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Phaedrus Audio Phusion

Stereo Channel Strip
Published August 2017
By Hugh Robjohns

Phaedrus Audio Phusion front panel.

Boasting an array of unusual and impressive features, this British high-quality channel strip really stands out from the crowd.

Phaedrus Audio are a small British company who specialise in building guitar amps and some unusual studio equipment, mainly with valve technology but they also re-interpret some classic vintage transistor designs. We’ve already reviewed a few of the company’s products, including the Hydra mic preamp (SOS March 2016), the Phamulus vari-mu compressor (July 2016), and the Shuphler stereo matrix (July 2017), each of which are from Phaedrus’ London Series of desktop units, with their distinctive blue hexagonal faceplates.

The Phusion, reviewed here, is closely related to those units but it’s fashioned as a much more conventional 2U rackmounting unit, with a battleship-grey front panel adorned with chicken-head knobs and toggle switches. As the name implies, it is a blend of several existing Phaedrus designs to create what is, essentially, a stereo channel strip. Two independent channels each comprise a configurable mic preamp (with four circuit topology options), a bypassable passive two-band low-cut equaliser, an unbalanced insert point, and a stepped output ‘fader’. There’s also a stereo matrix to convert a Left-Right stereo signal for Mid-Sides processing, and Channel 2 features a DI instrument input and polarity inversion. With this broad range of facilities — absolutely all of which are controlled by rotary switches or toggles, ensuring the repeatability of settings — the Phusion has been designed to serve equally well in tracking, mixing, and mastering situations. Linking these features back to existing Phaedrus products, the Phusion has borrowed from the Hydra mic preamp, the Philter EQ, the Phi DI stage, and the Shuphler stereo matrix, although there are some significant differences in their reincarnations within the Phusion.

Not only is the Phusion styled to resemble classic vintage equipment, it also incorporates one or two old-school features. For example, alongside the usual fused IEC inlet is a pair of 4mm binding posts with a thick piece of wire stretched between them. The green terminal is connected permanently to the unit’s metal chassis and the mains safety earth, while the black one connects only to the audio circuitry’s reference ground. Normally — and as shipped from the factory — these two terminals are connected directly together, joining the audio reference ground to the mains safety earth.

However, if used in a professional installation with a separate ‘technical earth’, the link can be removed and the audio ground referenced instead to the studio’s technical earth with no direct connection to the mains safety earth. This used to be a standard feature of professional studio equipment a few decades ago, and it’s a neat way of avoiding ground loops and noisy earth currents, but it seems rather a rare provision these days.

Another old-school idea is the way the front panel has been engineered. In most modern units the front-panel controls bolt directly onto the front panel of the chassis, leaving nuts and lots of mounting screws visible. The Phusion is built with a separate escutcheon plate spaced slightly in front of the chassis front panel, such that the toggle switches and switch shafts poke through machined holes and slots. In this way, all the mounting nuts for the switches and other fixing screws are hidden, giving a much neater appearance.

Signal Path

Working through the Phusion’s signal path, the two preamp stages accept balanced or unbalanced mic or line input signals via rear-panel ‘combi’ XLR connectors and, in each channel, the signal then passes through a toggle-switched 20dB pad (and 48V phantom power can be applied, if required) before reaching the input transformer’s primary winding. Curiously, the phantom-power LED is illuminated blue when phantom is not activated, and turns red when phantom is engaged. Jumper links inside the unit allow the pad to be increased to 30dB, the idea being to further reduce the signal level through the input transformer for a cleaner sound when working with hot line-level signals (as might be the case in mastering applications).

The transformer’s secondary winding feeds a rotary attenuator switch, whose 11 increments of roughly 4dB each span a 40dB range. In effect, this is the preamp gain control, but it controls the level going into all four mic preamp circuits in each channel simultaneously. Channel 2 features an extra toggle switch to select its exclusive front-panel high-impedance instrument input instead of the rear-panel ‘combi’ XLR.

In addition to the main inputs and outputs, the Phusion features an insert point, which allows, amongst other things, external processors to be hooked up to the M-S matrix.In addition to the main inputs and outputs, the Phusion features an insert point, which allows, amongst other things, external processors to be hooked up to the M-S matrix.

A four-way rotary selector switch picks up the output from one of the four preamp stages, and the signal is then routed through a stereo sum-and-difference matrix, which is engaged with another toggle switch. This matrix allows a conventional L-R stereo signal to be processed in the M-S format, but it can also be used to convert the signals from a stereo M-S mic array into the standard Left-Right format.

Immediately after the matrix, the signal is made available at a rear-panel TRS socket, which provides an unbalanced send/receive insert point (wired to the usual tip=send, ring=return convention). By locating the insert point after the matrix, external signal processing tools, such as compressors or more elaborate equalisers, can be operated conveniently in the M-S domain, which is a very useful feature, especially for mastering. As supplied from the factory, the maximum return input level is +18dBu, but internal jumper links allow that to be increased to +24dBu.

These Audio Precision response curves illustrate the maximum cut and boost settings when switched to the standard (red) and Lo (blue) modes. The green line is the response when the EQ is bypassed. The following two plots illustrate the full range of curves obtainable from the bass EQ section.These Audio Precision response curves illustrate the maximum cut and boost settings when switched to the standard (red) and Lo (blue) modes. The green line is the response when the EQ is bypassed. The following two plots illustrate the full range of curves obtainable from the bass EQ section.The insert return signal is routed straight into the Phusion’s passive (inductor/capacitor) EQ section for Channel 1, but Channel 2’s facilities differ slightly again, this time with an extra toggle switch to invert the signal polarity. By locating the polarity inversion facility here it can be used on the signal returned from external insert processors, giving the useful capability of flipping the stereo image around (Right-Left) when working on an M-S stereo signal.

Inspired by the EQ facilities implemented on EMI’s REDD mixing consoles in the late ’50s and early ’60s, the underlying design of this two-band passive EQ is almost identical to the Philter. However, this version has been augmented with an extra toggle switch to lower the bass section’s turnover frequency.

A set of Audio Precision frequency response curves to illustrate the wide range of tonal responses available using the bass control with the Lo setting engaged. The shelf reaches its maximum level around 50Hz in this mode, an octave lower than the standard setting.A set of Audio Precision frequency response curves to illustrate the wide range of tonal responses available using the bass control with the Lo setting engaged. The shelf reaches its maximum level around 50Hz in this mode, an octave lower than the standard setting.Two rotary switches adjust the amount of boost/cut applied to the bass and treble ranges in 2dB increments over a range of about ±10dB. The standard LF shelf curves normally affect signals below 700Hz, reaching the shelf level at around 100Hz; but the extra toggle switch reduces the corner frequency down to 200Hz, providing more focused control over the real low end with much less effect on the mid-range.

The high-frequency section features an additional rotary switch which alters the shape of the HF response when the signal’s being boosted — the cut side always has a simple shelf response (see plots). The two original EMI EQ responses are labelled as ‘Classic’ and ‘Pop’, the same as the separate plug-in EQ cassettes used in the REDD consoles. The ‘Classic’ setting applies a very broad HF shelf while the ‘Pop’ setting implements a relatively gentle bell-shaped presence peak, centred at 5kHz.

As above, but with the normal response which brings the shelf maximum level up to around 100Hz.As above, but with the normal response which brings the shelf maximum level up to around 100Hz.To supplement these rather basic EQ options, EMI also used an external ‘RS127 Presence Box’ with the REDD console, and this provided a range of additional bell responses with alternative centre frequencies. Consequently, the EQ mode in the Phusion/Philter provides extra bell responses centred at 2.5, 3.5, 8, 10, and 12 kHz. One final EQ option adds a second broad shelf response which starts higher up and has less effect on the upper mids compared to the ‘Classic’ setting.

A single toggle switch bypasses the whole EQ section for both channels, and the output from the EQ amplifier is passed straight into another switchable sum-and-difference matrix to re-convert from M-S to L-R. The output levels for each channel are adjustable independently, with more rotary switches offering a range of ±10dB in 2dB increments, and a pair of blue LEDs turn red when the signals get within 3dB of clipping. The XLR outputs can provide a maximum signal level of +24dBu, while the TRS output sockets are set 6dB lower at +18dBu.

A comparison between the two HF shelf response options. The blue curves are for the EMI Classic setting, while the red curves are for the standard shelf option, which has less effect on the upper-mid frequencies and behaves a  little more like an ‘air’ EQ.A comparison between the two HF shelf response options. The blue curves are for the EMI Classic setting, while the red curves are for the standard shelf option, which has less effect on the upper-mid frequencies and behaves a little more like an ‘air’ EQ.Over on the right-hand side of the front panel is a blue jewelled power-on lamp, and another toggle switch to power the unit on/off (which actually operates a relay in the power supply).

In Use

Like most Phaedrus Audio products, the Phusion is crammed full of character and offers a lot of flexibility in shaping signals in a very musical way. It takes a range of vintage circuit designs and blends them together in an interesting way to create a tool that really does offer something different and inspiring. The four preamps all have distinctly different but complementary sound characters, which are revealed particularly clearly if the signal level is pushed up — something which is made very easy by juggling the combination of input attenuator and output fader. The work I do tends to favour transparency above all else, but I found selecting different Phusion preamps and juggling the input and output levels allowed a wide range of tonalities, from pretty transparent, to subtle amounts of body and warmth, to really quite crunchy and edgy. This range of different sound characters combined with the easy controllability is what makes the Phusion so special.

A complete set of response curves showing every cut/boost setting for each response switch position.A complete set of response curves showing every cut/boost setting for each response switch position.The inclusion of sum-and-difference matrix facilities further expands the Phusion’s versatility, and I’m a great fan of M-S mic techniques and M-S signal processing — the ability to tweak sounds at the centre of a mix independently of those at the edges is incredibly useful, as is being able to control the stereo width at different frequencies, adding bloom and spaciousness at the low end, or sparkling diffusion at the top.

The handbook also includes details on how to configure a couple of useful ‘Shuffler’ modes using the Phusion’s EQ section in M-S mode. Settings are provided for both the original Blumlein Shuffler technique (to optimise near-spaced mic arrays for loudspeaker listening), and the EMI Stereosonic Shuffler system (to correct the HF image-width errors inherent in most stereo recordings).

A set of responses to illustrate the different bell response settings in cut and boost.A set of responses to illustrate the different bell response settings in cut and boost.Passive EQ often lends a more natural and attractive sound character compared with more modern active-feedback EQ topologies, and that’s certainly the case here. Although I was initially apprehensive about the asymmetrical nature of the EQ provision, I found that it actually worked very well indeed in a wide variety of practical situations, shaping both individual instruments and complete stereo mixes very musically and smoothly. Clearly, this EQ is not intended for surgical corrective work, but it’s hard to beat it for subtly shaping and polishing a mix, and with switched values allowing easy comparison of different settings I found it a real asset when mastering.

I’m not a huge fan of the over-bright blue LEDs, and in tracking situations the overall EQ bypass switch can become frustrating, but these are trifling issues. Overall, the Phusion is a superbly characterful and hugely versatile channel strip with a most unusual feature set. It takes the best of a wide variety of well-proven vintage technology and makes it accessible in a really creative and controllable way that’s ideally suited to modern workflows and production requirements. And it is built to exacting standards to deliver a solid performance, while also making it a joy to use and ensuring it should enjoy a long and reliable working life. This is a genuinely novel and inspiring product — I’ve been very impressed with what it can do.

Bench Tests

A view of the inside of the Phusion, with the two dual-triode valves for the RCA-style preamp stage clearly visible in the centre, with the input transformers below and to the left. Most of the remaining electronics are mounted on the vertical PCB directly behind the front-panel controls. The black cylinders sticking out from the top of the front board are the inductors for the passive EQ stage.A view of the inside of the Phusion, with the two dual-triode valves for the RCA-style preamp stage clearly visible in the centre, with the input transformers below and to the left. Most of the remaining electronics are mounted on the vertical PCB directly behind the front-panel controls. The black cylinders sticking out from the top of the front board are the inductors for the passive EQ stage.The internal construction of the Phusion is exemplary. A large PCB occupies the majority of the chassis floor, with a secondary board mounted vertically behind the front panel controls. Two dual-triode valves are mounted horizontally in the centre of the main board, and there are also a lot of op-amps and discrete transistors mounted across the two boards, using through-hole components on the main PCB and surface-mount formats on the vertical front-panel card. There are two mains transformers, a small torroid supplies the main power rails, while a second smaller transformer is used to generate the 48V phantom supply.

I ran my usual set of bench tests using an Audio Precision test set and, pleasingly, my measurements agreed closely with those published by the manufacturer; the performance is very good for a device of this type.

A comparison of the natural frequency responses for each of the four different preamps. The Tube, Cooker and SI-USA preamps share very similar responses, essentially flat between 20Hz and 40kHz, although the Tube amp gain is about 2dB less than the others. In contrast, the Germanium preamp (green) shows a much earlier LF roll-off and a distinct step-down in level above 2kHz.A comparison of the natural frequency responses for each of the four different preamps. The Tube, Cooker and SI-USA preamps share very similar responses, essentially flat between 20Hz and 40kHz, although the Tube amp gain is about 2dB less than the others. In contrast, the Germanium preamp (green) shows a much earlier LF roll-off and a distinct step-down in level above 2kHz.The maximum preamp gain, with the input attenuator and output fader both fully clockwise, varies a little with the preamp modes (see the ‘Preamp Flavours’ box), but is typically around 50dB (the measured range was between 49 and 51 dB). With identical preamps selected for each channel, the gains were matched within 0.1dB at all attenuator settings, so there should be no problem working with stereo mic arrays. With the output fader at its nominal centre/zero position and the input attenuator set fully clockwise, the gain measured 38.5, 40.5, 40.4, and 40.7 dB for the Tube, Germanium, Cooker, and SI preamps, respectively. With the attenuator fully anticlockwise the gains of all four preamps were +1dB. Clearly, this is not a ‘high-gain’ preamp — I wouldn’t recommend it for use with vintage ribbons mics placed far from the source, for example. But there is plenty of gain available for working with close-miked sources and vocals with a wide range of modern dynamic and capacitor mics.

The signal-to-noise ratio measured 89dB at minimum gain, and between 78 and 83 dB at maximum gain, depending on the preamp mode. This implies an EIN value of around -125dBu (150Ω source, 20Hz-20kHz bandwidth), varying slightly with the different preamp topologies. These aren’t the quietest preamps on the planet, then, but I don’t think anyone would expect them to be so, given their vintage circuitry. The fact is that they’re more than quiet enough when working with close-miked sources; this unit is intended to deliver sound character and controllable colour, rather than the ultimate in low-noise transparency.

Crosstalk between the two channels measured -62dB at 10kHz, which is a good result and means the two channels can be used to handle completely unrelated sources without one sound leaking into the other. The frequency responses (with the EQ bypassed) vary slightly with the different preamps, but are generally flat from 15Hz to 50kHz. The Germanium preamp is the exception since that rolls off earlier at the bottom end (-3dB at 30Hz) and has a distinctive 2dB step down at the high end from 2kHz-10kHz. Not surprisingly, the level of residual distortion varied with the different preamps, with the SI-USA preamp being the cleanest by far (0.003 percent). The other three preamps gave figures ranging between 0.1-0.3 percent with nominal output levels, although I found the Cooker preamp started to overload much earlier than the others. The SI-USA and valve preamps had the best headroom margins.

The frequency response via the DI instrument input is deliberately filtered to emulate the response of a  typical guitar loudspeaker, gently attenuating the low end and rolling off the top more severely above 4kHz.The frequency response via the DI instrument input is deliberately filtered to emulate the response of a typical guitar loudspeaker, gently attenuating the low end and rolling off the top more severely above 4kHz.I found that the DI input wasn’t affected by the pad switch at all, and that the maximum gain was about 10dB less than for the mic input (ie. >30dB). Also, in line with the company’s Phi DI box, the instrument signal is heavily filtered, rolling out the low end gently (-3dB at 25Hz) as well as filtering off the high end quite strongly (-3dB at 5kHz). This is intended to emulate the way a typical guitar speaker removes the ‘fizzy brightness’ that comes from the guitar pickups. This is great for recording guitar (and bass) instruments, making them sound far more like the acoustic sounds heard from a guitar amp without the need to equalise the DI signal separately, although it might take the shine and attack off some modern keyboard instruments.

Preamp Flavours

Like Phaedrus’ Hydra preamp, the Phusion offers four distinctly different mic preamps for each channel, selectable via a rotary switch. However, whereas the Hydra provided four transistorised designs derived from classic British studio consoles of the 1960s, the Phusion offers a wider and rather more eclectic range. The first offering is a classic American valve preamp, based on RCA’s BA-2C — a design which formed the basis of countless studio consoles on both sides of the Atlantic in the 1950s and early ’60s. The original design used dual 6J7 pentodes wired as triodes, but here each channel is serviced by a single ECC802 long-plate dual-triode — a close relative of the 12AU7/ECC82. This preamp has the greatest headroom of all four options and a rich, warm sound character.

Next up are two very different vintage British transistor preamp circuits. The first is based on the germanium mic preamp which Dick Swettenham designed for the original Olympic Sound Studio console in London — the forerunner of the later Helios consoles. This preamp employs discrete new-old-stock (NOS) germanium transistors, and it has a very distinctive ‘soft’ overload characteristic.

The alternative British offering uses silicon transistors and comes from a circuit design first published in Wireless World magazine as a hobbyist, general-purpose hi-fi project intended to amplify mic or record pickup signals! It was christened ‘the Cooker’ by Joe Meek, who apparently used it extensively for its interesting distortion characteristics.

Bringing up the rear is another American offering, this time employing a discrete transistor op-amp design with a lot of negative feedback. This preamp gives a very clean and tight sound character with plenty of headroom, and is well suited to recording percussive instruments.

Published August 2017