Audio Consulting Silver Rock -- Attenuator/Potentiometer

Originally purchased in 2006 but never used. Some might say this product is new, but since I don't have a manual and original packaging, it's used but in like new or excellent condition in my book.

Here's what Dick Olsher had to say about this product on the web:

"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 10 dB 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 series with 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 - 3 dB at 11 KHz" 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 65 kHz (- 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 +6 dB gain version of the Rock is
also available. However, Audio Consulting recommends you get the 0 dB
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 0 dB
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 100 KOhm, 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 favour 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: http://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 are 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've 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've 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 artisanal 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 combines 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!"