPhone 4 as well as a library computer which probably has the worst sound card known to man in it! 
 
Any advice much appreciated.

Sam

 

[agdr]

I've had my AKG K550's for two months now.   Some measurements revealed that with the K550's 114dB/V sensitivity, normal listening levels are around just 50mV rms (that is right, 50 MILI volts!) per channel, with "as loud as I can stand it" at about 80mV rms.  On the box they list the "94 dB hearing damage level" to stay away from at 135mV.  Which all means that not only will you probably not need an amplifier, you may wind up having to turn the volume down a bit!

That has been the case with me, anyway.  The output of a recent model Sony laptop is more than enough to hit over 80mV rms at full volume with some source material.  I usually only have to turn the volume slider on the laptop up about 2/3 of the way to hit comfortable listening levels. Same story with a DAC + headphone amp I have - had to turn the volume way down. 

I haven't tried the K550s with an iPhone, but I have tried them with a recent model HTC EVO 4G LTE cell phone (has a 3.5mm jack that fits the AKG K550s just fine), with results similar to the laptop.  With the volume level only about 1/2 way up the headphone volume level is comfortable, while 2/3 of the way up hits the uncomfortably loud range.

So, suggestion: get the AKG K550s first and try them with your sources.  If you can't hit full volume then consider an amp.  You probably won't need any voltage gain (voltage gain of 1X) with any source, just possibly some current buffering.  Which means setting the gain at 1 (no voltage gain) on whatever amp you buy, if you do buy one.  At 80mV rms the current through each channel to drive each headphone cup is just (80mV / 32R) = 2.5mA(rms), which shouldn't be much of a challenge for anything with a "headphone" output jack.  An amplifier may also have a lower output impedance (even if just set to 1x voltage gain) than the source which would improve the damping factor, another amp benefit.

A better DAC will usually improve things over the lower quality internal DAC in the case of a laptop or desktop PC.  A USB DAC + amp combo unit, one that you can set the voltage gain to 1x on, would be something to consider in this case.

Technology aside, the AKG K550s totally rock IMHO!

  I'm definitely enjoying the sound - and head fit - of these headphones.

 

[sam-c]

Thank you very mcuh for your detailed reply, it was a very interesting read. I'm going to listen to several headphones today so quite excited  

Do you know if the E17 has a x1 gain? 

 

[agdr]

Quote:

Do you know if the E17 has a x1 gain? 

 
I don't see anything in the E17 product description about adjustable voltage gains.  It probably has a fixed voltage gain, like 3x or 5x, with a variable resistor (volume control) on the signal going in, which is a typical way to do things.  That would still work OK, you would just wind up only using the first 20 - 30 degrees or so of the volume control. In other words, you wouldn't need to turn the volume control very far to hit normal listening levels.

 

[sam-c]

Well I demo'd and bought these today Got to say the sound quality is very good and I only used my iPhone as the source but it was more than fine listening to them on almost full wack. Amp is definitely going to be needed as I'm going to need to be able to go louder when I'm on the plane and as I said it was on nearly full wack as it was.
 
Question is which amp!? 

 

[agdr]

Congratulations on your purchase!  That does sound like you will need an amp with the AKG K550s on your iPhone.  You will probably need some voltage gain too in your case, like 2x or 3x, which is the common way most amps are set up by default.  I don't have any specific amp recommends, unfortunately.

 

[sam-c]

Thanks, I'm loving them so far! 
 
Looks like I need an amp/dac combo. Amp for the iPhone and DAC for netbook/crappy library computers lol 

 

[agdr]

I just took some pictures of actual AKG K550 listening level voltages, for anyone wanting to see some details.

  Instead of just sine wave testing I'm using a DMM that records maximum, average, and minimum true rms voltage levels, then playing an entire song to capture those levels after a reset.  I figure that gives a better representation of the voltage levels involved in listening to real world music on the phones.  I've run the measurements two ways here, at normal listening levels then again at a level that I could only keep the headphones on for about 1-2 seconds safely.
 
Here is the test setup.  Just a laptop with a breakout cable I made, going from the headphone jack to the phones, feeding a meter with full audio AC bandwidth capability and a scope.  The scope is there just to verify no clipping occurring in the laptop's audio output stage while driving the headphones.  Not likely at these low voltages and currents, but one never knows. The laptop is running on batteries to avoid any ground loops with the test equipment:
 

 

 
You can see from the photo here that the PC volume slider is somewhere between 3/4 and 7/8 (81/100 on the scale) for normal, comfortable listening levels on the AKG K550 headphones with this particular source material.  Over the course of a song the meter from the photo recorded true rms (AC coupled) values of: maximum = 48.2mV, average = 29.7mV, and minimum = 17.4mV.  Since these are true rms voltages and the impedance of the AKG K550s is very flat vs. frequency, the power input to the phones (per channel) is just E^2/R: maximum = [(48.2mV)^2 / 32R] = 72.6uW (yes that is MICRO watts!), average = 27.6uW, minimum = 9.5uW.
 
9.
 
 
And from the photo here, with the volume slider all the way up and the phones too loud for more than 1-2 seconds use, the meter recorded over the course of the same song: maximum = 147.1mV, average = 83.0mV, and minimum = 13.9mV.  Note that maximum voltage level on peaks, 147mV, is actually exceeding the "94 dB 135mV hearing damage level" on the box for extended listening.  So this one can be considered an absolute maximum set of numbers. Power input to the phones is: maximum = 676uW = 0.676mW, average = 215uW = 0.215mW, minimum = 6.0uW.
 

 
So for normal listening levels, the 50mV rms number I posted previously, above, is more of a maximum.  For this particular song the average is around 30mV rms!  I'm amazed these phones put out this amount of sound with so little input voltage.  Some pretty amazing sensitivity.
 
I will also have to modify what I posted above about an amp used with an iphone needing any voltage gain.  At those super-low drive voltage levels I would say it is unlikely the iPhone - or any other device - couldn't muster 50mV (rms) maximum.  Instead I'll go back to what I initially posted - what is needed is most likely a unity gain (voltage gain = 1) current buffer.  In other words, the problem isn't that the iPhone can't muster enough voltage swing for the AKG K550s, but rather it is much more likely that the iPhone can't supply even the small 50mV / 32R = 1.56mA (rms) maximum current requirement.  If the device's output stage can't supply enough current like this, it will drag the output voltage down (forms a resistive voltage divider with the headphones) and the net result is not enough maximum volume. 
 
So, in summary, back to suggesting an amp where the voltage gain can be set to 1 as purely a current buffer.