Portable Bluetooth receiver enhances headphone-jack-deficient smartphones


In my recent coverage of balanced headphone connections, I planned to mention the iPhone, despite the fact that neither it nor any other smartphone that I’m aware of—past, current, or announced future—even offers a balanced analog audio output. And ever since the iPhone 7 generation, iPhones don’t support analog audio outputs at all. So your puzzlement on this topic would be understandable, but I never said that the iPhone or other smartphone would be used standalone in balanced mode.

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To begin, let’s step back. If your device doesn’t have a built-in analog headphone jack (whether balanced or unbalanced, i.e., single-ended), you’re not out of luck with respect to being able to listen to (along with generate, if microphone functionality is also supported) music and other sounds with it. One option, if your device supports Bluetooth, is to pick up a pair of wireless headphones, but to do so you’ll need to shell out even more cash than you’ve already spent on any headphones you already own. The new ones may not be as high-quality as those you already have, either. And quality may be further compromised depending on which audio codecs are supported both by the Bluetooth transmitter (in the source device) and receiver (headphones).

Option #2: If your audio source device has digital I/O (various forms of USB along with, for iOS devices, Lightning, are the most common options here), and if that digital I/O supports the transport of digital audio (Apple’s Lightning-equipped iPod nano, for example, does not), then you can mate a set of analog headphones to the device in conjunction with an appropriately connector-equipped D/A converter. Apple’s Lightning to 3.5mm headphone jack adapter, for example, doesn’t support a balanced output option, but its sonic performance is reportedly still pretty good. And Google offers a conceptually-similar counterpart for USB-C-equipped gear. Plenty of third-party options also exist, at various price and claimed-quality points. But they’re all limited by a few factors that are out of their fundamental control.

Many audio-capable digital outputs in computers, smartphones, and the like, along with their analog audio outputs, are restricted to no better than “Red Book Audio” quality (i.e., 16-bit per-channel sample sizes and a 44.1 kHz sampling rate; 48 kHz is sometimes alternatively-or-also supported). Apple’s Mac computers’ USB ports can do 24-bit sample sizes and 48 kHz sampling rates, as can the company’s mobile devices’ Lightning ports, but audiophiles still sniff with disdain at such plebeian parameters.

These restrictions are not normally the result of system hardware limitations (after all, an uncompressed Red Book Audio bitstream is ~1.4 Mbps, whereas the USB 2.0 peak theoretical transfer rate is 480 Mbps), but are created by operating systems and drivers, which resample source audio streams prior to mixing them together and outputting them over the analog and digital audio output connections. Bluetooth system software suites frequently do similar undesirable audio transforms.

The third option, and the path I’ve chosen to travel, strives to bypass all of the above limitations as much as possible. It begins with, for my Android smartphones, an application called USB Audio Player Pro, also commonly referred to by its UAPP acronym (related discussion on the Head-Fi enthusiast site may also be of interest). As originally developed, UAPP is intended for use with smartphones that have built-in high-quality audio subsystems, like the LG ones I mentioned in my prior coverage. Reflective of the “USB” in the product name, UAPP can also be used with an external tethered DAC, subject to any interface hardware bitrate limitations. And UAPP’s proficiency has now also expanded to cover wireless (i.e., Bluetooth) connections to an external DAC.

In all cases, UAPP’s overall aspiration is to bypass Android’s various bundled audio processing facilities, thereby keeping the audio as bit-accurate and otherwise pristine as possible all the way from the streamed or file-stored source to your ears. As such, it even bundles an integrated Tidal client, including an optional MQA (Master Quality Authenticated) format decoder, for use with the Tidal HiFi service I mentioned a few months back.

Unsurprisingly, given the topic of this post, I’m using UAPP over Bluetooth. On one end of the wireless link (at least currently) are my two Google Pixel 3a smartphones, which ironically still include unbalanced analog headphone jacks. The eventual successor for one of them is the Pixel 4, which my wife got me for our recent anniversary and which I’m sure I’ll have more to say about in future blog posts. The Pixel 4, like its modern iPhone counterparts, does not have an analog headphone output, thereby bolstering the appeal of the Bluetooth-based alternative.

On the other end of the wireless link is a battery-powered portable Bluetooth receiver, the Earstudio ES100 MK2, from a company called Radsone (which currently also sells a USB-only DAC and a set of in-ear headphones). I first stumbled across the ES100 MK2 earlier this summer as I was browsing through Drop’s then-current product inventory (the related and extensive discussion on the product both there and at Head-Fi.org may also be of interest). I’ll start with some stock photos:

two photos of the Earstudio ES100 MK2 Bluetooth receiver

The ES100 MK2 can also operate in a USB-tethered fashion as a wired DAC:

photo of the Earstudio ES100 MK2 Bluetooth receiver USB-tethered as a wired DAC

Now it’s time for my shots, the first as-usual-accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison:

photo of the Earstudio ES100 MK2 Bluetooth receiver with a penny for scale

As you can see, the ES100 MK2 is pretty tiny: 1×2×0.5 inches, to be exact, with a weight of 20 grams. The packaging is equally diminutive, not to mention unpretentious. The ES100 MK2 product page includes links to multiple in-depth documents in PDF format, which I commend to your inspection.

photo of front of the Earstudio ES100 MK2 Bluetooth receiver box with a penny for scale

photo of back of the Earstudio ES100 MK2 Bluetooth receiver box with a penny for scale

photo of open Earstudio ES100 MK2 Bluetooth receiver box

photo of the Earstudio ES100 MK2 Bluetooth receiver documentation

Here’s the ES100 MK2 alongside my unbalanced Shure E5C in-ear monitors (IEMs), mentioned in my earlier balanced headphones post, my Etymotic Research ER4SR IEMs (complete with ear wax, I now see in re-looking at the photo, yuck sorry about that!) with the balanced cable configuration option, my unbalanced Massdrop X E-MU Purpleheart closed-back headphones (another recent anniversary gift), and my Massdrop x Sennheiser HD 58X Jubilee open-back headphones, also with a supplemental balanced cable:

photo of the Earstudio ES100 MK2 Bluetooth receiver and the Shure E5C in-ear monitors

photo of the Earstudio ES100 MK2 Bluetooth receiver and the Etymotic Research ER4SR in-ear monitors

photo of the Earstudio ES100 MK2 Bluetooth receiver and the Massdrop X E-MU Purpleheart headphones

photo of the Earstudio ES100 MK2 Bluetooth receiver and the Massdrop X Sennheiser HD 58X Jubilee headphones

Claimed battery life is 14 hours, dependent in part on what Bluetooth codec (and resultant required computational horsepower) is in use, the listening volume, what audio processing features are enabled, and other variables. This seems to jive with my limited usage so far, as well as the experiences of the HeadFi.org crowd. Speaking of which, the Android (shown) and iOS applications’ exposed feature sets are extensive:

screenshot of the Android Earstudio app

screenshot of the Android Earstudio app equalizer

screenshot of the Android Earstudio app sound control

screenshot of the Android Earstudio app ambient sound

The ES100 MK2 sounds great when paired with any of the earlier mentioned headphones, especially in balanced configurations, and can even drive my higher-impedance Massdrop x Sennheiser HD 6XX headphones to more-than-sufficient volume levels. It even embeds a microphone, so you can take (and make) calls while your phone is Bluetooth-connected to it. And the clincher is the price tag. I bought the ES100 MK2 from Drop back in October for $75 using a $10-off promotion coupon code (plus taxes, and $5 for shipping). As I type these words on Christmas Eve 2020, it’s selling on Radsone’s site for that same $75, and for $69.99 at Amazon. That’s a non-costly audio upgrade if I ever saw (and heard) one!

Do I wish the ES100 MK2 was made of metal, versus plastic? Conceptually, yes, although looking at the situation from an engineering mindset, I also realize that doing so would likely adversely affect the transmitter-to-receiver operating range (due to the well-known Faraday cage effect). And to that point, I should also point out that the “MK2” in the product name reflects a second-generation mechanical redesign with improved operating buttons and other enhancements (but the same electronics inside).

And do I wish the ES100 MK2 had a user-replaceable battery to avoid its sooner-or-later demise? Yes, but I also realize that doing so would adversely affect the unit’s size, weight, and cost. And pragmatically, after two years’ (or so) use, I’ll probably be ready for an upgrade anyway. The lead designer for the ES100 subsequently left Radsone for Qudelix; the company’s Qudelix-5K Bluetooth-and-USB DAC (whose artwork I used in my earlier balanced headphones piece, more foreshadowing!) has upgraded internals now also supporting, for example, the “Adaptive” variant of the aptX codec and currently sells for only $109 on Amazon.

All in all, I’m quite pleased with my purchase. With that, here’s even more foreshadowing: in my next post, I plan to further discuss the audio codec (for both archive, streaming, and Bluetooth transmission) topics I touched on here. And with that, I’ll wrap up for now and await your thoughts in the comments!

Brian Dipert is Editor-in-Chief of the Embedded Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

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