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Slate Pro Audio Fox

Dual-channel Microphone Preamplifier By Hugh Robjohns
Published November 2011

With two channels and four 'flavours' on offer, Slate's mic preamp promises plenty of flexibility. Does it also deliver on quality?

Slate Pro Audio Fox

Slate 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 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.


The front-panel controls are mainly pretty standard, but the Vintage/Modern and Normal/Combo switches hint at there being rather more going on...The front-panel controls are mainly pretty standard, but the Vintage/Modern and Normal/Combo switches hint at there being rather more going on...

The Fox is a neat, 1U rackmounting device extending about 260mm behind 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 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.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.The circuit boards carry a number of discrete small-signal and output transistors, along with three dual-FET, Burr-Brown OPA2604 op-amps, all 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 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 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).

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 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 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 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, 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.  


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.

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 available to view on the SOS web site at /sos/nov11/articles/slateprofoxmedia.htm


  • 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.


  • Varying gains when switching between the operating modes.
  • Susceptibility to ground-loop hums.
  • There's no calibrated unity-gain setting.


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.



Slate Pro Audio +1 323 656 2050.