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Reviewer: Frederic Beudot
Digital Source: Musical Fidelity A5 CD, Accuphase DP55, Atoll CD200 [in for review]
Headphone Amp: Musical Fidelity Xcanv3, Creek OBH11se, Rudistor NX-33 [in for review], HeadRoom Balanced Desktop [in for review]
Cables: Zu Gede (RCA and XLR), Consonance Billie interconnects
Headphone: Beyerdynamic DT911, AKG K701, Sennheiser HD650 [on loan]
Power Cords: Cobalt Ultimate
Powerline conditioning: Monster Power HTS5100mkII
Sundry accessories: Isolpads under electronics and speakers, Standesign stand
Room size: 15' x 30' opening to 3 other rooms. Short wall setup
Review component retail: Balanced AKG K701 by HeadRoom ($450 + $300 for mod), Rudistor balanced cable for Sennheiser HD650 ($150)

I owe 6moons readers an upfront clarification. Until recently, I wasn't a headphone enthusiast. Cans are not my main source for musical enjoyment and I used to view them more as a necessity than anything else. Until this assignment that is. I do listen through headphones every week, in the early hours of the day when I am awake and the whole family still sleeps in. Headphones are the only way to enjoy those few quiet hours before the house turns into a beehive; but they've always been a surrogate to a dedicated and insulated listening room as speakers are my preferred listening vessel, primarily because soundstage is so relevant to my appreciation of music and terribly lacking in most headphones.

Nevertheless, seeing my listening time through cans increase over the last 12 months, I decided to upgrade my 10-year-old Beyerdynamic DT911 and Creek OBH11-SE amplifier to something more comfortable and refined. That's how an AKG K701 and a Musical Fidelity Xcanv3 made it into my home a few months ago. Seeing this and some available time in my schedule, Srajan suggested a review of Rudistor's NX-33 headphone amplifier, which I gladly accepted to see what an amplifier costing 3 times as much could do against the Musical Fidelity. But the Rudistor is actually a balanced amplifier designed to drive balanced earphones. Testing it single-endedly only would have painted a very partial picture of its abilities. Enter the good folks from HeadRoom who sent me a pair of balanced AKG K701s to compare to my single-ended stock version as well as a pair of Sennheiser HD650s, just because one cannot really judge an amplifier with just one set of cans – and I probably needed another balanced reference amplifier to compare the Rudistor to so I also signed up for a review of the HeadRoom balanced desktop amplifier. I am not quite sure quite how it all happened but within two days after accepting the Rudistor assignment our Editor had solicited from the team, I had 2 amps and 2 sets of cans on my to-do list, in addition to getting accustomed to my personal new gear. And I am not even a headphone geek. But boy am I glad destiny played this little trick on me.

I don't know about you but until I looked into the specs of the Rudistor amplifier, I had never heard of balanced headphones and all their supposed benefits. To make things easier and manageable for all of us, I have divided this review into three installments. This first part will look into the headphones, the technology and the differences I heard between single-ended and balanced operation. In a few weeks I'll follow up with the formal review of the Rudistor NX-33 amplifier and after that, I'll wrap it all up with the review of the fully upgraded HeadRoom Balanced Desktop.

Balanced headphones, technology and expected benefits
Over 5 years ago HeadRoom was the first company to postulate that driving headphones in balanced mode should yield sonic benefits and they have been working on improving the recipe ever since, now joined by a few other manufacturers like Rudistor bringing their own stone to the edifice. HeadRoom has published a number of very good articles on this technology and I am using their material as a reference in the following introduction. You can also visit the balanced section of their website to find out more about it.

The initial assumption was that getting rid of the common ground between left and right channels in a single-ended earphone would reduce cross talk and improve sound quality. As it turned out, balanced topology in earphones does that and much more too.

A normal headphone cable plug has three connections on it: the tip is left; the ring is right; and the sleeve is ground. The tip connects to a wire that goes to the positive (+) lead of the left headphone driver coil; the ring connects to a wire that goes to the positive lead of the right driver. The sleeve connects to a wire that goes to both negative (-) terminals of the drive coil; this wire usually has a solder joint in the "Y" or in the earpiece where the ground wire from the plug splits into separate wires that are connected to the negative terminals of the driver coils.

Bold lines indicate common return of both left and right channels

The most important thing to note here is that as the left and right channels of the headphone amplifier drive the left and right driver coils, the return current from the drivers gets joined together and travels some distance before returning to the amp's audio ground. This common pathway has some -- possibly significant -- electrical resistance from the wire, solder joints, contact resistance at the plug/jack, and so on, which causes a common signal to appear at the negative terminal of both driver coils. This common signal (a low-level summation of the left and right channel) will generate low level cross talk and distortion in the sound heard on headphones.

Balanced headphones are just regular headphones that have been re-cabled in a special way. The normal three-conductor cable with the common ground connection is replaced with a cable that has four conductors: right positive and right negative conductors to the positive and negative connections of the right driver coil; and left positive and left negative conductors to the left driver coil. The cable is terminated in two XLR male connectors, one each for the left and right channel. A special headphone amplifier is used that has "balanced" outputs where each channel has a normal audio drive signal and a mirror image inverted drive signal. The trick here is that there is no "ground" to the headphones anymore, so there is no opportunity for the crosstalk distortion described above.

The really interesting thing though is that removing the common ground is only the beginning of the possible improvements from balanced operation. There are different ways to implement a balanced headphone amplifier, the easiest being a single-ended internal design with transformer- coupled outputs. In that case the sonic improvements come from the natural noise rejection of XLR connections (not a major factor in single-ended headphones) as well as the common ground removal. But it is possible to go further in the design and operation of balanced headphone amplifiers by using four completely separate amps for all four legs of the signal (right normal, right inverted, left normal, left inverted) as described in the graph below.

The immediate benefit of such a design is that each amplifier only drives half of the coil allowing for a much better control of that coil. It also doubles the effective slew rate (the voltage an amp can swing per second) as both amplifiers are operating in opposite phase to increase the dynamic realism of the music reproduced. Such a design can get fairly difficult to implement with all discrete transistors as they require careful manual matching to ensure that both halves of the signal are amplified identically but integrated chips do remedy this in a fairly elegant and cost effective fashion, allowing entry-level balanced amps to remain quite accessible. If the theory of balanced headphones seems all rosy, there are a few hurdles you need to be aware of. They all relate to the implementation of a balanced headphone system from start to finish.

First, no stock headphone is balanced. Your choices will be reduced to modifying headphones yourself or through a specialized shop. If you are a Sennheiser HD650 or HD600 owner, consider yourself lucky as Sennheiser actually design their cans with miniature terminals on the cord which can be easily replaced, turning one of those to balanced operation into something no more difficult than swapping the stock cord with a balanced one purchased from HeadRoom ($300 for a 10ft balanced Cardas cable), Rudistor (€150 for a 3m cable), Stefan Audio Art ($389 for 10ft) or Moon Audio ($275 for 10ft). Those are the four sources my online searches turned up but I'll reference others if readers are aware of more.

If you never fell for Sennheiser's sonic aesthetic or Spartan comfort, your options will be reduced but HeadRoom does offer modified AKG K701s and beyerdynamic DT880s for $300 above their single-ended counterparts and HeadRoom can also modify your own pair of those models for a similar charge. Moon Audio offers a very broad range of balanced modifications for headphones including but not limited to AKG, Sennheiser, Beyerdynamic, Grado, Sony and Audio Technica (they also offer to examine any other cans you would like modified and assess feasibility and cost). I have not heard their modifications so cannot vouch for the quality but they unquestionably offer the broadest range of options. Stefan AudioArt also modifies AKGs and Sennheisers for balanced operation and while I have not heard their mods either, based on Srajan's past experience with his AKG K1000s, I would expect quality to be top notch.

The second obstacle awaiting the upgrading listener is the choice of amplifier to drive his or her now balanced headphone. Rudistor and HeadRoom both offer complete lines of balanced headphone amplifiers and Ray Samuels very recently announced its new flagship Emmeline II B52. Rudistor's line ranges from €950 ($1200) for the NX-33 to €3500 ($4400) for the fully-discrete, quad-mono RP010-B MkII. HeadRoom offers balanced headphone amplifiers in 3 of its 5 lines (Desktop, Home and Max), ranging from $899 for the basic balanced Desktop to $5597 for a fully upgraded Max balanced amp with DAC. Ray Samuels' $5350 B52 is a combined pre-amplifier and balanced headphone amplifier with separate power supply but might suffer one handicap versus Rudistor's and HeadRoom's top offerings. The B52 does not have a phase splitter on its single-ended inputs, meaning that despite its balanced topology and connections, it will operate in single-ended mode and shorts pin 3 to ground unless fed a balanced input signal, therefore losing most of the benefits of a truly balanced design. It's not an issue i if you have access to a balanced source but a lot more problematic if like me you enjoy one of Musical Fidelity's CD players devoid of balanced outputs. Maybe the improved sound quality of the B52 is worth this small sacrifice, I don't know. I have not found any other manufacturer of balanced headphone amplifiers but if readers know of others, I'll be happy to add them in future installments to highlight as many choices as available.

The final challenge for the candidate to balanced bliss is to pick one or more sources to listen to. As indicated already, Rudistor and HeadRoom make it easy for you by incorporating a phase splitter after the unbalanced inputs of the amplifier, allowing fully balanced operation of the amp and fully balanced drive of the headphones regardless of the source used. My Accuphase has both balanced and unbalanced outputs which I ran separately into the Rudistor NX-33 with balanced and single-ended lengths of Zu Gede interconnects. I could detect some very faint differences between both types of connections, mainly a more relaxed top end and a slightly lower noise floor in favor of the balanced connection - but I sincerely doubt I would notice it in normal operation outside of critical listening. A significant fraction of modern CD players have balanced outputs, some sounding much better than their single-ended outputs (think Esoteric and their fully balanced players), most sounding from just different to absolutely identical in both modes. One I heard a few years ago actually sounded worse in balanced mode (I'll keep the name of this now defunct brand unmentioned as you have little risk of running into it even on the used market).

Phono cartridges are a naturally balanced source but unless you have the arm rewired, most today come with RCA outputs. Balanced phono preamps are about as rare as balanced headphone amps and cover an even wider price range. With PS Audio, AQVOX and Moon offering balanced phono preamps for less than $1500 and BAT's $8,500 VK-10SE and FM Accoustics' €19000 FM222 MkII, I have pretty much exhausted my knowledge of balanced phono preamps.

One of the most elegant ways to provide headphone listeners with a balanced source may actually have been implemented by HeadRoom who offer a fully balanced DAC upgrade for their three lines of balanced amplifiers. By feeding the digital signal to a separate DAC on each side and allowing them to generate both the normal analog output and its anti-phase equivalent, they generate a perfectly matching balanced signal for the twin amplifiers in each channel. As the Desktop amplifier sent to me for review is equipped with this option, I'll be able to report on
how efficient this solution is. And I left the best part for last since this DAC also has a USB input which will allow me to feed it directly from my laptop, jitter free.

All this is to say that going balanced will cost you at the very least $1200 (amplifier plus balanced cable for a Sennheiser), and another $200 if you want to add a balanced DAC (still the cheapest option to go balanced from source to headphones, excluding a computer which I assume you own since you are reading this review). A more advanced system will set you up north of $2000 to $2500 and a top-of-the-line system close to $6000 excluding a source which can cost as much as your pockets are deep. Yes, it is possible to spend even more on a single-ended system but it is also possible to set up a satisfying single-ended amp and headphone combination for less than $1000 with greater choices. So are balanced headphones really worth it?

Balanced headphones - how they sound.
As you all know, a key to reviewing is to only change one element at a time to assess its sonic impact. In this particular case, it was not possible as both the topology and the headphone cables were different between balanced and single-ended versions of the headphones I had. To keep it simple, I'll talk about the sonic characteristics of the balanced or single-ended version but keep in mind that it entails the sonic signature of the associated cables as well.

To assess the differences between SE and balanced operation I listened to both the AKG K701 (two sets of cans, one stock, one with Cardas balanced cable) and Sennheiser HD650 (only one set of cans, swapping between the stock and Rudistor's balanced cable) through both the Rudistor
NX-33 and Desktop balanced (as both amps can drive both types of cans but obviously only using one internal amp per side in SE mode versus two in balanced operation). I took notes of the differences I heard between both modes with both amps – the differences that were common regardless of the amplifier I attributed to SE vs. balanced operation, the differences I heard only with one amp I considered to be amp specific and not a result of the operation mode (and I'll highlight those in my reviews of the amplifiers). All listening was done using the Atoll CD200 in for review as a single-ended source.

The first thing to highlight is that the nature of the changes was mostly identical between the Sennheisers and AKGs but the relative intensity of the changes varied between both headphone brands. The stock AKG K701 has been reviewed by multiple publications so I am not going to describe its sound and performance in many details except for a few elements that are relevant to this evaluation. The first one and most often voiced criticism of the AKG is its relative lack of bass (or its extremely tight bass depending on your optics and preferences). Balanced operation was able to extract much deeper bass from the K701 without losing grip or control over it. There was simply a far more physical feeling to the lower registers, probably still insufficient to satisfy a bass hound but certainly enough to satisfy anybody feeling the AKGs to be just a little lean and dry in the lower octaves, without going overboard and becoming loose or wobbly. The Rudistor actually went a little further than the Desktop balanced in its ability to pull bass out of the AKGs but both amps made a very substantial improvement). It was very obvious on all recordings with any sort of bass extension but it hit me particularly when listening to Saint-Saens' Carnaval des Animaux [EMI CDM 7691122]. Its double basses represent an elephant and for the first time I actually felt the pachyderm's heavy footsteps shake the bones of my head.

A second trait of the AKG is its musical transparency and again I found the balanced cans much superior in their ability to retrieve musically relevant details (as opposed to brightly enhancing all details) and position them properly in the musical flow. I have been wondering if this was a result of the increased control over the voice coils or just the naturally lower noise floor provided by balanced operation. The last element I noticed is that the stock AKG is the headphone with the widest headstage I have heard so far but the balanced version goes much further on that aspect, with a very wide stage but still very little sense of depth. Whether this is due to the lack of cross talk or the deeper bass better rendering ambient cues or the increased level of details (or all of the above) I can't say but the improvement was significant and returning to SE operation felt as though walls were closing in around my head.

Another critic often leveled against the AKG K701s is their somewhat recessed midrange. I could not agree more. The Rudistor NX-33 in balanced mode did a fantastic job giving flesh and life to their otherwise timid midrange but I did not detect quite the same level of improvement with the HeadRoom amp although there was some improvement too. One thing that balanced operation did not affect is the incredible elegance of the AKGs, their refined treble, their talent with tones and their uncanny ability to reveal music hidden deep in any recording, all which makes them such unique earphones in my mind.

Sennheiser owners are in for a treat too with balanced operation. Swapping cords is a 30- second deal and not permanent. Your headphones will still work on other single-ended amps you may own, not the case for the AKGs or Beyerdynamics which get permanently rewired. The flip side (there has to be one) is that the Sennheiser contacts between cable and phones aren't soldered and I heard static in one ear on a few occasions as I was moving about (it was a rare occurrence in normal use but fairly easy to reproduce by wiggling the cable at the contact point, raising the question of durability). Besides that, the major impact was better control of bass notes. The balanced HD650 went deeper than its SE version and bass got tighter, a little faster and quite more detailed even though not quite of the AKG caliber in that last department. Overall it lost most of the excessively laid-back (some might say boring) character which the HD650 exhibits in SE mode. If you think the HD650 a little ripe at the bottom or lacking dynamic and leading edge emphasis, balanced operation may just be what your doctor ordered. All in all the Sennheiser gained speed and dynamic contrast as well as detail in balanced mode but not to the point of changing overall character. It was more a tightening of bolts here and there and injecting a serious and welcome shot of adrenaline (the HeadRoom Desktop balanced pulled that act in a far more convincing fashion than the Rudistor).

Shostakovich's 11th Symphony's final movement will tax any speaker or earphone-based system with its gigantic orchestral masses roaring forward while a bell strives to be heard above the mayhem. To add to the challenge, Inbal's version with the Wiener Symphoniker [Denon CO-78920] is somewhat congested but the HeadRoom and balanced HD650 did reproduce the full weight of the orchestra without flinching while allowing the bell to resonate in the distance with a clarity I had never heard before. Finally, balanced mode allowed the HD650 to broaden its soundstage quite significantly, nothing as spectacular or wide as the AKGs but clearly less localized between the ears as the stock phones do.

Unfortunately from my perspective, balanced operation did not solve all the quibbles I have with the HD650, namely a nasal midrange and a top end devoid of elegance. Even the Rudistor that worked wonders with the AKG's midrange was not able to bring life and credibility to this part of the HD650's musical reproduction, squeezed as it is between a deep and slightly booming bass at one end and a somewhat bright and stiff treble. Neither did balanced operation change the fact that those cans are plain uncomfortable after a couple hours, mostly because the pressure they exert on one's temples is far too great. The AKG by comparison is almost non-existent.

The bottom line of these few weeks spent with various pieces of balanced headphone equipment was that the improvements brought about by balanced operation were real and very significant. If you own a Sennheiser HD600 or 650, a beyerdynamic DT880 or an AKG K701 and are considering spending money on upgrades (of cans or amp or both), I strongly encourage you to look into balanced operation. The reason is fairly simple. You will never enjoy this kind of improvement by working your way up the curve of diminishing returns unless you spend considerably more than it will take to jump to the beginning of another curve, this one with the tremendous potential of balanced operation. If you are a Grado, Audio Technica or Sony lover, your options are more limited but I would predict you would have a lot to gain by going balanced as well.

All in all, balanced operation sounds so much more real and present, more dynamic and detailed. If you are looking for a dramatic improvement to your headphone, I can't see a more cost-efficient way to do so. In the next two installments, I'll tell you a little more about the balanced amplifiers - two very different sounding animals, two contestants worthy of your attention.
HeadRoom website
Rudistor website
AKG website
Sennheiser website