head injury said:

  Made a GIF in Gimp. I copied the screenshots of the spectrums into layers in Gimp to make cropping them easier, so I just saved the result as a GIF too.
 
OP measured both a 30 Hz square wave (which shows the larger dip in the middle on balanced) and a 300 Hz square wave. I was discussing the 30 Hz one because the 300 Hz ones look practically the same.

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Oops my bad.  Somehow overlooked the 30 hz one. 
 
Still pretty sure it is something transient unrelated to the equipment.  If you look at the 15-17 second mark in both files you can see the low end noise is pretty much identical and lower than most of the balanced signal.  The unbalanced stays generally lower in the low frequencies while the balanced one fluctuates though when quieted down is the same as the unbalanced.  Look at the 20-23 second segment in both files which is in the 30 hz squarewave section and see that both have virtually identical noise floors at that point.  The balanced is actually a touch lower there. 

mikeaj said:

  Sorry, only have time for a quick skim through, so this response will be shallow, but you use the headphone output compared to the balanced line output of the same device?
 
I'm pretty sure you'll find the output impedance to be different for the two outputs (and probably not the balanced being the same or double the other), or is that the point? Any difference in behavior between the outputs is not necessarily due to the state of being balanced or unbalanced. It's probably different drive circuitry. What are the measured impedances, anyway?

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Focusrite claims <10Ω on the output impedance. I am not sure if it's the line balanced output or the headphone output impedance, but I will note this in the next few measurements that I make.
 
Worthy of note is that these are not hi-fi devices, but rather are purpose-built to be reference sound. It is their design to match sound as close to what it should sound like; as such I am currently assuming that the drive circuitry should reflect that. This I recognize as flawed; perhaps I should try a mono left-right adapter for balanced output emulation through the headphone out alone, playing inverted copies of the same file through a "stereo" channel (I'm unsure how the circuitry would work here though). Unfortunately that means a new adapter and more time...
 
I do have another unbalanced/balanced source and we'll see what that does. 300Ω balanced impedance, unlisted headphone impedance. I will try another test later with this source instead to see if it's just this interface that exhibits this behavior.
 

esldude said:

  To those looking at the low end differences.  Having done room measurements for Room correction this is likely just a change in background noise.  A truck a block away pulling up a hill in one instance and not being there in the other will do that.  Those low frequencies travel a long way.  I have done impulse response tests that way and seen the range below 50 hz swing wildly just a few minutes apart. 

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I have also seen wild variations in sub-bass levels with different generated samples of noise alone. I closed all windows and such to avoid any environmental factors, but some things are just not within my control. I will try different samples and see what I get.
 
Thanks for reading, I'll try to post more data here for your opinions.

esldude said:

 
Oops my bad.  Somehow overlooked the 30 hz one. 
 
Still pretty sure it is something transient unrelated to the equipment.  If you look at the 15-17 second mark in both files you can see the low end noise is pretty much identical and lower than most of the balanced signal.  The unbalanced stays generally lower in the low frequencies while the balanced one fluctuates though when quieted down is the same as the unbalanced.  Look at the 20-23 second segment in both files which is in the 30 hz squarewave section and see that both have virtually identical noise floors at that point.  The balanced is actually a touch lower there. 

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I admitted as much about there being differences unrelated to gear. My point being, those differences seem to appear in the frequency response as well, not just in the square waves and saw waves as the OP suggested. They're both related, and both are very sensitive to changes in the environment. Repeated tests would be great to see. There's no disagreement between us.

head injury said:

 
I admitted as much about there being differences unrelated to gear. My point being, those differences seem to appear in the frequency response as well, not just in the square waves and saw waves as the OP suggested. They're both related, and both are very sensitive to changes in the environment. Repeated tests would be great to see. There's no disagreement between us.

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My observation with the 30Hz square was that throughout the entire 15-second interval, the same difference consistently showed up. The 1dB difference might cause this, might not, we'll see. The saw wave is slight enough to be environmental, though.

IMO, I think you'll find with, say, a differential (balanced) headphone amp designed to be used with the balanced output that the SE output will be inferior. In other words, I think it is more the amp's design than that that the connection is balanced. What might be interesting is to compare the square waves with different amps, including SE and balanced from differential amps. If the differential amps consistently gave similarly different results from SE amps, that would be interesting.

I think you need to look at the implementations of balanced output amplifiers a bit more.
 
Generally there are 2 kinds, those which are 2 separate amplifiers being driven from
a balanced source (or an unbalanced to balanced converter) And the amplifiers
that are fully differential, and possibly symmetric and or complementary symmetry.
 
In either case, the output slew rate is double that of a single amplifier.
 
In most cases, the output impedance is double that of a single amplifier.
nelson pass style super symmetry reduces the output impedance by about 6db.
 
The output power can be up to 4 times that of a single amplifier.
 
The second harmonic distortion can be at least 6db, and typically 10db lower
with no resultant change in 3rd harmonic distortion. Comes from the fact that
there is no such thing as perfectly matched pnp and npn devices.
 
In fact things like the crown macro reference do fully balanced output with
only npn devices. Same thing with some of nelson pass creations, as well
as circlotrons. Virtually all solid state electrostatic amplifiers do exactly the
same thing.
 
its not really about ground loops, or common mode etc.

kevin gilmore said:

  I think you need to look at the implementations of balanced output amplifiers a bit more.
 
Generally there are 2 kinds, those which are 2 separate amplifiers being driven from
a balanced source (or an unbalanced to balanced converter) And the amplifiers
that are fully differential, and possibly symmetric and or complementary symmetry.
 
In either case, the output slew rate is double that of a single amplifier.
 
In most cases, the output impedance is double that of a single amplifier.
nelson pass style super symmetry reduces the output impedance by about 6db.
 
The output power can be up to 4 times that of a single amplifier.
 
The second harmonic distortion can be at least 6db, and typically 10db lower
with no resultant change in 3rd harmonic distortion. Comes from the fact that
there is no such thing as perfectly matched pnp and npn devices.
 
In fact things like the crown macro reference do fully balanced output with
only npn devices. Same thing with some of nelson pass creations, as well
as circlotrons. Virtually all solid state electrostatic amplifiers do exactly the
same thing.
 
its not really about ground loops, or common mode etc.

Click to expand...

 
So would the doubled slew rate possibly be responsible for the kinds of results prodo123 got in his headphone square wave measurements? Or am I connecting the wrong dots?

currawong said:

So would the doubled slew rate possibly be responsible for the kinds of results prodo123 got in his headphone square wave measurements? Or am I connecting the wrong dots?

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Slew rate has nothing to do with the headphones themselves. And it's only relevant to an amplifier if you're slew limiting. And I'm not aware of any modern amplifiers that slew limit. So while "double the slew rate" is technically accurate, from a practical standpoint, it's meaningless.

se

kevin gilmore said:

In most cases, the output impedance is double that of a single amplifier.
nelson pass style super symmetry reduces the output impedance by about 6db.

Click to expand...

I don't belive I have ever seen a difference in impedance expressed in decibels. :confused_face(1):

se

Um... so is no one going to point out that these comparisons are rather moot since the output devices are different on the headphone output vs the balanced output?

armaegis said:

Um... so is no one going to point out that these comparisons are rather moot since the output devices are different on the headphone output vs the balanced output?

Click to expand...

We were waiting for you.

se

armaegis said:

  Um... so is no one going to point out that these comparisons are rather moot since the output devices are different on the headphone output vs the balanced output?

Click to expand...

 
Currawong and I basically said that. ^_^
 
(Although experiments for the sake of experimentation are kinda cool nonetheless.)

if you had a theoretically perfect 1:1 output transformer (there is no such thing)
then you could measure the amplifier as single ended (a pair, or more than one
pair of (complementary, or not) transistors in parallel) vs a balanced output
(bridge) which is 2 or more pairs (complementary or not) of transistors in
a H bridge formation. And the measurements would definitely show significant
differences between the two. And the sound is different too.
 
For electrostatic use, there are already many amplifiers out there that
are slew rate limited at the voltage levels required to drive the headphones.

I have created some frequency response and THD plots from the sine sweeps (click on the images for higher resolution):
   
The "THD" is actually mostly just noise, as it can be seen from FFT analysis of the sweep at various positions. So, it is not particularly helpful at all. The large spike under 4 kHz is because of some interference or ambient noise that produced constant peaks at multiples of that frequency, so that is not actual distortion either. From the low level of distortion that is masked by noise, I guess the SPL was fairly low.
 
The frequency response is basically identical between the unbalanced and balanced output above 100 Hz on the left channel, but there are minor differences on the right. Maybe the positioning of the headphone or the microphone has changed during the tests, as the cable was unplugged and moved (for this reason, in my output impedance tests I have used a relay controlled by the PC to switch between low and high source impedance).
 
One difference that seems to be consistent is that the balanced output has a low frequency roll-off, which also explains the visibly different 30 Hz square wave responses. The most obvious explanation is that the balanced output is capacitor coupled, and the capacitors are too small. Is the balanced output of the Focusrite even designed for driving headphones, or is it originally intended to be used as a line output ?

great read, thanks to the OP for starting it, and to ... well all the others bringing ammo to the discussion.
 
I always thought that all things being equal(which they are not) the main difference on amps would be in crosstalk and maybe channel imbalance. but I guess that's me trying to connect the dots as to why I felt a different headstage with some amps when going from BA to SE.
 
my own very subjective experience is indeed that a given amp in BA sounded better to me than the same amp in SE. but that's a little like saying that 2 of one thing is better than 1. it doesn't really say anything about how good another SE amp is compared to a balanced one. and I kind of expect the manufacturer making a BA amp, to make it with BA in mind. and add the SE output only as a safety option for people who won't recable(I never understood why someone would buy a BA amp if not to use it in BA, but I know for a fact that it is done).