Review written by Andrew Park (@Resolve)
Review units provided by iFi and Drop.com
The Sennheiser HD650 (now produced by Drop as the HD6XX) is of course the legendary headphone that's served as both an introductory headphone to this hobby for many budding audiophiles, and also as potentially the best value option in headphones ever. Many consider the HD6XX to be better than anything under $700 or more. This is partially because of how well this headphone scales with higher end gear.
I do feel there is a limit to this scaling, but it's impressive how shockingly good this headphone sounds when I pair it with something like an SPL Phonitor X and a Matrix X-Sabre Pro. Of course, a setup like that costs something over $4000, and at that price you could just get a higher end headphone. So the question becomes, what's the best value to performance setup to get the most out of the HD6XX before the prospect of just going with a different headphone for the same amount of money starts to become more salient? iFi may have an answer in collaboration with Drop to make the perfect source for what many consider to be the perfect headphone.
The following is going to be an evaluation of the Drop Zen CAN and Zen DAC Signature editions, but specifically in comparison to their non-Signature edition counterparts that come in at more modest price tags. Effectively, this article aims to differentiate the different units pictured in this stack and then decide if the Drop Signature versions are worth the price increase specifically for the HD6XX.
The Different Models
iFi Zen DAC - $129.99
This is a DAC/Amp combo - and for most headphones this is all you’ll need. The DAC uses the Burr Brown chip.
Features: ‘TrueBass’ button, Power Match button, balanced out
- Supported Formats: DSD256/128/64, Octa/Quad/Double/Single-Speed DSD DXD(384/352.8kHz), PCM(384/352.8/192/176.4/96/88.2/48/44.1kHz) MQA
- DAC: Bit-Perfect DSD & DXD DAC by Burr Brown
- Output Power: (@1% THD) 4.4mm Pentaconn (BAL) 330mW@32 Ohm; 6.6V@600 Ohm; 16 Ohm - 600 Ohm Headphone 6.3mm S-BAL (SE) 230mW@32 Ohm; 3.3V@600 Ohm 16 Ohm - 600 Ohm Headphone Note: Using 5v power. If powered from USB, power output may vary.
- Output Impedance: < 1 Ω (BAL/SE)
iFi Zen CAN - $169.99
This is the standalone headphone amplifier counterpart to the ZenDAC with more power and extra features.
Features: 4 gain settings, ‘XBass’ button, 3D button, balanced input and output
- Inputs: 4.4mm Pentacon BAL, RCA L/R, 3.5mm Jack SE
- Headphone Output: 6.35mm, 4.4mm
- Maximum output power @ 600 Ohm: 15.1V / 385 mW (Balanced)
- Maximum output power @ 64 Ohm: 11.0V/1890 mW (Balanced)
- Maximum output power @ 300 Ohm: 7.6V / 196 mW (SE)
- Maximum output power @ 32 Ohm: 7.2V / 1600 mW (SE)
- Gain: 0dB, 6dB, 12dB and 18dB
iFi Zen DAC Signature - $249 ($229 with the Amp)
This is a standalone DAC with no headphone output. It also features the Burr Brown chips commonly found in iFi’s DACs. The main difference between the Signature Zen DAC and the regular Zen DAC - apart from the headphone amplifier - is iFi’s use of premium components found in their higher end products.
Features: Balanced out, variable or fixed volume control
- Supported formats: 44.1 / 48 / 88.2 / 96 / 176.4 / 192 / 384 kHz, PCM
- 2.8 / 3.1 / 5.6 / 6.2 / 11.2 / 12.4 mHz, DSD
- 353 / 384 kHz, DXD
- DAC: Bit-perfect DSD and DXD by Burr Brown
- Audio RCA output (unbalanced): 2.1 V fixed, 1 V – 3.3 V maximum (variable)
- 4.4 mm Pentaconn output (balanced): 4.2 V fixed, 2 V – 6.2 V maximum (variable)
- Z output (unbalanced): ≤ 100 ohms
- Z output (balanced): ≤ 200 ohms
iFi Zen CAN Signature - $249
This is the standalone headphone amplifier counterpart to the Signature Zen DAC from Drop. The main point of this unit is to pair with the very popular Sennheiser HD6XX headphones. The Signature edition features a specific ‘HD6XX’ function that adjusts its frequency response (not just in the bass). Note that this is all analog and not digital signal processing or software EQ.
Features: 4 gain settings, ‘HD6XX’ button, 3D button, balanced input and output
- Inputs: RCA, 3.5 (single-ended), 4.4mm (balanced)
- Headphone output: 6.35 mm, 4.4 mm
- Maximum output power (16 ohms): 3.0 V / 600 mW (balanced), 4.0 V / 1000 mW (single-ended)
- Maximum output power (300 ohms): 15.1 V / 756 mW (balanced), 7.6 V / 196 mW (single-ended)
- Maximum output power (600 ohms): 15.2 V / 385 mW (balanced), 7.6 V / 98 mW (single-ended)
- Zout (headphone out): 0.25 ohms (single-ended), 0.5 ohms (balanced)
- Zout (balanced line out): 200 ohms
- Gain: 0 dB, 6 dB,12 dB,18 dB adjust
iFi Zen Blue
The Zen Blue is a Bluetooth streamer that allows you to stream wireless to your iFi stack. This unit will not be part of this evaluation since there is no Drop version and it's really just there to be able to make it all bluetooth capable, but it is pictured in the top image.
Let's talk first about the difference between the DACs. I think iFi is well aware of how strange it might seem to prospective customers that the Drop Signature Zen DAC has fewer features than the regular Zen DAC (which also has a headphone output), costs twice the price, and is housed in what essentially looks like the same box. This may spark some cognitive dissonance, because on the surface it looks like you're paying more for less stuff. Additionally, both units are based around the Burr Brown chip, so it should be a somewhat similar sounding device.
For anyone unfamiliar with the typical Burr Brown sound, you get a somewhat smoother and more laid back type of character as opposed to the more sharp and analytic ESS-based implementations (occasionally known as 'sabre glare'). For the HD6XX, I generally prefer higher end ESS-based DACs or AKM chips for mid-level equipment - at least for off-the-shelf chips - just because I find that running it of a system that includes the Burr Brown smoothness is kind of like doubling down on what's already there with the HD6XX, but I can certainly see why many prefer the Burr Brown sound overall.
When it comes to the differences between the DACs, here's where we have to take iFi at their word when they say they've used more premium components in the Signature DAC that have trickled down from their pro line (the excellent Pro iDSD and Pro iCAN). It should also be noted that the original Zen DAC did create some controversy by not measuring as well as the competition (even though it's still better than any audible threshold), and this may have also been due to the lack of a separate power cable, something iFi didn't include in the original Zen DAC (it's powered by USB, but there's a 5v option as well).
In any case, premium components or not, what I like about the Zen DAC Signature is that they've focused less on trying to cram everything into one box, and instead focused exclusively on the DA conversion function. I'm always a big fan of separating equipment to dedicated functions - better to do one thing really well than many things poorly - and in a way that's what's going on here. The big question, however, is does it sound any better? Because it's twice the price of the original.
The takeaway for me, at the moment, is that while the Signature DAC may sound better, it's not significant enough to warrant the price increase of the original - for the HD6XX specifically. If you think about the fact that this headphone costs $220, to get this plus the stack that includes both the amp and the DAC, it would run you in the ballpark of $700. And yes, I am aware of how well this headphone scales and that the original HD650 cost a lot more, but at $720 you could get into more high end headphones like the Focal Elex or a HiFiMAN Ananda - both of which don't really require much amping. So while the Signature Zen DAC may be one of the better implementations of the Burr Brown chip, certainly at its price point, in my mind that price point is a bit of a tough pill to swallow.
To me, the Signature Zen DAC sounds a bit more 'analytical' than the original Zen DAC or perhaps a more typical term would be more 'resolving', but it's a small enough difference where I'm not confident I'd be able to correctly identify it in a blind test. This perceived difference may also be within the bounds of sighted biases or other framing effects, with the knowledge that it is a standalone unit, and if anything it just goes to show that in spite of the original Zen DAC not being the king of SINAD, it's still good enough for most headphones.
It should come as no surprise to us that, while there may be some sonic differences among DACs in this price range, when they're in the same chip family, those differences aren't as significant as what you might find at the higher end - or between different chip families (or going R2R for example). So in my mind, yes, you do technically get a better performing and better sounding DAC with the Signature edition, but is it really twice as good as the regular version? My answer at the moment is no, but it'll have to be up to prospective buyers to decide how important the knowledge that it's dedicated to digital to analog conversion alone is, and that it's made with more premium components.
Ideal Setup For Sennheiser HD6XX?
The most important comparison in my mind is between the Drop Zen CAN Signature and the regular iFi Zen CAN. For the Zen CAN Signature edition the main highlight is the 'HD6XX' button, which is meant to improve its frequency response. To anyone familiar with the HD6XX (formerly the legendary HD650), this may come to a surprise, since many consider it to already have an excellent frequency response. This is a headphone that's widely regarded as one of the best value options for anyone looking to get into high end audiophile headphones. The other key claim to fame is how well the HD6XX scales with higher end upstream equipment - something that this iFi Signature Zen stack aims to provide.
While all of this may be true, the HD6XX on its own has one potential shortcoming, even on higher end equipment. Its bass response rolls off quite a bit below 100hz, meaning that while you do get some decent presence in the upper bass, for genres that token those low sub-bass frequencies it can sound a little bit lacking. Now it's interesting in this case because while the sub-bass extension on the HD6XX is not good, I imagine this is also one of the reasons why some gravitate towards the HD6XX's with its midrange emphasis in the first place. But nonetheless, the iFi Signature amp has an 'HD6XX' switch on the front that impacts the frequency response.
Here we can see that not only does the 'HD6XX' button boost the bass to fit fairly closely with the Harman shelf from 2013, it also boost the treble significantly around 5-6khz. It should be noted that all of this is achievable through EQ software for free using something like equalizer APO or other system-wide EQ software. Really the Zen CANs are all there to just make that process easy for us with the push of button - and while it's all analog on the amplifiers, if 'fixing' the HD6XX's bass is your goal, this is something you can do regardless of the source.
In my mind, the primary allure of the Zen CAN Signature is that its 'HD6XX' button is something listeners can trust to optimize the headphone's frequency response rather than having to rely on their own ears (and risk boosting things too far, causing distortion) or the inconvenience of having to learn how to do it manually. But as we can see, whether or not its optimal is a more interesting question.
Let's first look at the bass boost. Importantly the Zen CAN Signature's 'HD6XX' bass boost doesn't increase the bass as significantly as the 'TrueBass' function on the Zen DAC, which would make things line up a bit closer to the 2018 Harman bass shelf. This seems like a good move for iFi, since that kind of substantial bass shelf will introduce a certain amount of distortion. With the more modest bass boost of the 'HD6XX' button, in my opinion this is the sweet spot.
Now let's look at the treble boost. This is where I think things get a bit controversial. Apparently iFi has done quite a bit of research to conclude that the HD6XX could benefit from a bit more energy around 5-6khz, and maybe this is the case with the added bass boost that the 'HD6XX' button gives you. In my opinion, however, adding a bit of a peak to this range wasn't necessary.
This is absolutely a matter of preference more than anything else, since the reference target being shown in these graphs is based on a consumer preference curve after all. But, one of the things I love about the HD6XX in the first place is how smooth it is in the treble. Adding a 5-6khz peak like this may brighten it up a bit and give it more energy that's immediately noticeable, but for my taste, this kind of ruins a good thing. I even know some who say the only real issue with the HD6XX is that it has a 5-6khz emphasis by default, and that it's a bit much for their liking.
This is the important consideration with the HD6XX in my opinion - it's a headphone that you're able to just throw on and love your music with for long sessions without being fatigued or have anything be tiresome, and this is in part due to the smooth treble. This raises an interesting question, namely whether we want something that 'grabs' our attention like we might get with the 5-6khz peak that the 'HD6XX' button introduces, or if it's better to aim for long-term listening balance and smoothness. I can see merit to both ideas, but for me I tend to prefer - or at least appreciate - the latter in the long run.
Thankfully, there's an option for those of us who want the bass shelf but also want to retain the smooth treble response of the HD6XX, and it's the regular non-signature Zen CAN - and this costs quite a bit less.
Notice how the bass boost function on the non-signature Zen CAN is identical to that of the 'HD6XX' button on the signature version, without the treble emphasis that was added specifically for the HD6XX. Regardless of your thoughts on what's ideal for the HD6XX, at the very least it's great that there is an option for each side of the fence regarding its treble performance. So for my preference, I would end up going with the non-signature version of the Zen CAN to keep the smooth treble performance, but I can easily see if someone would prefer the Signature version.
Of course, that's not all you get with the Zen CANs, you also get a '3D' effect, which is something the HD6XX can really benefit from. The headphone may have a fantastic frequency response with great detail capabilities, but it's also famous for having a very narrow and intimate soundstage with the dreaded '3-blob' effect. This means that while you get some lateral definition for left and right, and you get some center image definition, front left and front right aren't as filled in, leading to a somewhat uneven image presentation.
Both of the Zen CANs have a '3D' function that does a fantastic job at fixing this issue - at least to my ear. I'm really impressed by how this improves the staging and imaging of the HD6XX - from both amp units, and it makes enough of a difference to where I generally preferred having it on. I think this is also a bit more dialed in than what iFi have done with their '3D' effect in the past, offering a more subtle yet much more appropriate sense of space. There is a tradeoff, however, and it's that when you use the '3D' function, you end up boosting the treble significantly on both (much more than just the 'HD6XX' button did on the Signature amp).
Because of this, I recommend only using the '3D' function on both in conjunction with the bass boost. Without it, it makes everything much too bright. With both functions enabled, it's still reasonably balanced overall - still brighter than the default tonality - but nothing is significantly out of place, at least on the non-signature version. And here again is where I'm left feeling a bit puzzled about the 'HD6XX' function on the Signature Zen CAN. Because when you enable both function buttons, the 5-6khz emphasis is made that much more noticeable due to the '3D' effect's treble boost. Here's where I yet again have a clear preference for the non-signature edition when using both functions - and the HD6XX does really benefit from the '3D' function as mentioned earlier.
So are the Zen DAC and Zen CAN Signature editions the perfect companion source for a Sennheiser HD6XX system? My answer to this is going to have to be a nuanced one. Recognizing first that much of what's been done with these units can be achieved for free with EQ (not the '3D' function, that's really cool), for the much larger segment of audiophiles who aren't comfortable doing that, the Zen stacks are an interesting proposition.
While I love what they've done with the Zen CAN Signature in principle - as in the ability to push a button and 'fix' certain shortcomings with a headphone through an analog adjustment - I find that the adjustment you get from the non-signature edition's 'XBass' function that doesn't boost 5-6khz as well is more to my taste. You may find that you prefer the HD6XX with more lower treble energy there and then the Zen CAN Signature edition would be the better choice, but I worry that this throws off the balance for the 'magic' that this headphone already has. Moreover, both units are quite a bit more expensive than their non-signature counterparts.
In fact, while I do find them to be excellent units that perform well, a $500 stack for a $220 headphone seems like a bit of a stretch, even if it does scale well. It may make a bit more sense for anyone who already owns an HD6XX, and maybe this is the strategy here. But in my opinion, for my taste, I prefer the analog adjustment from the considerably less expensive Zen CAN non-signature edition. So the bottom line for me is that they've already produced their ideal companion amplifier for the HD6XX in the much less expensive non-signature Zen CAN.
-Andrew Park (@Resolve)
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