Visually, the Vision phonostage appears to be a pretty simple affair: a small black box powered by an external transformer. The latter is a small 15V AC “wall wart,” which by virtue of its location manages to decrease internal AC-induced hum. The chassis interior is dominated by a large main audio board and 15V regulated power supplies. Its compact size and low cost ($499) are made possible by the use of operational amplifiers. Being integrated chips, op-amps are the product of intensive research and development by major semiconductor manufacturers. The end result typically offers superb and repeatable technical performance in a sub-five-dollar part that fits neatly into an eight-pin socket. Historically, high-end audio has shunned op-amps in favor of discrete circuits, and while that made sense some 30 years ago there’s no longer a valid rationale for it. In fact, op-amps have become ubiquitous in the recording chain. A typical mixing console used to master multi-track recordings may contain dozens of op-amps, as do mike preamps, mixers, limiters/compressors, and eq and reverb effect devices. Even so, op-amp-based audio products are still rare on the high-end scene. One notable exception has been Junji Kimura of 47 Lab, who has elevated op-amp designs to new sonic heights.
Enter Frank van Alstine. Since op-amps can never be discrete, his vision was to keep them as sonically inconspicuous as possible. And Frank has successfully accomplished that with the Vision. I should note that the Vision phonostage is also available as an option that can be included in all-new AVA preamps, at $329 for the moving-coil version and $250 for the basic moving-magnet stage. The design is based on a schematic he drew up in the 1970s during the Jensen’s Stereo Shop days. He says that the original version never saw production because the integrated circuits of those days just weren’t quiet or linear enough, and that trying to null DC offset for each IC using trimming resistors would have been a nightmare in production. Frank decided to revisit that old circuit because of its potential, this time with much better modern ICs. The Vision uses a split, passive, RIAA de-emphasis network. One section provides the necessary 20dB of bass boost between 500 and 20Hz, while the second section provides the required treble cut above a frequency of 2.21kHz. Both of these frequency-shaping networks are first-order 6dB/octave types, and each uses a single WIMA polypropylene capacitor.
Frank relates that weeks were spent rolling in and listening to almost all the modern linear ICs available, including some surface-mount chips, before finalizing the design around a pair of gain stages per channel using the highly regarded (and rather expensive) Burr-Brown OPA627 op-amps, the first OPA627 being selected for low noise. There is no third gain stage; instead, a National LME49600 current amplifier—featuring a high slew rate of 2000V/µS, THD of 0.0003%, and an output current capability of ±250mA—is used as an output current buffer to isolate the circuitry from the outside world. The Vision’s overall gain is a tad below industry standard at 38dB for moving-magnet and a nominal 58dB for moving-coil cartridges, as measured at 1kHz. The mc gain can be boosted by an additional 6dB, but even so, plan on mating the Vision with a line preamp with at least 15dB of gain.
Because of the OPA627’s exceptionally low DC offset and bias current, it was possible to direct-couple the circuit from input to output, meaning that no coupling caps are used. As Frank puts it, “You have no worries about which brand or type of coupling capacitors to use; there are none.” But I’m not so sure that DC-coupling a phonostage is necessarily a good idea. It opens up the possibility of passing subsonic garbage downstream to the power amp and loudspeaker. In an imperfect world, there are the usual subsonic suspects to contend with, most notably record-warp energy and tonearm/cartridge resonances. But the Vision’s design has addressed this issue; the active stages are driven to unity gain at DC, and have no input or output voltage or current offset, minimizing the chance of energy from very low-frequency record warps appearing at the output.
A flexible, user-adjustable mc-cartridge loading scheme has been implemented. Two dual, in-line, package (DIP) switch banks are located on the main board. Combinations of switch settings can provide various cartridge loads. Mercifully, there are only four chassis cover screws that need to be removed to access the board. You can select one of the five fixed resistors (1000, 475, 220, 100, and 47 ohms), or a parallel combination of several resistors. For example, switching all of these resistors on in tandem (positions 7, 8, 9, and 10) gives an effective 30-ohm load. Frank says that if you’re not up on the algebra involved in calculating the loading of several resistors in parallel, AVA will be glad to assist. [The total resistance is the reciprocal of the sum of the reciprocals of the individual resistances.—RH]