Leckerton UHA-6S MKII Portable Headphone Amplifier
Portable headphone amps are an interesting segment of the market. On one hand they are quite popular, generating a large amount of discussion on HeadFi. On the other hand, a lot of headphone enthusiasts have never used one. I suspect these people do the bulk of their listening at home on a dedicated setup, and just go straight from the headphone jack of an iPhone or other player if they end up listening on the go. I can identify with that mindset - a simple, lightweight, compact solution is very desirable for mobile use.
There are other factors keeping people from using portable amps aside from the extra bulk. For starters, many portable situations don't lend themselves well to critical listening. Activities like jogging, carpooling, or mowing the lawn aren't exactly times where you concentrate heavily on your music. So even if a portable amp has potential to offer a nice upgrade in sound, for many people it just isn't worth it. And then when you are at home in a quiet environment where it would make more of a difference, a portable amp will almost always have less to offer than a comparably priced desktop unit. I can certainly identify with people who are not the least bit interested in these devices.
But what if the portable amp had more to offer? How about a nice DAC section with multiple input options? What about adjustable gain to ensure it pairs well with a variety of headphones? How about socketed opamps to facilitate customization? Features like these transform an ordinary portable amp into a more useful multi-purpose device.
Jack of All Trades
Based in Austin, Texas, Leckerton Audio is a small but increasingly popular firm that deals solely in portable amps. By day, owner Nick Leckerton works as an Application Engineer for Cirrus Logic, helping other companies implement Cirrus chips into their products. He's also been offering portable DAC/headphone amp units under his own name for over 5 years. Unlike some competitors, Leckerton does not release a new model every few months, which could be seen as a good thing or a bad thing depending on your perspective. The UHA-6S MKII is his newest model - building on the already well regarded UHA-6S, the MKII adds several important features while keeping the same $279 price.
The new model is just 70mm x 84mm x 20mm and has rounded edges for a sleeker look. It's got the usual 1/8th" jacks for input and output like nearly all portable amps. Charging is done through the rear-panel micro-USB port, which can also link to a computer for USB DAC functionality. SPDIF digital connections are also offered in both coaxial and Toslink form, an array which is not often found in a portable unit like this. The SPDIF inputs are functional up to 24-bit/96kHz while USB tops out at 16-bit/48kHz. The somewhat limited USB implementation is used to maximize iPad compatibility - using the Camera Connection Kit, a digital stream can be extracted from an iPad without needing a dedicated unit like the Cypher Labs Algorithm Solo. There are some hi-res capable USB chipsets that will interface with the CCK but these older 16/48 types tend to have better compatibility.
Power is supplied by a Lithium ion battery which has a life of approximately 30 hours using the analog input, or 10 hours when fed through an SPDIF input. A switch allows charging to be disabled in order to preserve laptop battery life. Gain can be set at +0dB or +18dB to fit a wide range of headphones.
The guts of the device are packed rather tightly. Considering the amount of features, I'm impressed at how small Leckerton was able to make the enclosure. The original UHA-6S was comprised of two separate boards sandwiched together - one for the power supply and DAC, the other for the amp section. This update fits everything on a single PCB which makes for a thinner device overall, though it is slightly wider.
USB signals are received by a TI PCM2706, transcoded to SPDIF, and sent to the Cirrus Logic CS8416 digital interface receiver. The DIR also handles incoming signals via coaxial or optical connections. The DAC section is based on the flagship CS4398 chip from Cirrus Logic.
The power section is well done---a DC-DC power supply generates isolated + and - 6 VDC power supply rails. This means no "virtual ground" is necessary and no DC-blocking capacitors in the signal path are required. This bipolar conversion is the most expensive way to do a battery-powered DAC. It also takes up more space and draws more power than other methods such as inverted charge pumps or rail splitters. But for pure audio quality this design is as good as it gets.
The amplifier output section is centered around a pair of single channel socketed opamps. The stock configuration uses the OPA209 but other options like AD797, AD8610, or OPA627 can be had for an extra fee. The original UHA-6S came with the AD8610 stock, but the OPA209 upgrade became so popular that Leckerton is now using it by default. Output impedance is suitably low at 0.4 ohm thus preventing any impedance-related interactions.
From a technical perspective, this all adds up to a device that should sound excellent. But does it? Read on to find out.