Promised to start shipping in June, Alo Audio’s new Continental dual mono headphone amp with USB DAC was already the talk of the show at the Headphonium section of T.H.E. Show Newport. Alo Audio will not be alone with the idea of designing an hybrid tube/solid-state “transportable” headphone amp and USB DAC but it certainly did it with an interesting design.As the company explains, “we examined each element of the amplifier - 6111 tubes, Wolfson DAC, Power Supply, Volume Potentiometer, Battery, Capacitors - to find the best sounding component. As we put them all together we ended up with The Continental Dual Mono.” So that sums it up and we could stop here, but the challenge of creating a battery operated tube amplifier is not that simple. Fortunately Alo Audio explains a little more.
The Continental Dual Mono is a “tube/solid-state hybrid” amplifier, combining “the low output impedance, tight bass and sonic precision of a Class AB solid-stage amp, but with the added midrange richness, expanded soundstage and spaciousness of a tube amp.” The machined aluminum chassis features a Gorilla Glass window to show the tubes without exposing them and expanded vents along the sides to remove the heat.
At the center of The Continental Dual Mono are a pair of New-Old Stock 6111 Vacuum Tubes which are expected to provide 5+ years of operation and are user replaceable. “Each tube is individually soldered to a custom PCB. This provides mechanical isolation of each tube, insulating them from unwanted microphonics. It also provides a quick and easy solution for tube-rolling,” they say.
Directly following the volume potentiometer, the dual 6111 vacuum tubes provide the current gain (ie tube “buffer”) to the amplification. According to Alo Audio, “this provides a clean, strong and dynamic signal to the headphone output stage at a lower output impedance.”
Running in Class A, the Dual Mono implementation of the vacuum tubes “provides improved stereo separation and an expanded soundstage. This is because of lower levels of crosstalk achieved between Right and Left channels. The tubes are auto-biased so there is no need for any time consuming manual biasing.”
Alo Audio used Panasonic 18650 batteries, combined into a three-cell pack in series to deliver the voltage the tubes demand, while also providing better overall dynamics from the amplifier. As they explain, “we selected this battery because it has a high cycle life, approximately 1,000 cycles. It also provides short circuit and overcharge protection. It is also safely user replaceable. So when you reach the end of the batteries life, you can swap in a new one and keep the music going.”
Each stage of the CDM has its own dedicated linear voltage regulator. This means the internal voltage of each stage is independent. This minimizes the sharing of differing internal voltages across each stage of amplification and the digital to analog converter. Alo Audio explains this decision to use dedicated linear voltage regulators, which have lower output noise, because it produces a cleaner voltage rail than typical switching regulators with less crosstalk between the stages and better isolation of the analog and digital sections.
But, as we expected, using a tube stage imposes other challenges which Alo Audio had to deal with. “The design of the tube circuit in The Continental Dual Mono minimizes the inherent microphonic tendencies of the tubes. Utilizing the tubes as a buffer, handling the current gain and not the voltage gain reduces microphonics. Mechanical isolation of each tube and the component layout of the PCB further assist eliminating any distracting microphonics.”
Digital Section and Analog OutputThe Wolfson WM8741 DAC chip occupies the heart of The Continental Dual Mono’s digital section, selected by Alo Audio 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.”
The power feed to the digital section is turned OFF when the USB cable is unplugged. When the DAC is active, the internal Li-Ion battery and linear voltage regulators provide the power. The noisy 5 V rail from the USB is not used for any part of the power.
For lowest amounts of jitter and cleanest digital to analog conversion Alo Audio used a high quality clock IC in The Continental Dual Mono. For both PC and MAC computers, PCM 16/24 bit - with 44.1, 48, 88.2, 96, 176.4, 192 kHz sample rates - as well as DSD64 are all supported. MACs require the driver provided for DSD playback. PCs require a driver for both PCM and DSD playback.
Android devices can pair with the DAC using a micro USB OTG cable and the ALSA driver. Apple iDevices can pair with the DAC using an Apple Camera Connection Kit and micro-USB to USB-A cable.
The Continental’s front panel lights provide indication of the DAC’s current sample rate. Red = 44.1/48 kHz, Green = 88.2/96 kHz, Blue = 176.4/192 kHz, White = DSD64. The connections include an analog “Line Out” on the rear panel, fixed 2 V rms, from the output of the DAC’s analog output stage. It is tapped before going through the volume control or headphone output stage. In this way, Alo Audio intends mainly to showcase the DAC qualities while driving a preamp, integrated amp, or another headphone amp.
The control panel also features a decent sized volume control potentiometer which also acts as on/off switch, with a high/low gain switch close to the headphone output, driving 80 mW into 32 Ω up to 60 mW into 300 Ω. On the left side, there’s a balanced output to drive 125 mW into 32 Ω up to 95 mW into 300 Ω. On the back panel we can find the mentioned line out, a connection for the external power supply (12.6 V/1.5 A), micro USB digital input connector, balanced jack input and 3.5 mm stereo input. There’s a switch to allow selecting which input is being used.
Each Continental Dual Mono amplifier is hand assembled and tested in Portland, OR, and sells for $1,495.
www.aloaudio.com
