Audio Consulting's Silver Rock Transformer Potentiometer by Dick Olsher

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February 2003

Audio Consulting's Silver Rock Transformer Potentiometer
Review By Dick Olsher
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Audio Consulting's Silver Rock Transformer Potentiometer Let me paint a picture for you of what is a pretty common scenario in an increasingly digital audio world. Consider a system that consists of a CD player and power amplifier. The problem is how to best interconnect these two components? If the CD player's output voltage is 2 volts and the power amplifier's input sensitivity is on the order of 1 volt or less -- all pretty typical specifications -- then voltage gain is no longer an issue. Add the additional proviso that the CD player's output impedance is no higher than about 1 KOhm and cable drive issues also go away. At this point, an active preamp with 10dB or more of gain makes little sense. Why bother with all that complexity and gain when all you really need now is a purist means of adjusting or padding volume. That is why "passive preamps" have become popular, though I take offense at the name, which is clearly an oxymoron because such devices have no gain - they do not amplify.

So let's talk volume control. I'm sure that over 99.99% of audio preamplifiers out there use a conventional potentiometer (pot) for volume control. Carbon types used to dominate the scene until about 15 year ago when conductive plastic pots became popular and the latter have by now become an industry standard. Ultra high-end passives may use a fancy stepped attenuator for volume control and may also provide function switching for accommodating multiple inputs. Some passives, however, include no output buffer. That is a bad thing as is aptly pointed out on the Audio Consulting Web page. Simply inserting a stepped attenuator or potentiometer in the signal path without an adequate buffer can lead to serious frequency response deviations due to the RC low-pass filter effect of the passive's output impedance and any cable capacitance. If you are mathematically inclined, the formula for determining the -3dB point for an RC filter is 1/6.28RC, where R equals the source impedance in Ohms and C is the capacitance in Farads. The example given is for a 100 KOhm pot set at medium position, which results in a 50-kOhm resistance in series with the signal. Quoting specifics: "With a 300pF cable or input capacitance, this situation leads to an attenuation of -3dB at 11kHz" and "The lower you go with the volume setting, the narrower the bandwidth becomes right in the audio band."

Enter the Silver Rock and with it a new paradigm for volume control. It consists of two transformers (one per channel) with 24 logarithmic secondary windings. Volume is adjusted up by selecting an increasing number of secondaries. The action of the volume knobs is similar to that of a stepped attenuator, but rather than picking off a different point on a resistor ladder, the variable in the Rock is the selected number of secondary windings. It is interesting to note that unlike a conventional potentiometer, the Silver Rock's source impedance actually decreases as volume is lowered. And since the winding ratio of the primary to secondary is approximately unity (in standard model), the overall voltage gain is also unity. The unity gain also implies a low source impedance, which is primarily determined by the DC resistance of the windings, and is a maximum of 250 Ohms. Frequency response, as usual, is a function of primary inductance and leakage capacitance, and is extended to 65kHz (-3dB). One of the advantages of a transformer-potentiometer is the ability to control bandwidth. With RFI being a major issue in most urban environments, I'm a great believer in only opening the "window" so far. A +6dB gain version of the Rock is also available. However, Audio Consulting recommends you get the 0dB version for all digital sources rated at a nominal output voltage of 2 V or greater when used with amplifiers which are rated at a sensitivity of 1 V or lower. Apparently, 95% of all Silver Rocks sold are the 0dB version.

The Silver Rock (SR) is the brainchild of Audio Consulting's Serge Schmidlin - one of the World's greatest tweakers. He tells the story of the Rock's evolution. Believing in the beginning that a stepped attenuator was the way to go, he decided to build and audition the ultimate attenuator. Everyone seems to have a favorite resistor type/brand these days, Holco and Vishay metal film being two examples. Well, Serge decided to opt for what is regarded by many to be the ultimate resistor for audio applications, and that is the tantalum film type, endorsed by Mr. Kondo of Audio Note Japan, and about as widely available as Kryptonite. A 100kOhm, 24-position stepped attenuator was painstakingly assembled using these tantalum resistors. The shocking truth, according to Serge, was that listening tests showed the finished attenuator to be inferior to an old-fashioned carbon pot. Let me re-enforce that observation by stating for the record that I've never been bowled over sonically by stepped attenuators. They're expensive and they sound OK, but I've never found them to be a critical factor in the overall sound of a well-designed preamplifier. I think that the Audio Sheep Factor is very much in operation here. A few anecdotal findings spread and start a buzz. Others follow suit without critically evaluating the options.

With Serge in the loop, by definition, the SR promised to be no ordinary transformer. For starters, the iron core is a custom design using very thin laminations. High-quality silver wire is used for the windings, and multiple electrostatic shields minimize RFI. An unusual design choice involves the avoidance of conventional metallic or plastic chassis materials exclusively in favor of wood. A painted MDF cabinet is used in the lower priced versions and oak or other prime woods are given the ultimate preference in the cost-no-object versions. Even more exotic is the use of Dieter Ennemoser's C37 lacquer to treat the iron core's laminations. The treatment is even used for the wood cabinet and knobs of the ultimate Rock. For more information about the eclectic Mr. Ennemoser and his magic mojo, surf over to www.ennemoser.com. The intent behind all of these details is of course to minimize vibrational resonances. My review sample included the C37 treatment for the iron core and a resonance treatment coating (granito finish) for the painted MDF cabinet, which unfortunately gives the unit a pretty pedestrian look. Finally, ground lifts are provided on the back panel that is useful in minimizing ground loop hum.

The end result is a very expensive solution for volume control whose final makeup is based on extensive listening tests in the finest tradition of high-end audio. So was the effort worth it? Stay tuned...

The Sound

Much of the time, the SR was inserted between my Balanced Audio Technology VK-5DSE CD Player and several SET power amplifiers whose input sensitivity was 1 V or lower. I also tried it in conjunction with the LAMM Industries LP2 phono stage, but with less success. What follows pertains to using the Silver Rock in the context of a compatible digital front end and power amplifier.

My attention was immediately drawn to the incredibly low noise floor, at least relative to the tube-based line stages I'm familiar with. In addition to being quiet, its sound was also free of any audible distortions. High-frequency glare, solid-state dissonance, electronic glaze, inter-modulation smearing of textures, edgy transients - the sort of gremlins we have been complaining about for years - were totally absent. As a result, the ebb and flow of harmonic textures was supremely pristine. This brings to mind the vision of a crisp and clear Alpine lake unadulterated by the encroachment of civilization. The SR's purity of expression made a lasting impression.

Tonal character was entirely absent. By that I mean that what came previously in the chain was faithfully transmitted to the power amplifier. Substitution of active devices in the chain always revealed the addition of various editorial tonal and textural effects. My favorite tube line stages were "guilty" of warmer and sweeter sound. Low-level detail resolution also suffered by comparison. The ability of the SR to dig deeper into the noise floor of the recording allowed me to more clearly follow the decay of musical transients and resolve complex harmonies. Keep in mind, however, the flip side or potentially negative aspect of neutrality. Your front end has nowhere to hide, as its character flows unimpeded down stream. If you've got an exceptional tube-based CD player, such as my BAT, then you have got nothing to worry about. But if your digital front end suffers from digititis - that annoying bright and crisp sound that music lovers hate - then a bit of active line stage euphonics may not be a bad thing. In my book it is always permissible to "sugar coat" a bitter pill. Ultimately, of course, the best solution is to nail down the sonics of the front end, or you will forever be a slave to its whims.

That is not to say that everything was well right out of the box. I made the decision, probably wrong in hindsight, to periodically listen to the unit during its break in period. For the record, Audio Consulting recommends a 50 to 100 hour music break in period. Serge Schmidlin emphasizes that this is in addition to a two-week polymerization process for any C37 lacquer treatment that may have been applied. During each of these episodes I would marvel at what the Rock did so well but I would eventually come away disappointed with its lack of dynamic conviction. There was, however, incremental improvement in this area, especially after the first 20 hours or so. And then one afternoon I settled down for another audition and discovered to my amazement that the fledgling had finally taken to full flight. Microdynamics were given complete scope of expression. The emotional intensity of the music pent up in the dynamic nuances of the signal was allowed to explode within a wide and deep soundstage. The sense of speed and control was also very much in evidence, as transients unfolded with laser like speed and focus. The three strong suites of the Rock, clarity, purity of expression, and dynamic conviction were blended into a coherent whole. Needless to say, that ended up being an extremely long listening session, and I have been glued to the Silver Rock since.

Conclusion

Kudos to Audio Consulting for having the courage (and good sense) to follow the road less traveled in developing the Silver Rock Transformer Potentiometer (SRTP). In the old days high-end was synonymous with the concept of a artisan company with a new idea taking risks the big boys would not, and in the process nudging the state-of-the-art forward. Audio consulting fits that mold extremely well. By offering an alternative to the resistive type of volume control, a whole new perspective on sound quality has now become possible.

As I mentioned to Serge Schmidlin early on in this project, to my mind the SRTP inaugurates a whole new audio category for volume control. Here is a passive device (no power cord), which requires no active buffer stage. It is also one of those rare components that combine the sonic trinity of clarity, purity, and dynamic conviction in one box. If you own a compatible system, an audition is absolutely mandatory. Trust in the Rock - a five-star product in my book!

Specifications

Inputs: one RCA (Can be special ordered with multiple inputs using selector switch. However, this is said to degrade the overall performance

Outputs: one RCA (Can be special ordered with two outputs for bi-amplifying)

Dimensions: 12 x 5.25 x 10 (WxHxD in inches)

Weight: 13 pounds

Price: $2,900 - $5,300 (depending on version)

Review Sample: MDF cabinet - Granito/C37 Iron Treatment: $3,370