Slate Pro Audio Fox 11.11 layout.indd

on test
Slate Pro Audio Fox
Dual‑channel Microphone Preamplifier
Hugh Robjohns
S
late Pro Audio are the
professional recording and
audio equipment manufacturing
arm of an American company called
Yellow Matter Entertainment, who’ve
been in business for about five years.
The very impressive Dragon compressor
(reviewed in SOS July 2010) was their
first hardware product, and the Fox
dual‑channel mic preamp is their second.
There are countless mic preamps on
the market, across all price levels and with
myriad different designs, topologies and
active elements. I’m often asked “Which
mic preamp should I buy?” but there’s
no simple answer: different preamp
designs inherently sound quite different,
and sometimes those differences are
important in achieving the required sound
character. That’s why every recording
engineer I know has a selection of mic
preamps to choose from.
Some preamp designs attempt to
address the requirement for tonal variety
by including selectable filters or harmonic
distortion facilities (the latter are very
much in vogue). Others, such as the
Millennia Twin Topology STT‑1 Origin, go
to the trouble of incorporating a couple of
completely different preamp circuits in the
same device, enabling the user to select
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With two channels and four ‘flavours’ on
offer, Slate’s mic preamp promises plenty of
flexibility. Does it also deliver on quality?
very different tonalities and characters at
the flick of a switch. The STT1 includes
both discrete solid‑state and all‑valve
preamp stages, for example.
The Slate Pro Audio Fox falls into the
same general camp as the STT1, in that
each channel can employ either of two
completely separate preamp circuits.
Essentially, the two circuit designs are
based along the lines of the Neve 1073
topology (complete with input and output
transformer), and a bespoke, ultra‑fast
and linear topology based on op‑amps.
A unique feature of the Fox, though, is
that the input and output stages of these
two circuits can be mixed and matched,
giving four distinct variations in total.
This inventive circuitry was designed by
Tim Caswell, who designed the Dragon
mentioned above, as well as various
hardware synths, guitar amps and many
others things for Studio Electronics.
Overview
The Fox is a neat, 1U rackmounting
device extending about 260mm behind
November 2011 / w w w . s o u n d o n s o u n d . c o m
the rack ears. It weighs a substantial 7kg,
thanks largely to four audio transformers
and a medium‑sized mains torroid. It
features an internal linear power supply,
and the front panel is painted, with a
subtle black leaf motif incorporated
in the surface finish, as in the Dragon
compressor. Although the visibility of
this unique paint job varies considerably
depending on the ambient lighting and
viewing angle, it’s always attractive and
attention‑grabbing.
The rear panel is neat and simple,
each channel being equipped with an
XLR mic input (1.2kΩ input impedance)
and a balanced line output wired in
parallel on both an XLR and a quarter‑inch
TRS socket. (The front panel hosts an
unbalanced high‑impedance (>1MΩ)
instrument input.) The IEC mains inlet
incorporates a fuse holder and voltage
selector (110/220VAC).
Internally, the Fox is very well
constructed, with a metal screen running
front to back, to separate the chunky
linear power-supply section on the right from the
audio electronics on the left. The latter are mostly
accommodated on large, separate circuit boards for
each channel, mounted securely on the base of the
unit, with some smaller daughterboards mounted on
the front‑panel controls. The main circuit boards were
apparently revised after serial number 24 to resolve
a number of issues with gain and polarity mismatching
between the Vintage and Modern topologies, and
a signal-grounding issue. Units before 024 can be
upgraded, and if you have one, you’ll need to contact
Slate Pro for more information. (The review unit was
serial number 047.)
Pretty much all switching functions are handled
by sealed relays, and the preamp mutes briefly when
operating modes are changed, to prevent pops and
clicks from reaching the outputs. American‑made
Altran C4018 mic input transformers in mu‑metal
screening cans are present near the input XLRs, and
large Altran C4000 gapped‑core output transformers
are available to the output stages. Although there’s
no explicit information in the manual as to when these
transformers might be in circuit or not, the vintage
topology employs the output transformer to provide
a fully floating balanced output, whereas the modern
topology provides an impedance‑balanced output
that carries signal on pin 2 of the output XLR, with pin
3 grounded via a 10Ω resistor. Assuming appropriate
connector wiring, these arrangements allow the Fox to
be used to drive balanced or unbalanced destinations
equally well and with consistent output levels. The
input impedance remains constant at 1.2kΩ, regardless
of whether the pad is engaged, or which mode
(vintage or modern) is selected.
The circuit boards carry a number of discrete
small‑signal and output transistors, along with
three dual‑FET, Burr‑Brown OPA2604 op‑amps, all
Slate Pro Audio Fox £1695
pros
• Excellent overall performance.
• Four different circuit topologies, with distinct sound
characters to choose from.
• Solid and well built, using premium components.
• Includes instrument inputs.
• Attractive styling.
cons
• Varying gains when switching between the
operating modes.
• Susceptibility to ground-loop hums.
• There’s no calibrated unity-gain setting.
summary
The Slate Fox is an innovative, dual‑channel mic preamp
with instrument inputs, offering user‑selectable vintage or
modern circuit topologies that are uniquely configurable
in four separate modes. The Fox thus provides the user
with an unrivalled variety of sonic texture and character,
its build quality is solid, and high‑quality components are
used throughout. It performs superbly well, and is priced
extremely attractively.
w w w . s o u n d o n s o u n d . c o m / November 2011
121
on test
S l at e P r o A u d i o F o x
socketed. Full‑size components are
used throughout (no hard‑to‑service,
surface‑mount micro‑components here)
and the construction is neat and tidy
throughout. The use of plugs and sockets
for all the control and transformer wiring
looms means that servicing should
be very straightforward.
The front panel is neat and clearly
labelled, with the two channels’
controls being laid out identically, the
only exception being an illuminated
mains‑power rocker switch occupying
the extreme right-hand side. To the left
of each channel section’s controls is
the instrument input socket mentioned
more like 11.5dB) and select the rear mic
or front instrument inputs. Apparently,
the vintage preamp circuitry doesn’t
require an input pad to handle very
hot mic-level signals, but the modern
topology does, so when activated, the
pad switch reduces the vintage preamp
gain too, to maintain matched signal
levels with the modern topology stage.
The last two switches provide phantom
power (a red warning LED illuminates
when active) and a signal polarity
reversal. (In the pre‑production versions
of the Fox, the pad switch was labelled
‘Range’ and apparently engaged an
additional gain stage).
specifications and check the overall
technical performance. The Windt
Hummer Test, which injects a ground
current between XLR pin 1 and chassis
ground, revealed that the Fox does have
something of a ‘pin 1 problem’ and is
therefore susceptible to ground-loop
hums. The test hum signal raised the LF
noise floor by almost 10dB. I also noticed
that the Vintage and Modern/Combo
modes suffer roughly 8dB more crosstalk
than the Modern mode, which is probably
due to capacitive coupling in the wiring
to and from the output transformers, or
perhaps even some magnetic coupling
between them.
The front-panel controls are mainly pretty standard, but the Vintage/Modern and Normal/Combo switches hint at there being rather more going on...
above, followed by a 12‑position Grayhill
rotary gain switch and an output level
control that acts like an output fader.
The gain switch increments are typically
5dB or less, while the output control is
intended to operate fully clockwise. It
applies about 20dB of attenuation at the
middle position, and either 64 or 67dB
attenuation when fully anti-clockwise,
for the Vintage and Modern modes
respectively. A green LED gives some
indication of the signal level immediately
before the output-level control. With
the input gain set to minimum, this
LED starts to illuminate when the input
exceeds ‑20dBu, reaching full illumination
by -11dBu. There’s no overload or clip
warning, but extremely high signal levels
do cause the transformers to ‘sing’ quite
audibly — as they also do on the Dragon.
To the right, six toggle switches
configure the unit’s various operating
modes. The first two switches introduce
a pad (marked as 10dB but actually
The really interesting switches are
located in the middle, and these select
the Vintage or Modern preamp circuit
topologies, and Normal or Combo
configurations. When in Combo mode,
the circuit’s output stage is swapped
to the alternative topology, so that
Vintage/Combo means the 1073‑like input
section driving the modern output stage,
and Modern/Combo provides a modern
input section driving a 1073‑like output
stage. Flipping any of the toggle switches
produces a reassuring click from the
internal sealed relays, accompanied by
a moment of muting as everything settles.
I noticed that the performance seems to
improve subtly for a minute or two after
changing modes, too, as if the circuit
operating conditions take a while to
stabilise to their optimum conditions.
Tech Talk
As usual, the first thing I did was run a few
bench tests to confirm the published
Audio Precision Test Results
In the course of my tests, I took a number
of measurements using an Audio
Precision Analyzer. The resulting plots are
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available to view on the SOS web site at
www.soundonsound.com/sos/nov11/articles/
slateprofoxmedia.htm
November 2011 / w w w . s o u n d o n s o u n d . c o m
The published specifications claim
a frequency response from 9Hz to 101kHz,
but without giving any tolerance limits.
In my bench tests, the bandwidth in
Modern mode extended between 4Hz
and 80kHz at the -1dB points, while the
Vintage mode has a narrower bandwidth
extending between 6Hz and 20kHz at
the -1dB points. A typical transformer
resonance generates a modest and
well-controlled LF peak in Vintage mode.
With the gain control at zero, the
minimum gain for the Vintage topology
is 14.5dB, but 18.4dB for the Modern
topology (reducing to 3dB and 7.5dB
respectively with the pad switches
engaged). There’s no calibrated unity‑gain
setting, although the output control can
be used to set a unity-gain condition
if required. The Vintage and Modern
topologies have closely matched gains
across much of the available range, but
at the extremes there are significant
gain mismatches. The Modern mode is
significantly louder than the Vintage mode
at minimum gain, for example, while the
Vintage mode is slightly louder than the
Modern option at maximum gain. At high
gain settings, the Modern mode also
exhibits a slight HF boost. The maximum
available gain is 64.5dB for the Vintage
Alternatives
The UK list price puts the Fox amongst the
likes of the Benchmark MPA1, Universal
Audio’s 2‑610S valve preamp, and the BAE
1073MP. Pushing the budget a little more
brings the Chandler TG2, the GML 8302, and
the Neve 1073DPA into reach. These are all
superb mic preamplifiers, but none offer the
sonic flexibility of the Fox.
mode and 63dB for the Modern mode.
These characteristics can be seen in the
Audio Precision test plots we’ve placed
on the SOS web site (see box, left).
The maximum input signal level (gain
zero, pad engaged) is around +23dBu for
the Vintage mode and +15dBu for the
Modern mode, giving output levels (and
THD figures) of +26dBu (0.1 percent) and
+22dBu (0.003 percent), respectively.
Not surprisingly, the Vintage topology
has a higher overall level of distortion
compared with the Modern topology,
with a significant saturation ‘bulge’ for
output signal levels between -15 and
0dBu (see AP plots). At higher levels, the
transformers clearly saturate, compared
with the obvious clipping of the Modern
mode. At low and medium input signal
levels, the Modern mode has extremely
low distortion, but pushing the input
harder elicits some second-harmonic
distortion at first, progressing quickly
to strong third-harmonic distortion. The
Vintage mode has much higher overall
distortion levels anyway, but when
pushed, it is the second harmonic that
tends to dominate.
The claimed Equivalent Input Noise
(EIN) measurement of -128dB seems
a little optimistic to me, and Vintage
mode seemed slightly quieter than
Modern mode, but I didn’t find amplifier
noise a problem in practical applications.
The signal-to-noise ratio for both modes
at the minimum gain setting (taking into
account their different maximum input
levels) is an impressive 111dB.
All Ears
The Vintage topology has very
definite overtones of a classic Neve
1073‑style preamp — obviously warm,
‘phat’, rounded and rich, with a solid,
big‑bottomed character complemented
by a smooth top-end sheen. Switching
to the Combo mode (substituting the
Modern output stage) gave it a tighter,
The Fox’s internal construction is to very high
standards, with the linear power supply isolated
behind a metal screen to the right, and the
output transformers mounted on the left‑hand
side wall. The three small socketed ICs on each
channel’s circuit board are OPA2604 op‑amps,
while the two large ICs are logic control chips
for the relays.
more refined character, with a little
less weight and a touch more focus.
The Modern topology mode is very
different, with razor-sharp transient
attacks, immense presence and detail,
and a very extended, open bandwidth.
It sounds similar to a GML or Millennia
preamp: clean, articulate, neutral and
detailed, but never sterile, anaemic or
thin. Switching to Modern/Combo mode
brings in the output transformers, with
the expected bass enhancement, along
with a slightly tamed top end and a subtle
grunge or richness.
The instrument inputs worked exactly
as expected when I tried them with my
bass and my daughter’s guitar. In both
cases, the signal was clean, bright, and
with a very low noise floor. The minimum
gain in Vintage mode is 10dB and 14dB
in Modern mode (‑1.5 or 3dB with the
pad engaged), rising to 66 and 60dB
respectively at maximum — so there’s
plenty of range to cope with the quietest
passive and loudest active instruments.
The instrument signals follow the same
signal path options (except that phantom
power isn’t available on the instrument
sockets, of course), making this a hugely
versatile instrument preamp.
Unfortunately, deciding which
operating mode is best for any given
source and situation is made slightly
harder by level changes between the
different modes. For example, working
with line‑level signals (gain control at
zero) and starting in the Vintage mode,
the level rises by 4dB when switching
to Combo or Modern modes (and vice
versa when switching from the Modern
topology to the Combo or Vintage
modes, of course). At maximum gain,
the level offsets apply in the opposite
directions: Vintage to Combo or Modern
elicits a 1.5dB drop (and vice versa from
Modern to Combo or Vintage).
These level shifts are very distracting
and make objective evaluation of the
different sonic textures much harder than
it should be, because the human brain
tends naturally to favour ‘louder and
brighter’. However, having said that, the
four topology options do actually sound
quite distinct from one another, and have
clear characteristics, giving real tonal
choice to the user. From a tactile point
of view, all the controls feel solid and
reliable, and the overall sound quality
is extremely good.
Fantastic Mr Fox?
The Fox is an innovative preamp that
enables a very useful range of tonal and
sonic characters to be realised in a single
device. The price is attractive, too. The
gain differences between the various
modes are disappointing and potentially
frustrating, but I suspect most users will
be able to overlook such idiosyncrasies,
given the unit’s immense versatility and
overall performance. The Fox is well worth
auditioning, and if you can only afford one
decent mic preamp, this should be a very
strong contender. ££ £1695 including VAT.
TT Sub 6 +44 (0)845 481 8149.
EE [email protected]
WW www.sub6distribution.com
WW www.slateproaudio.com
w w w . s o u n d o n s o u n d . c o m / November 2011
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This article was originally published in
Sound On Sound magazine, November 2011 edition.
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