In search of the perfect volume control – that's an appropriate subtitle for this article. And it should be your primary concern, especially if your front end is restricted to a CD player. In that case, line level is a nominal 2V – plenty of signal to redline typical power amps with an input sensitivity between 1 and 2 V. Given such a scenario, it would not only be wasteful but also harmful to insert a traditional line preamp into the chain. The waste aspect has to do with the fact that active preamps usually provide 12 to 18 dB of line level gain. That's way too much gain for a digital front end. The signal therefore would have to be attenuated before it is amplified to its previous level. Gloriously inept! Injecting an unnecessary gain stage with its inherent nonlinearities and distortions into the signal path can only be characterized as dumb. And to add insult to injury, that $1000 to $2000 line preamp of yours only had a budget for an inexpensive volume potentiometer. Upscale conductive plastic pots such as Alps are step in the right direction but hardly approach the state-of-the-art in volume control. What about a stepped attenuator, you ask? Right, they definitely represent a logical upgrade, especially if care is taken in selection of resistors. That's the path I recently took with my First Watt B1 buffer preamp. Phil Marchand of Marchand Electronics installed a pair of 100K model at24-1-a 24-position attenuators, assembled with Takman Rex carbon film resistors. These replaced the stock inexpensive Chinese pots and made for a noticeable improvement in transient clarity and textural smoothness. But resistive ladders are hardly the omega in volume control. That accolade in my experience lies in the realm of the inductive volume control.
Dave Slagle's goal was to offer an affordable inductive volume control that measures really well, is easy to install, and provides sufficiently fine volume steps. It makes much more technical sense to use an auto transformer (aka autoformer) for this application. Since there's no need to isolate the primary and secondary windings, and both windings may be grounded, it is perfectly fine to combine the primary and secondary into a single winding. That's exactly what McIntosh has been doing for decades with their output transformer based solid-state amplifiers. Dave uses an 80% nickel core for increased permeability. As far as the switch, production has recently shifted from the British Lorlin to an Alpha switch made in China for improved reliability. Dave says that he really wanted to keep the thing US and Euro made but sadly the Lorlin switches were causing 1 in 10 pair to come back to with issues. While the original and cheaper modules ($200) were apparently a royal pain to wire up, the new AVC module only requires a three-wire hookup per channel, In, Out, and Ground, much like a conventional pot. Dave would like to give credit and special thanks to Bent Audio's John Chapman for his help in designing the current circuit boards.
One residual side effect of this design is the need to use two discrete switches (coarse and fine) to control volume level. The left switch has 12 positions in 3.75dB increments while the right switch has three positions allowing for +1.25dB, 0dB, and -1.25dB modification of the overall level. The -3.75dB tap is skipped, so the attenuation steps in dB proceed from +1.25, 0, -1.25, -2.5, -5, -6.25, -7.5, and then in -1.25dB steps to -41.25 and off. This scheme takes a bit of getting used to, but with a bit of practice even an old hand such as myself got the hang of it. And of course the ability to adjust each channel independently provides the functionality of a balance control.
To complete the AVC development story, it should be mentioned that a remote version was also investigated. Dave credits Stephie Bench with introducing him to the possibility of using logic and nonlinear switching to control the tap selection and nudging him along this engineering avenue. John Chapman entered the picture again, and to quote Dave, "we did the really slick +7 to -51dB units that have remote and balance control. I think these are the ultimate turnkey solution but at the cost of over $1000 a pair they were well out of the reach of the majority of the DIY community. The problem at hand was a affordable easy to wire Autoformer Volume control unit and the units you have are the solution John and I came up with. I am very pleased with the results from both the sonic and operational perspectives and at the price feel they offer a very nice bang for the buck."
Later listening sessions did nothing to diminish my initial enthusiasm. Transient clarity and speed were consistently strong suites. Struck cymbals decayed with startling filigree and clarity. Blessed by a ridiculously low noise floor, no low-level detail was obscured. Flawless reproduction of ambient clues resulted in a believable spatial impression, and this with no less than solids-state amplification. Bass lines were well defined with no apparent loss of slam. If you're waiting for if or buts, some form of qualification, there really isn't anything major to report on. I could quibble about the lack of super fine volume control or the fact that primary inductance was only matched within 10% between modules, but I'm not going to. After all, if this is the Mona Lisa of volume controls; it would be undignified to look for zits in the face of perfection.
Price: $350 per pair fully assembled and tested.