There seems to be no end in sight for the significant gains portable audio has seen in the past few years in terms of both sound quality and feature sets. The last big hurdle seems to tackle the elusive integration of tubes. A few desktop units from Woo Audio and others have trickled out into the market but a more portable solution hasn’t really gained significant steam in the public consciousness. Portland-based ALO has introduced several variations on this theme with their original Continental and subsequent Pan Am models, but has since halted production to focus on their new portable flagship called the Continental Dual Mono ($1,495).
The feature set is a hefty one, and appears to be an impressive collection based on learnings and observations from the company’s time in the field.
At first glance, the most obvious (and perhaps surprising) changes to the ins and outs is the inclusion of a 2.5mm balanced connection and the exclusion of the traditional 4-pin RSA port included previous generations. ALO’s amplifiers have always looked to complement the current lineup of portable players, so it may come as no surprise that the substitution has been initiated to correspond the current rise of Astell and Kern players. AK not only provides the source output, but also sells a two pairs of headphones for the receiving end of the chain; additional headphones may require an adapter. Other versatile niceties include a fixed 2Vrms analog output (3.5mm) for direct access to the included DAC section and both SE and balanced ins and outs on the front and back panels. The whole presentation is laid out extremely well and doesn’t skimp on the options. The previously mentioned DAC section can be tapped via a micro USB connection, charging capabilities are exclusive to the included 12.6V wall wart. ALO founder Ken Ball and his team pay careful attention to the current audiophile trends so it is no surprise that the CDM includes both DSD compatibility and an output impedance of less than 1 ohm on both the SE and the balanced headphone output. The front panel also includes 4 colored indicator lights to let you know what file resolution you are hearing as well as a 2-way gain switch for variable headphone output.
As is the case with most amplifiers that feature both SE and balanced output, the balanced connection had more gain that its SE counterpart. As far as the SE headphone output is concerned, the total volume from the high gain stage might be a little light for loud listeners with extremely hard to drive headphones like the HiFiMAN HE-6. There was no issue driving my reference pair of Audeze LCD-3s or the HE-560s, but it was possible to drive the volume pot to its near maximum with quiet tracks (with the HE-560s). This gain appeared to be on par with the SE output from the AK240, but didn’t provide much additional muscle beyond that. This point is a small one, if even a null one when it comes to the overall appeal of the amplifier. Perhaps a subtle hint from the name, the intent of the Dual Mono seems very much wrapped up in the balanced connections and this becomes even more apparent in use. In broad strokes, the balanced connections are where the Continental really shines.
In-ear monitors can be tricky to amplify in concert with full size headphones. They usually require separate amps to pull the best out of their corresponding partners. From the SE output the noise floor on low gain was relatively inaudible through sensitive IEMs (like the JH Audio Layla) and the volume sweep from low to high was extremely manageable. With the increased gain of the balanced output the 2.5mm connection of the Layla’s produced a very slight buzz. None of the amplifier’s outputs were plagued by any micro phonics or the dreaded “ting ting” sound (like a small pebble being throw against a glass jar) that can occasionally creep into tube amplification on this scale. The unit did warm up slightly during use, but kept surprisingly cool considering the encased tube design at play. There have been a few portable pieces that have entered the market that threw off incredibly high amounts of heat (most of which did not employed tubes) so considering the circumstances the outgoing temperature of the CDM seems very well done.
Ken partnered with Vinnie Rossi of Vinnie Rossi Audio for the battery implementation and design. Power supply design is imperative to keeping amplification dynamic and clean, Vinnie elaborated on its application with the CDM:
“The Continental uses a battery pack containing three of the Panasonic NCR18650 cells (same as used in the Tesla Model S) connected in series for a 11.1V nominal battery pack. These are known as the finest 18650 Li-ion cells on the market. The battery pack (and tubes) are user-changeable. What makes the CDM's implementation special is that we are not using a step-up transformers or DC-DC converters for the tube stages. We feed the clean power from the battery pack directly to the tubes' B+ (anode) and achieve remarkably low noise floor and microphonics for a tube-based amplifier. As far as I know, CDM is the only portable tube amp/dac that is all linear-voltage regulated. Therefore, we were able to meet our goals of getting a good taste of ALO's reference amplifier The Studio Six, in a portable package and at a much lower price point.”
While some audio items don’t vary much with playback exposure, the CDM sample I received did change slightly with a burn in period. The mids opened up and leveled out and the overall presentation picked up quite a bit from the initial plug in after a few days of use. Once a balanced playing field was achieved, it became easy to visualize suitable applications for the new flagship. Partnering with an AK device became an interesting proposal. The AK240 ($2,499) isn’t the cheapest player on the market, but it comes packed with a real pretty sound. Through the player’s 2.5mm balanced output via ALO’s SXC 24 cable, the Dual Mono added just a hint of tube to the mix. The stock tubes that come with the amp are a pair of new, old stock Phillips military 6111s. The resulting sound is delightfully linear and refined from this glass. The mids round out just a hair and the bass stays tight and doesn’t get even slightly mushy. It’s a very interesting and appealing approach for those who are adverse to intense tuby-ness. For those who love an even fatter sound, tube rolling is an option. Ken has experimented quite a bit with different combinations and a significant range of alternatives is available directly from ALO’s site. According to Ken, the stock tubes lean on the light side of “Tube-ness” scale while many others take a deeper dive. To my ears the stock tubes hit the sweet spot perfectly. The CDM is even auto biasing to make the rolling process more user accessible. You can see Ken explain the process in more detail here:
While the CDM’s single ended side is both transparent and linear, I found myself drawn to the balanced connection’s texture and range. If you are willing to put down the funds to get this amplifier, I highly suggest you invest just a little more in your headphones and take advantage of the balanced output. The dual mono configuration really shines in this implementation. Even IEMs appeared to take advantage of the situation. The JH Audio universal fit Laylas sounded exceptional when listening to the 24bit/192kHz version of Cat Steven’s Where Do The Children Play?. Vibrant and dynamic, the Continental did an amazing job of creating an organic sound while maintaining a clean window to see through. The Laylas do a top-tier job of creating an out-of-head experience for the restrictive in-ear driver technology. Through the CDM the organ sounds from the track sounded even more precise and natural, a very healthy acoustic combination.
While the amplifier allows for SE and balanced crossover headphone to source, full size headphones appear to see a slight benefit from keeping with the same type of connection, and between the two a fully balanced setup end-to-end again appeared as a preference. An A/B between the AK240 source produced no frequency anomalies, although some tinkering can most likely be achieved with tube rolling. The CDM masked no detail.
While still a great performer for the price, the $500 AK Jr, doesn’t quite have the resolving power of its bigger brother. Through its SE line out connection, the amplifier section of the Continental remained true to the source with no artificial sweeteners or preservatives, just a mild injection of the previously mentioned well-placed hint of tube. Compared to the AK Jr’s headphone output the advantage of CDM is even easier to pin point. Expanded soundstage, a more delicate dimensionality - the entire presentation feels enhanced with increased levels of energy and dynamics.
The digital section of the unit is driven by a Wolfson 8741 chipset married to a latest-gen CMedia 6632A USB interface. The connection requires a driver install for DSD usage to a MAC, but all other Apple interactions are driver-free. This includes connectivity for lighting-based iDevices. Both an iPhone 6 Plus and iPad Air worked perfectly out of the gate via a lighting to USB camera adaptor with no additional fuss. From ALO’s site:
“We selected this chip after extensive examination of all available reference DAC chips. The WM8741 has exceptional signal to noise ratio and extended dynamic range. It also provides low noise, low distortion and superior linearity. The Wolfson provides high-resolution DSD and PCM playback and offers musically compelling digital filters. WM8741’s minimal phase digital filter is more natural sounding because it has no ‘pre-ringing’ of its impulse response. Just as a piano doesn’t produce sound before a key is pressed, the minimal phase filter doesn’t ‘pre-ring’ its impulse response.”
At the market matures around audio DAC chipsets, it seems manufacturers are looking beyond the ESS SABRE 9018 for more natural presentation and easy implementation/programing. The multi-platform compatibility here is a nice touch. The size and heft (not to mention the use of tubes) would probably prevent a consumer from strapping the CDM to an iPod and putting it in a pocket, but its Walkman tapedeck shape is wildly appropriate for any desktop solution. It is portable and can be easily moved, but I wouldn’t recommend just throwing it around like you would a phone due to the tubes.
Comparing the digital section to Auralic VEGA ($3.5k) produced interesting results. The much higher price of the VEGA isn’t quite a fair cost comparison, but those extra dollars do an excellent job of manifesting themselves as natural, lived-in musical reproduction. The comparison here revealed much of the same. The VEGA was whimsical with a softer edge but made the already solid digital/analog pairing even more believable. The 8741 gets high marks for resolving power and musicality on its own. The overall feeling of the chipset seems to be in line, if not a step forward from most digital sections in amp/dac combinations that have made it to market in recent years. There were absolutely no aberrations in frequency response that could be pinned on the Wolfson and the level of detail was very impressive. Bonus points to ALO for including a line out to tap into a full stereo or another headphone amp, a feature that seems grossly overlooked in the portable sector.
At $1,495 the Continental Dual Mono is the most expensive portable amp/dac combo I have ever reviewed. But with portable players beginning to hit the $3.5 mark the move seemed almost inevitable. It adds just the right amount of fun to the mix without ever being overbearing or colored. It is feature rich in all the right places and leaves very little else on the table. The included stock tubes feel right on the money but allow for tube rollers to fine tune the device to their heart’s content. If the CDM falls in your price range suit up, grab yourself a pair of respectable headphones and lose yourself in the music. The best is only going to continue to get better.
- Product - ALO Audio Continental Dual Mono DAC / Amp
- Price - $1,495
- Product Page - Link
Source: MacBook Air, Astell and Kern Jr., AK240, iPhone 6 Plus, iPad Air
DAC: Auralic VEGA
Headphones: Audeze LCD-3, HiFiMAN HE-560, JH Audio Layla (Universal), JH16 (Custom), Beyerdynamic AK T5p
Playback Software: Audirvana Plus, iTunes
Cables: AudioQuest Victoria, Zu Mission RCA Mk.II-B, ALO SXC 24 2.5mm to 2.5mm balanced
Where To Buy:
Addicted To Audio (Australia)
About The Author
I’m a recovering musician turned audio reviewer. I currently manage and write reviews for Audio-Head.com and freelance with several other publications. I love tech and the tools of music, especially the ones involved in reproduction. After I finished my undergrad degree in business I went to the local community college and got one in photography, which was way more fun. I like it when people have unbridled enthusiasm for something and I have the utmost respect for individuals who try to create, even more for those who are good at it.