[HDMan]

	Active balanced headphone ground
	Each driver of a headphone is connected to the amplifier by two wires. Normally one of these wires (the ground wire) is connected to ground and thus carries a constant zero-Volts signal. The other wire carries a varying voltage, the electrical audio signal. The driver is driven by the voltage-difference between both wires. The voltages of the 4 wires change continuously and the external electrical fields generated by the electrical charges on the wires change accordingly. There is energy associated with these fields and thus energy is constantly stored and released. The properties of the insulating material have a strong influence on this process and thus effect sound quality. The CONCERTO does not keep the signal of the ground wires fixed but adds an extra signal to all the four wires (signal wires and ground wires) such that the sum of the voltages of the 4 wires is constantly zero. This extra signal is the same for all wires so the voltage differences between the two wires of a single driver do not change. The effective audio signal is still the same. However, since the sum of the voltages is zero, the total electrical charge on the wires is also zero. The external potential field is strongly reduced and much less energy is stored and released when an audio signal is applied. The purity of the sound signal is much less effected by the properties of the insulating material. The pictures below show an example of a typical headphone cable driven by a conventional headphone amplifier (left picture) and by the CONCERTO (right picture).
	At the left below we see the signals as generated by a conventional headphone amplifier. The ground signal is kept constant. Left and right channel signals vary independently. At the right we can see the signals generated by the CONCERTO. The ground wire now carries a signal that is also added to the left and right channel signals. The difference between Left and Ground and between Right and Ground is the same as in the left picture.
	The left picture below shows the electrical potential field of the positively charged headphone cable when both signal wires carry a positive signal at the point of time indicated by the yellow bar at the picture above. The right picture shows the potential field for balanced ground operation. It can be easily seen that field potentials are much lower than at the left picture.
		The differences are even more pronounced in the far field, as shown in the next pictures. Since the "balanced" headphone cable has zero netto charge there is hardly any energy stored in the far field.
	Apart from these principle benefits there are also some technical advantages offered by active balanced headphone ground. When a positive signal is applied to a headphone driver by a conventional headphone amplifier current passes from the output stage of the amp, through the headphone driver towards the ground plane of the amp. During this process the current is "taken" from the positive power supply line whose voltage, as a result, will not be entirely constant. (In the pictures below the positive and negative supply lines are depicted as two large storage capacitors.) The currents that run through the ground plane do induce small voltage differences. Ground is thus polluted and since it acts as a internal reference for all signal amplification processes sound quality is impaired. Also variations of the powersupply lines are not entirely rejected/neglected by the output stage and are partly transferred to the output signal. This reduces sound quality even further. When a signal is applied by the CONCERTO, current passes the signal output stage, the driver, and the ground output stage. Both positive and negative powersupply lines are "loaded" simultaneously. No driving currents pass through the internal ground plane. There is no pollution of the internal referfence. The voltage variations of the two powerlines are mirrored. Their mean value is roughly constant. This strongly increases their rejection at the output stages.
	If voltage variations are found in one of the powerlines only, then these have a similar effect on the signals of both the signal output stage and of the ground output stage. However, it is the difference between the two output signals that drives the headphone. This difference is not changed and the effective audio signal is not influenced. PSRR (Power Supply Rejection Ratio) is very high.

 

[HDMan]

	CROSSFEED
	In normal daily life people use various mechanisms to locate sources of sound. Firstly, the sound of a source to the right side of the listener (e.g. the right loudspeaker) not only reaches the right ear but, attenuated and delayed, is also heard by the left ear. The level of attenuation and the delay time of this crossfeed signal provide important directional information. Secondly, the soundwaves are partly absorbed and partly reflected by the tissues of the head. Reflections at the oracles (pinnae) interfere with the soundwaves that directly enter the ear-channel and amplify or attenuate specific frequency components. Since these reflections depend on the direction of the soundwave the "color" of the sound changes with the direction of the source. Thirdly, reflections of the soundwaves from the walls, ceiling and floor of our listening room produce reverberation that conveys an extra feeling of space. The information obtained by these mechanisms is further refined by movements of the head. Changes in sound levels, delay times and sound color refine the sense of direction. For a demonstration, blindfold a friend and ask him to locate a ticking clock that you have hidden in the room. He will start turning his head although he can't see anything. With his head in a fixed position an exact localization is much more difficult.
	Listening by headphones
	All the mechanisms of directional listening are missing when we use headphones. The sound at the right ear will no longer reach the left ear and pinnae-reflections no longer interfere with the original soundwave. Moreover, the headphones are directly attached to our head, and so head movements no longer add information. Reverberation is also not present. As a result, the sound heard by headphones seems to stick to the inside of our head and to our ears and an unnatural soundfield is created. The brain misses logical clues for direction and this subconsciously results in mental stress. Some people cannot tolerate this stress and are unable to use headphones.
	The natural crossfeed filter
	In principle, digital soundprocessors can simulate all the mechanisms for directional listening but the results are, thus far, not very satisfactory. In particular, pinnae-reflections are very complex and listener-specific and impossible to simulate accurately. Fortunately, the mean directional information is provided by the time delay and level of attenuation of the sounds that reaches the opposite ear. The CORDA headphone amplifiers can electronically simulate this process and, with appropriate attenuation and delay, add some of the right audiosignal to the left channel and vice -versa. This considerably reduces the adverse symptoms of headphone listening. A unique feature of the crossfeed circuitry of the CORDA headphone amplifiers is that it "recognizes" the virtual positions of the instruments and singers in a recording. The sound of an instrument in the middle of the soundstage will be equally present in both audio-channels and isn't given any crossfeed. A crossfeed signal is only generated for instruments that are not placed at the center. The more off-center the instrument is placed, the stronger the crossfeed and the longer its delay. This feature is called "natural crossfeed".

 

[HDMan]

Inside the ROC:
 

 
 
Inside the Concerto:
 

 
 
 
Don't know about you but I sure like the look of the PSU inside the Concerto. Skylabs compared it to the Pheonix, a higher model and according to the headphones was preferable for him.

 

[Ypoknons]

Yes, the Concerto has active balanced ground. But crossfeed is another technology. 

 

[realmassy]

Skylab said, correct me if I'm wrong, the meier was better than the Phoenix, using the SE output... With the balanced output is a different story. I have to agree the SE output of my Roc is simply OK.

 

[TigzStudio]

Yep ACSS/balanced is where its at on the Roc (but I have noticed SE isnt too shabby at all with my LCD-2s)

 

[BigMango]

 
Don't know about you but I sure like the look of the PSU inside the Concerto. Skylabs compared it to the Pheonix, a higher model and according to the headphones was preferable for him.

As others have said above, Skylabs prefered the Phoenix to the Concerto when used with balanced out, but if using only SE out the Concerto was better to him.
 
And to me this seriously is a non-issue. On my HD650, the soundstage and instrument positioning improvements of the Roc's balanced mode (with or without balanced source) are making the music sound so much better to my ears that it doesn't make any point for me at all to go with SE when I am spending so much money on an amp. Add to this that an SE amp would be limiting my future source upgrade paths.
 
The next upgrade for me after the Roc/Phoenix would probably be a Beta22, but certainly not an SE amp. Anyway, I currently don't have any plans to upgrade.
 
The only reason I am still keeping an SE amp is my receiver's dolby headphones / headphones theater mode which accurately transfers multichannel audio positioning to the headphones when I am watching movies. But for music? No way.

 

[FauDrei]

Nice active ground marketing HD.
 
Still, full differential drive, or as we mainly call it "full balanced" drive, is further advance to active grounding:
 
Active ground (like in Concerto) is like the "third" amp channel where you "mix" both, left and right, signals together, invert them and feed such signal instead of plain "flatline" ground to both headphone drivers. Wires are: Left signal, Active ground, Right signal, Active ground.
 
Full differential amp (like Roc) must have four amp channels, you do not mix left and right signals - you invert both of them. So the connection goes: Left signal, Left inverted signal, Right signal, Right inverted signal.
 
Stereo separation must by definition be better with full differential drive - the active ground "mixing" introduces certain amount of music information from other channel and as such can be seen as some kind of crossfeed effect per se. Whether greater separation or "mixing" is preferred - it is a matter of personal taste and/or music material.
 
If we oversimplify: which would you like better - 3 channel β22 or 4 channel β22?
 
Finally, while 30W incapsulated torroid transformer might look fancy, "shielded" 80W R-core just might pull it better.

 

[HDMan]

At the end of the day when you listen to it then you know which is better. Good review thanks Skylab.
 

 

[TigzStudio]

HDMan I am sure you will be supremely happy with your purchase of the Concerto, looks like it would be awesome for the LCD-2s as well.  

 

[Solude]

Quote:

 
If we oversimplify: which would you like better - 3 channel β22 or 4 channel β22?
 

 
b22 3rd channel is active, not balanced.  That said would you rather two channels for $1000($500per) or four channels for $1000($250per)  My b22 is two channel because though everything that is said about active/balanced ground is true its just not that important in the result.  Some have even said its a bad idea because while it does what it claims... it reduces other performance metrics.

 

[Cya|\|]

Quote:

Yep ACSS/balanced is where its at on the Roc (but I have noticed SE isnt too shabby at all with my LCD-2s)

Did you try the lcd 2 with balanced outputs too? If yes, how much improvement did you notice with the lcd 2 balanced?

 

[FauDrei]

b22 3rd channel is active, not balanced.

Either you did not understand my post or I do not understand yours. Have I said something different?

 

[Solude]

Just see a lot of people compare the balanced ground of the Meier amps to the active ground of the M3/b22 but they don't mention that they are not the same.  Ie the 3v4 channel b22 is not the same as balanced ground v traditional balanced drive.

 

[Canuck57]

per the Meir website "Active balanced headphone ground offers most of the advantages of a true balanced headphone setup without the need of balanced connections."
 
The Concerto is not a true balanced headphone amp., the question is, what % is "most" of the advantages of a true balanced headphone setup.