Tip #76: The Slagle Autoformer Volume Control Modules
Reviewed by Dick Olsher
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 $1K to $2K 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.
Inductive controls as a category covers both transformer and autoformer based designs. It turns out that the such designs were first patented during the early radio years in the 1920’s but otherwise lay dormant for many years due to cost considerations, except for a foray by Luxman in the 1980’s. My first exposure to this genre was in 2003 when Serge Schmidlin’s Silver Rock landed in my listening room. The Rock, consisted of two transformers (one per channel) with 24 logarithmic secondary windings. Volume was adjusted up by selecting an increasing number of secondaries. The action of the volume knobs was similar to that of a stepped attenuator, but rather than picking off a different point on a resistor ladder, the variable in the Rock was the selected number of secondary windings. Of course the Rock cost mucho dollars and was out of reach of the DIY community. Interestingly enough, Dave’s own interest in the idea started in the late 90's by Thomas Mayer's praise of an "Autoformer Silver Rock" he had Serge wind for him. With that inspiration, he “tried a pair of 2A Variacs as volume controls and as wrong as it seems, there was some magic.” Dave remarked that about 6 years ago he met Serge at RMAF and took the opportunity to thank him for the inspiration and apologize for "stealing" his idea because he could not afford the Silver Rock. To which Serge replied that he wound his own while in college because he couldn't afford the Luxman. It’s fair to say that DIYers beget DIY products – it’s in their genes.
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’s 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.”
The first order of business was to anchor the modules to a suitable surface. I decided to drop the modules into a polycarbonate enclosure which is no thing of beauty, and sure enough it has already engendered appropriate feedback as that “ugly ass box.” However, I did spend the big bucks on Cardas RCA jacks for the input and output connectors. I considered but rejected the addition of an input selector switch in order to keep the signal path as simple as possible. And besides, with a single digital component, I had no need for switches. I have to confess that my drilling technique and drill bit selection were less than perfect for polycarbonate, which helps explain the large knobs on the front panel, selected in order to hide the resultant ugly holes. Wiring was a piece of cake with the only moment of angst being the result of a dyslexic blunder - reversing the In and Out wires on one channel.
The Slagle modules were asked to compete against expensive line stages in the context of a serious high-end system. The front end consisted of the Weiss Engineering Jason transport and updated Medea DAC. The Volent VL-2 SE loudspeakers were used for most of the listening tests together with a variety of power amps. I should add that these speakers feature fabulous clarity and are extremely revealing of front-end and amplifier sonic differences. To set the stage for the first listening impressions, the petite modules were directly driving the Lamm Audio M1.2 Reference monoblocks; as simple as it gets. What a surprise! There was magic in the air! Absolutely spectacular textural sweetness and purity – as if layers of electronic haze were being washed away. By contrast, resistive volume controls were made to sound grainy. Image outlines were full-bodied as if on steroids and soundstage depth perspective was well fleshed out. Dynamic nuances that previously lay dormant in the fabric of the music were resurrected with conviction. In general, the ebb and flow of musical lines appeared to be dramatically enhanced. What a first impression!
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, This is the Mona Lisa of volume controls; it would be undignified to look for zits in the face of perfection.
The sonic benefits of the Slagle AVC accrue from the elimination of an unnecessary electronic gain stage and replacement of resistive attenuation from the signal path. Bottom line: a superb volume control and the best $350 investment in audio!
Intactaudio aka Dave Slagle
Price: $350/pr fully assembled and tested