Tube Audio

Installation

Vacuum tube amplifiers were the only amplifiers until the transistor came around. The tube consists of a filament (heater), cathode, grid, and anode (plate). When voltage is passed through the filament, it boils the cathode and the cathode emits electrons through the grid to the anode. This electron flow is what amplifies the signal. That’s why vacuum tubes glow from the filament and why they need to warm up to sound best.


The 6922 tube that AOpen uses is readily available with the Russian made Sovtek tube retailing for about $10. The rated lifetime of the tube is 5000-6000 hours, used 4 hours a day, it should last about 4 years. A multitude of 6922 tubes are available, at many price points, each with characteristic sonic qualities. The advantage of using different tubes is the ability to truly customize the character of our sound.

The tube

The supporting hardware for the tube amplifier is also first class. The use of large 400V polypropylene capacitors and 1% tolerance resistors are only found on finer audio components. It is worth noting that one of our samples used 200V caps for the smaller set of capacitors. The tube socket is ceramic with gold plated contacts. Even the power supply to the tube has been engineered to reduce interference and increase sound quality. The AOpen manual frequently references the use of Cardas cables in the design, although I could not find any wires with the Cardas name printed on it.

Tube mounted on the motherboard

Close-up of the hardware

Bundled with the tube setup is a small daughter board that fits into an empty PCI back plane. This card might have been a last minute design, as it unfortunately does not mirror the same diligence to detail that created the motherboard. First, the use of a large ¼ inch headphone is a nice, but the off center placement of the jack on the back plane makes fitting it into a standard case very difficult. I tried in 3 different cases, and none would fit the daughter card smoothly. In addition, the size and orientation of the connector dictates that the card have the adjacent slot free. Normally, this wouldn’t be a factor if you use the slots that are occupied by the tube, unfortunately, AOpen put too many connectors and a capacitor on the motherboard next to the PCI back plane, so the add-on card won’t fit without touching the motherboard. The slots farthest away from the CPU are not usable as well because of space constrictions. A better solution would have been to offer a simpler back plate with an external box with the needed connections.

Tube's daughterboard

Tight fit

This daughter board has connections for a mini-jack line-in, a ¼” headphone output, dual RCA outputs, and a SPDIF optical output. The motherboard ships without the tube installed so you get the fun of putting the tube in yourself. AOpen also provides a separate manual for the tube features with detailed listings of compatible vacuum tubes. This manual is informative, but reads like it has been secondarily translated into English.

Another shot of the daughterboard installed

Feature-wise, there aren’t as many input and output options as with premium soundcards, but it should be more than adequate for just about everyone.