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.쟔o 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
href="http://www.ennemoser.com">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.쟕he 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.잸s 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
poundsPrice: $2,900 - $5,300 (depending on version)Review
Sample: MDF cabinet - Granito/C37 Iron Treatment: $3,370
Company Information
Audio Consulting14 b Chemin des Vignes1291
CommugnySwitzerland
Website:
href="http://www.audio-consulting.ch">www.audio-Consulting.ch
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