Probing a USB analog audio adapter
How do engineers squeeze all the necessary circuitry (and what is it?) into one of these devices, and do so this inexpensively?
With the demise of analog audio line out, headphone (output-only), and headset (adding mic-in) jacks in modern electronics devices—computers, smartphones, tablets, and the like—alternative methods of connecting analog audio sources and destinations are becoming increasingly common. Bluetooth-based wireless mating is certainly one option:
but the audio peripheral must also be battery-powered (and therefore potentially charge-drained when you try to use it) in this case. And quality can also be hit-and-miss depending on the lossy codec options supported (and selected) at both ends of the connection, not to mention degradation resulting from other spectrum-overlapping broadcasters.
Diminutive wired adapters
The other common option involves instead leveraging the digital audio (plus power, along with other functions) connections that are still present in these devices. Admittedly, the Earstudio ES100 MK2 shown above can alternatively operate this way, too:
but that’s not the prevalent use case for this particular peripheral, which, anyway, is also no longer seemingly available for sale (I’ve got its successor queued up to discuss in the future). Plus, it was bulky and priced at $99; the Apple Lightning-to-3.5mm Headphone Adapter, shown below as usual (as well as with photos that follow) accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes:
was only $9 when Apple was selling it (when I caught wind of the pending closeout, I bought up not only the one shown above but also a few others before inventory was depleted), not to mention being self-powered over Lightning and delivering remarkably solid audio performance (and squeezing in not only the ADC and DAC but also the necessary MFi certification circuitry).
Now that Apple has transitioned its devices to USB-C, both it and Google, along with others, offer(ed, in Google’s case) diminutive, cost-effective, and performant USB-C-based successors:
I found a two-pack of them on sale for $2.09 the other day, believe it or not:
And my wife even bought me a balanced headphones-supportive USB-C adapter for Christmas!
Size-simplified dissection
That said, with iFixit’s “rough” teardown results as a guide (after seeing how challenging a community member’s experience was, iFixit staff stuck with x-ray analysis for their own coverage), I was loath to tackle the dissection of one of these diminutive devices myself. Instead, today I’ll be showcasing something a “bit” bigger, albeit presumably based on the same fundamental building blocks; Sabrent’s USB to 3.5mm Jack Audio Adapter, which claims to support up-to 24-bit and 96 kHz high-res audio and cost me only $6.98 on Amazon last summer:
As the above stock photo shows, and unlike one of the earlier adapters that merges both headphone and microphone functions on a common connector, this one (akin to a computer sound card, which is its target use case) splits them into two jacks; a stereo one for audio out (96 dB SNR claimed) and a separate one for the mono audio input (90 dB). Plus, the manufacturer conveniently provided a preparatory conceptual cross-section diagram, too:
From past similar experience, however, I’ve learned that such graphics don’t necessarily match reality, so I’m still going to dig inside going to satisfy my curiosity. Some box shots to start:
Inside is the adapter, safely ensconced by rubberized foam padding:
along with a few snippets of literature:
The one on the left is just the usual legal gobbledygook, in multiple languages:
Here’s our patient, first the body:
Now both ends:
See, two connectors!
Don’t overcomplicate the disassembly
The body is a mix of plastic and aluminum…I didn’t realize at first:
that the latter went all the way around the outside:
No, Brian, there’s no screw holding the chassis pieces together; it’s a single-piece assembly from the start:
Duh:
That’s much easier:
With the front panel now popped off:
the PCB now pushes right out the front, following right behind it. Connectors on top:
And…whaddya know…for a pleasant change, the C-Media CM3271 USB audio controller shown in the earlier conceptual diagram actually matches what’s on the PCB underside!
It’s no longer listed on the supplier’s website, but I still found a datasheet (PDF).
I still don’t know how other USB audio adapter manufacturers squeeze all the necessary electronics into their even more diminutive devices, but I’m also still not confident that I would have gotten the answer to that question if I’d tried (versus simply obliterating the product in the process). I’m happy with this alternative approach and end result, and I hope you are too. Agree or disagree, let me know what you think in the comments!
—Brian Dipert is the Principal at Sierra Media and a former technical editor at EDN Magazine, where he still regularly contributes as a freelancer.
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