Mojo 2 & Sennheiser HD660 S2

A Subjective Review of the Chord Mojo 2 DAC Paired With Sennheiser HD660 S2 Open Back Headphones

This review is purely subjective and based on my experience with the equipment. There was no attempt for any objectivity or comparison with any other equipment. This review does not replicate all the information readily available in the promotional literature for the reviewed products. Go read that yourself! This review is my experience in respect of what is not advertised.

The sound source is Tidal with maximum fidelity subscription played from the Tidal app running on and Android Pixel 4a mobile phone.

Mojo 2 Features

The Mojo 2 is very similar in style to the original Mojo. The new version has an additional illuminated menu spherical button to access the new features of a 4-band equalisation and what Chord call crossfeed. Personally, I do not like the fourth button. The user interface is not immediately obvious. I had to refer to the user manual. OK after a few days, I have learned my way around, but as a computer UI designer, this just feels like it has taken the aesthetic one step beyond what makes sense.

The Mojo2 has been designed to retain plug compatibility with the Chord Bluetooth Poly. This means that the Mojo retains the micros USB plug layout of the original Mojo. This means the new additional USB-C socket is rather awkwardly placed and out of place. Worse still is that the USB-C socket is data only and you still need to connect a power source via the mini USB socket, This seems to really go against the grain of USB-C being the declared European standard socket to rule them all.

Operation is similar to the original Mojo with the additional menu button allowing you to cycle through the new crossfeed and band equalisation options with each option working in conjunction with the up/down volume buttons to increase and decrease the value for each option.

The device feels very solid (similar feel to the original Mojo). One problem I experienced is whether using the USB-C or micro USB is that these small connectors can be easily knocked resulting in a momentary loss of connection and this can cause problems such as the playback being paused.

Sennheiser HD660 S2 Features

They are headphones. You put them on your 'ed.

They are light and comfortable to wear and I had no problems wearing them for a couple of hours at a time.

Sound Quality

Again, this is a purely subjective opinion based on my listening experience solely with this Mojo/Sennheiser combination. No comparisons with other equipment were made.

Bottom Line - I love the sound! And the more I listened, the more I forgot about trying to listen to the equipment and the more I enjoyed the music. The experience was thoroughly seductive. This combination is beautifully neutral. Instruments are well-defined both spatially and tonally. The timbre and detail are excellent. It felt as if I were up close and personal with the music. Vocals can be so darned good that you'd believe the singer is in the room standing next to you.

After initially playing with the four-band equalisation, I found no equalisation was necessary on any of the material I listened to. Crossfeed on the other hand was a genuine and surprising revelation!

The Mojo 2 manual says of crossfeed "to improve spatial effects for a more ‘speaker-like’ sound-stage when using headphones".  As I understand crossfeed mixes a small portion of the stereo channels together a small amount of left going into the right ear and visa-versa. The actual implementation is not revealed and it may indeed be smarter than a simple mixing.

I was initially sceptical about why I would ever want to drag my stereo recording to be closer to monaural, but now I get it. For anyone as sceptical as I was I will explain my experience with crossfeed.

Crossfeed comes into its own when listening to early stereo recordings where sounds were directed exclusively, or almost exclusively, to the left or right channel. When listening to this on a speaker setup in a room, even if the sound comes from one speaker only, the sound will reach both your ears. there will be a minor phase and amplitude difference but you will simply perceive the sound as coming from the left or right. There is no way to produce a sound from the speakers that will result in sound only reaching one of your ears.

With headphones, things are different. If there is a sound on the left channel only, only the left ear gets to hear it. This sounds very weird and disconcerting! It's a bit like sticking your finger in one of your ears.

By analogy, the perceived effect of this one-channel-only sound is rather like when you cover one eye and lose depth perception.  Two eyes allow the brain to use the parallax difference to gauge depth. When you take that extra parallax information away, things can get a bit weird.

This classic Ames' room optical illusion only works when viewed from one specific viewpoint, with one eye (or through the single lens of a camera).

So it is with crossfeed, it seems that mixes just enough information between the channels to avoid that uncomfortable finger-in-the-ear sensation when listening to early, overly enthusiastic, stereo recordings.

Interestingly, it makes little difference when the program material is a more modern recording with better-balanced mixing. It perhaps, just brings the entire sound stage ever so slightly more front and centre, which I like.

I also found that the three levels of crossfeed had only barely detectable differences between them. As a result, I have set my Mojo2 to the middle crossfeed setting and have left it there permanently.

My music source is Tidal with the "Max" subscription offering up to 24-bit, 192 kHz. Not all the music on Tidal is available at this resolution some is available at a lower resolution and bit rates. Reassuringly, even when played directly from my Google Pixel 4a phone, a relatively old and inexpensive smartphone, the Mojo 2 sample rate indicator showed blue, indicating it was indeed detecting 24 bit, 192 kHz when that was the Tidal advertised bit rate. However, the sample rate indicator would sometimes lie or take time to switch over indicating a lower sample rate when the program material was changed.

Overall, soundwise, the Mojo 2 / Sennheiser 660HD S2 does it for me.

Gripes - Or What I Would Have Liked... for the Price

1. Plug compatibility with the Poly resulted in too much of a compromise with the overall case configuration. Two micro USB ports one for data one for charging and a USB-C port that does not charge just feels entirely wrong. It means I have bought this fab new kit and I have to have one cable style to charge, and another one for data. Not only that, but it would have been so much more convenient if the Mojo just had two USB-C ports with power through, so that you could daisy chain a power adapter, Mojo 2 and 'phone and have the data connection while simultaneously charging both the Mojo and 'phone. (you only need one to run out of juice to spoil the fun).
   
   
2. I would have loved to have seen a built-in low-latency Bluetooth. It need not have been of great quality, but it would have provided easy connectivity for my many devices that I route through my active speakers. When I want top quality, I am happy to go with a hardwired connection.
   
   
3. A dedicated MoJo Android/IOS Bluetooth-connected app that would provide a decent graphical user interface for the control of the Mojo settings. As said, the coloured balls are pretty but as the need for more control increases the user interface becomes confusing. I also wonder if the decision to have just 4 bands of Eq is largely down to the awkwardness of the coloured balls menu interface. Subject to the FPGA clever magic being capable, I suspect that full parametric EQ could probably be offered or at least more bands save for having to work out the user interface with coloured balls.
   
   
4. I'd have liked the USB (C or micro) to have an option for locking screws. USB-C and micro-USB are small plugs carrying large amounts of data. I found it all too easy to momentarily lose the connection between the Mojo and my Google Pixel 4a resulting in Tidal pausing. Also, I fear that continual strain on these small sockets could easily result in solder joints coming loose.

So there it is. I love the sound but I have some gripes.

24-JUL-2024 Edited to Add: 

My son has bought the identical Mojo2 and Sennheiser HD660 S2s. I use a Pixel 4a whereas my son is using a Pixel 7a. We both share the same Tidal family subscription at the highest "MAX" quality. We both connect the Mojo to the phone using a short high-quality USB-C USB-C cable (and tried other quality USB-C-USB-C cables)

The difference is that when I select a "MAX" track, the Mojo bandwidth indicator lights up blue indicating 192KHz detected (the advertised bitrate for Tidal MAX), whereas when my son plays a MAX track his Mojo 2 only ever illuminates as green indicating 96KHz. We can find nothing online that explains this difference in the maximum bit rate produced by the two Pixel 'phones. We can also find nothing in the specs for the two phones, or in any Tidal documentation describing maximum audio bit rates.

Does anyone else have anything with verifiable sources on the matter?

1-AUG-2024 Edited to Add:

Eventually after contacting Tidal technical support, I received a reply informing me:

...please note that we don't support bit-perfect on Android 14 yet.

I was recommended (by Gabriel Herrera Manzanilla) on the "Chord Mojo 2 user group" on Facebook to try the Android app "USB Audio Player PRO" This did indeed allow the playing of Tidal MAX quality tracks on the Pixel 7a. Very subjectively, my son and I also feel there is a general improvement in quality using this app over the Tidal app even with CD quality tracks.