sonic Farm Silk Road
on test
Sonic Farm Silk Road
Microphone Preamplifier
The latest preamp from Canadian
company Sonic Farm is a classy and versatile
affair, thanks to its wide range of tonal options.
Hugh Robjohns
I
’ve had a few Sonic Farm products
across my test bench in recent years,
including the Creamer (SOS May
2013) and Berliner (SOS May 2016) valve
mic preamps. Sadly, I didn’t get to try the
Silkworm 500-series solid-state mic preamp
module, as that pleasure instead befell my
colleague Bob Thomas. However, it seems
he was very impressed with it (SOS October
2014), and that’s good to know because
the Canadian device that’s sat beside me
now — the Silk Road dual-channel desktop
preamp — is quite clearly derived from
the same design. So much so, in fact, that
clicking on the ‘download manual’ button
on Sonic Farm’s web site delivers the
Silkworm’s manual!
It’s not unusual for manufacturers to
‘re-house’ 500-series modules as desktop
or rackmount products. After all, why go to
all the trouble of redesigning PCBs when
166
often all that’s needed is the addition of
a power supply? However, that’s not quite
what Sonic Farm have done here. The
Silk Road clearly employs bespoke new
circuit boards, as well as a slightly different
control layout and the welcome addition
of a configurable high-pass filter. At its
core, though, this is the same high-quality
solid-state preamp, complete with its
fully discrete gain stage constructed from
carefully hand-matched transistors, and
with a DC-coupled signal path to avoid any
possible capacitor distortion and unwanted
phase shifts. Of course, the downside of
DC-coupling is a risk of internal DC offsets
(which can cause asymmetrical clipping)
and nasty pops and bangs when switching
the output signal (or editing the resulting
audio in a DAW). However, a dual-stage
DC servo system takes care of all that, and
the end result is a preamp which enjoys
a fundamentally clean, neutral, and fast
character, with extremely low distortion and
July 2016 / w w w . s o u n d o n s o u n d . c o m
Sonic Farm Silk Road
$1800 CAD
pros
• Fundamentally fast, clean and
quiet sound character.
• Vibe mode and selectable output
transformer allow a wide range of
sonic tailoring.
• Useful high-pass filter options.
• Musical instrument input mode.
• Integrated universal power supply.
• Excellent technical specifications
and build quality.
• Handy dual-channel package.
cons
• The on-off position of the toggle switches
is unclear.
summary
A dual-channel re-packaged and extended
version of the Silkworm 500-series module,
featuring a discrete transistor gain stage,
interesting input impedance variations, and
a high-pass filter, along with Sonic Farm’s
classic feature of selectable transformer or
solid-state output configurations.
massive headroom. Having said that, in the
traditional Sonic Farm way, this preamp still
has plenty of options for introducing some
musical colour too, if required.
Road Trip
The Silk Road is constructed as
a self-contained desktop unit, housed in
a bright-red, brick-shaped steel case with
a flexible retracting carry-handle on the top.
If you’re eyeing a convenient space at the
side of your desk you’ll want to know that
it measures 150 x 110 x 305mm (WHD) and
weighs 2.9kg. The rear panel contains four
XLRs for the two sets of mic inputs and line
outputs, along with the usual IEC mains
inlet and fuse holder, plus two small rocker
switches for mains on/off and a ground-lift
facility. The internal switched-mode power
supply module accepts 100-240 Volts AC
and generates ±24V audio power rails
— and since these are rather higher than
those of the Silkworm 500-series module,
the Silk Road desktop preamp enjoys
a greater headroom margin and higher
maximum output level (all the way to
a massive +32dBu).
The maximum level for the microphone inputs is an unusually high +30dBu.
Moving to the front panel reveals
a veritable forest of miniature toggle
switches, each channel bearing no fewer
than nine. There’s also a rotary gain trim
control operated with a white vintage-style
knob. The two channels’ control sets are
clearly separated one above the other, and
it appears that the ‘off’ position for the
toggles is towards the right (something the
labelling fails to make clear!).
w w w . s o u n d o n s o u n d . c o m / July 2016
167
on test
Sonic Farm Silk Road
Each channel features a quarter-inch
unbalanced instrument input socket at the
extreme right-hand side (with a 1MΩ input
impedance), while to the extreme left of
each channel is a pair of toggle switches
to configure a 12dB/octave high-pass
filter. The upper, three-position toggle
selects turn‑over frequencies of 80, 160, or
320 Hz, while the lower toggle turns the
high-pass filter on or off. This facility does
not feature at all on the Silkworm 500-series
module (presumably due to panel space
restrictions), and it’s extremely useful. The
published specifications align to my own
Audio Precision measurements, the plots for
which can be found on the SOS web site at:
http://sosm.ag/jun16media.
All of the remaining controls are
identical to those of the Silkworm, even if
some are in different positions. The more
familiar functions include an input selector
for the rear-panel mic or front-panel
instrument connections, a 15dB input pad
(pre-transformer), output polarity reversal,
and a soft-start 48V phantom power (with
red LED indicator). The phantom power
voltage measured comfortably within
specifications even when delivering the
maximum current. An adjacent bi-colour
LED provides basic level metering, showing
green with variable brightness for a healthy
output signal, changing to red at +29dBu as
clipping approaches.
Anyone familiar with other Sonic Farm
products will recognise the ‘OT/SS’ switch,
which allows the user to route the output
signal through either a Cinemag output
transformer (with 100‑percent iron core)
or via a standard solid-state balanced
output driver chip. As you’d expect, the
transformer output sounds a little rounder
and fatter, thanks largely to that iron core,
and this effect is sensitive to the output
drive level. The solid-state output is, in
contrast, very clean and neutral at all levels.
Coarse gain is set with a three-position
toggle switch (labelled ‘H, L, M’) in roughly
18dB steps, although it’s worth noting that
the ‘H’ and ‘M’ modes provide the same
gain range for instrument inputs (because
the high-gain mode is not appropriate in
that case). The conductive-plastic rotary
control provides a continuous trim for the
selected gain mode over roughly the same
range. In this way any required gain can be
achieved easily for the mic input between
+3 and +67 dB.
For the technically minded, the precise
mic input gain ranges are 53.1 to 67.6 dB
(H), 37.5 to 52.8 dB (M), and 18.3 to 38.5 dB
(L), with the pad switch reducing everything
168
The toggle-switch-festooned front panel
provides access to all the normal — and some
unusual — mic preamp features.
by a further 15dB. In practical terms, the
maximum mic input level to generate
a +4dBu output is +2dBu with the pad
engaged, but as clipping doesn’t occur until
+32dBu the absolute maximum input level
is a whopping +30dBu. The minimum input
level necessary to generate a +4dBu output
is -63dBu.
For the instrument input, the H and M
settings share the same gain range of 26.9
to 42.1 dB, while the L mode offers 7.6 to
27.8 dB, and the pad is not active on the
instrument input. The maximum instrument
level for a +4dBu output is -4dBu (gain set
to L), and the minimum is -38dBu (with gain
set to M or H). Again, given the massive
headroom margin, the absolute maximum
input level is +24dBu, which is more than
enough for any electric instrument.
An unusual feature, borrowed from the
Silkworm, is the Vibe switch, which was
originally going to be labelled ‘character’ —
apparently there wasn’t space on the panel
for that so it was rechristened Vibe! It uses
capacitor-resistor (CR) networks to alter
the impedance of the microphone input
in a frequency-selective way. It has three
modes, identified as Smooth, Present and
Warped (S, P, and W, on the switch), where
the ‘P’ mode provides a nominally flat
response. In contrast, the ‘S’ mode rolls off
the extreme highs (giving the impression of
warmer lows and mids), while the ‘W’ mode
adds some extra ‘air’ around 8-10 kHz.
As is always the case with variable
input-impedance designs, the effects are
July 2016 / w w w . s o u n d o n s o u n d . c o m
most apparent with dynamic (moving-coil
and ribbon) microphones, and least audible
with capacitor mics or those with active
outputs — although the inherent frequency
response characteristics of the CR networks
will exhibit a much more apparent effect
with capacitor mics than more conventional
variable-impedance preamp designs would.
As well as altering the frequency
response, these CR networks also affect
the phase response: ‘W’ mode introduces
considerable phase lag at high frequencies
in comparison to ‘P’ mode, while ‘S’ mode
(green) brings in a noticeable phase lag
in the mid band. Engaging the output
transformer while in the default ‘P’ mode
(orange) also brings in some modest phase
lead at LF and lag at HF, of course.
I tested the input impedance (with
phantom power off) for the various Vibe
modes using an NTI Minirator and, given
the reactive circuits involved, I tested at
100Hz, 1kHz and 10kHz. The default ‘P’
mode presented an input impedance of
3.5kΩ at 100Hz rising to 5.2kΩ at mid
and high frequencies. So it’s not quite
the 8kΩ mentioned in the handbook,
but still usefully higher than most stock
mic preamps. Switching to ‘S’ mode, the
impedance remained at 3.9kΩ at 100Hz
but fell dramatically to 760Ω at 1kHz and
560Ω at 10kHz. The ‘W’ mode values were
3.6kΩ at 100Hz, 4.1k at 1kHz, falling sharply
to 470Ω at 10kHz. These radically different
input impedances at different frequencies
alter the loading on a dynamic microphone’s
capsule, and thus affect its frequency
response, and that effect is compounded
by the frequency dependent responses
of the CR input networks themselves. Not
surprisingly, the pad switch reduces the
input impedance for all Vibe settings, and
engaging phantom power brings the input
impedance back to around 2.2kΩ for every
mode (because of the extra phantom feed
resistor loading).
Checking the distortion measurements,
the Silk Road produced 0.01 percent THD
when generating an output level of +24dBu
(at 1kHz), regardless of which output mode
was selected. Reducing the test frequency
to 100Hz revealed a THD+N figure of
0.05 percent for the solid-state output
and a much higher 0.26 percent for the
transformer, which is entirely as expected...
and desired!
In Use
The Silk Road preamp is, as I’ve come to
expect of Sonic Farm, very well-designed
and constructed and performs to
exemplary standards. Whereas most of
the other SF products are clearly designed
to impart some musical colour onto
the microphone’s signal, the Silk Road
is fundamentally a much more neutral
device, although the ability to select an
iron transformer for the output allows
some vintage character to be dialled in,
and that is supplemented by the ‘S’ and
‘W’ Vibe modes which tailor the signal
even more strongly. So in reality, the
Silk Road offers six distinctly different
sound flavours — many of which will
also vary interactively with your dynamic
microphone collection!
Whereas the valve-based Sonic Farm
preamps I’ve reviewed rely heavily on
various flavours of harmonic distortion for
their sonic interest, the Silk Road is much
more about frequency sculpting, which
opens alternative doors to sonic creativity.
There’s no doubt at all that the Silk Road
dual preamp is an unusually versatile and
very high-quality preamplifier, exhibiting
all the qualities we normally associate with
discrete transistor designs. It’s fast and
clean, quite API-like in many ways, yet
the use of Cinemag input and (selectable)
output transformers, combined with
the switchable Vibe tonalities provide
everything from clean, snappy and
modern sounds to coloured, smooth and
Alternatives
Surprisingly, I can’t think of many
dual-channel solid-state preamps with
instrument inputs in a desktop-format, let
alone any with discrete transistor gain stages
or the tonal versatility of the Silk Road!
distinctly ‘vintage’. The instrument input is
also very clean and quiet, but certainly not
bland or sterile in any way.
The only niggle I would raise — but one
which may well not apply to other potential
customers — concerns the aesthetics
of its forest of toggle switches and their
ill-defined on/off positions. Nevertheless,
I enjoyed using the Silk Road very much
indeed. In fact, of all the Sonic Farm
preamps I’ve tested to date this is probably
my favourite and the one I’d be most
likely to buy myself, as it’s ideally suited
to the kind of classical and acoustic work
I prefer.
££ $1800 CAD (about £1000) plus shipping,
import duty & VAT.
TT Sonic Farm +1 310 402 2390
WW www.sonicfarm.com
w w w . s o u n d o n s o u n d . c o m / July 2016
169
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This article was originally published
in Sound On Sound magazine,
July 2016 edition
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