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Phaedrus Audio Hydra

Microphone Preamplifier
Published March 2016
By Hugh Robjohns

Phaedrus Audio Hydra

This many-headed beast offers you the flavours of four classic ’60s London studio consoles.

Phaedrus Audio, based in Maidstone in the UK, are an intriguing company who manufacture a small but impressive range of valve guitar amps and combos, as well as a number of interesting studio products inspired by the bespoke and ground-breaking British mixing consoles of the 1960s. This was the transitional period from sophisticated valve designs into the early solid-state circuitry that used germanium and silicon transistors, and it coincided with the pioneering heights of the British music industry, when the likes of EMI, Decca, Trident, Helios, Neve and Calrec were all developing their own unique recording console technologies.

The company’s London series comprises a number of (mostly valve-based) products inspired by the leading British studios of the late ’50s, and includes the Phab valve mic amp, the Philter passive three-band inductor EQ, the Phamulus delta-mu valve compressor, the Shuphler stereo imaging processor, and the subject of this review, the solid-state Hydra ‘multi-headed’ mic preamp. These all share the same unusual form factor, being housed in an extruded aluminium case with a thick hexagonal front panel, painted in a bright blue. The dimensions of the case are roughly 110 x 50 x 220 mm (WHD), with the front plate being 150mm at its widest point. Although they can each be used independently as desktop modules, powered by an unusually heavy 12V AC wall-wart supply with screw-locked coaxial connector (all of the circuitry’s required power rails are synthesized internally from this 12V AC input), any two modules may be mounted in an optional 2U 19-inch rackmounting ‘keeper plate’. If the idea of a wall-wart PSU puts you off, an optional (but expensive) a Phuel power-supply unit is available, and this can power up to two London-series modules. It shares exactly the same form factor as the modules, with a matching front-panel rotary on-off switch, and features a high-quality toroidal transformer.

Heads Up

Named after the ancient mythical multi-headed serpentine monster, the Hydra is a single-channel mic preamp which, uniquely as far as I am aware, boasts four separate vintage mic preamp ‘heads’, any one of which may be selected via a front-panel switch. The mic and line inputs are connected via a single combi-XLR socket, while the electronically balanced output is on a standard XLR.

The Hydra’s front panel is simplicity itself, with just two rotary switches operated with vintage-style, cream-coloured knobs, separated by a miniature, illuminated round VU output meter, calibrated for the traditional British alignment of 0VU = 0dBu. The 11-step input-level switch is on the right-hand side, with a black button below to select phantom power (indicated by a blue LED). To the left is the four-way ‘head’ selector, with another black button below to engage a 20dB input pad (which doubles up as a line-input mode). A recessed screwdriver trimmer is also provided to adjust the input ‘drive’, controlling the input transformer’s saturation and hysteresis, and modifying how hard the preamp stages are pushed.

Tour Of London

Before delving into the details, it’s worth noting that the Hydra was designed to complement the company’s Phab valve mic preamp, which is derived from a revered design used at Abbey Road. By the 1960s valve technology was very mature and so there was a lot of commonality in the designs of the mic preamps in valve mixing consoles, and therefore also in their sound character. However, the same couldn’t be said of the early semiconductor-based consoles that were emerging at this time, promising better technical specifications and facilities, while also saving cost and space!

The Hydra’s preamp ‘head’ options are denoted by four London postcode districts, SW1, SW13, NW8 and W1, each being the location of a famous studio back in the day. As each recording studio (and broadcaster) essentially had to design and build most of their own equipment to meet their own needs, these early console designs tended to be quite individualistic and idiosyncratic. Some employed germanium transistors and others silicon, and all employed different circuit topologies and power-rail voltages. Consequently, they all had distinctive tonalities which inevitably shaped the sound character of the records produced in that era. Phaedrus Audio decided that no single transistorised preamp could ever recreate the full range of sounds associated with those ’60s studios, and so this unique four-headed design was conceived, combining four completely separate and discrete preamp sections in the one unit. All four designs share the same mic transformer to boost the input level, but via different primary and secondary taps. Intriguingly, all four mic preamp heads happen to use three-transistor gain stages, too — albeit in radically different configurations.

The NOS germanium transistors for the Olympic Sound preamps are at the bottom, and the BC109s for the Abbey Road preamp just above, with the associated band-pass filter stages to the right. Below the input transformer is a  gyrator circuit — a  form of synthesised inductor — which is part of the Drive circuitry — and the VU meter driver sits above the transformer.The NOS germanium transistors for the Olympic Sound preamps are at the bottom, and the BC109s for the Abbey Road preamp just above, with the associated band-pass filter stages to the right. Below the input transformer is a gyrator circuit — a form of synthesised inductor — which is part of the Drive circuitry — and the VU meter driver sits above the transformer.

Beatles fans will probably already realise that the NW8 mode relates to EMI’s legendary Abbey Road Studios in the St John’s Wood area of London, and this preamp is a recreation of the design employed in the famous transistorised consoles introduced in 1968 — consoles used to record the Beatles’ Abbey Road album and Pink Floyd’s Dark Side Of The Moon, among countless others. The circuitry employs three DC-coupled BC109 silicon NPN transistors, and a 20dB headroom margin was achieved by employing a low operating level around -8dBu. The main power rails are ±12V, although the first two common-emitter transistors, which provide most of the gain, actually work on a lower +8V sub-rail.

Despite the low standing current (below 2mA), the noise performance from this simple circuit is actually very good (the best of the four Hydra options, in fact), which is just as well given the depressed operating level! The gain stage also incorporates high- and low-pass filters with second-order slopes to replicate the full console’s conditions, and these play a major part in defining the sound of this revered design.

Moving a little further south towards London’s Soho, the W1 mode is a recreation of the microphone amplifier employed in Trident Studios’ A-Range console design. Trident Studios started in 1967, recording the likes of the Beatles, Queen, David Bowie, Black Sabbath, Elton John, Rod Stewart, Genesis and Supertramp, to name just a few, and the custom-built console had some very advanced features which proved extremely popular. A number of customers asked Trident Studios to build similar consoles for them, which led to the formation of Trident Audio Developments as a separate manufacturing company. Thirteen A-range consoles were built in total, and the mic preamp featured a very high-quality Marinair 1:6 input transformer driving an all-silicon three-transistor amplifier circuit. A common-emitter PNP input transistor feeds a Class-A push-pull NPN output stage, with a standing current of around 20mA — the greediest of the Hydra options. The high standing current in this design should make it very quiet, but a resistive divider circuit on the input degrades the noise performance to some extent.

The remaining pair of head options relate to two different incarnations of the Olympic Sound Studios. The SW1 mode is based on the mic preamp developed for the world’s first transistorised professional recording console, which was installed in Olympic’s Carlton Street Studios in London’s West End. This console was built in 1960 by Dick Swettenham, who was formerly a service and design engineer at Abbey Road, and it was used to record the Jimi Hendrix Experience and the Rolling Stones, among many other famous artists. This circuitry is unique in the Hydra in using new old-stock germanium transistors running on a negative sub-rail, configured as two PNP common-emitter stages, the second as a Darlington pair. The noise performance is quite good, notwithstanding the use of germanium transistors, and it has a unique sound character not least because of its rather curtailed high end!

Olympic Sound relocated to the London suburb of Barnes in 1966, and the SW13 head is a recreation of the preamp designed for a brand-new console installed in Studio 1 there. This console quickly gained a reputation as the best in the world (at that time), and it was used to record the Rolling Stones, Led Zeppelin, the Small Faces, the Beatles, Traffic, Hawkwind, the Moody Blues, Deep Purple, Procol Harum... and many more. An evolution of the Carlton Street design, the new mic preamp was an unusual hybrid silicon/germanium transistor combination, and it became the basis of preamps in Swettenham’s later and much-praised Helios consoles. This ‘Barnes’ circuit operates on a single +24V power rail with a 10mA standing current, and achieves a moderate noise performance. Again, the output section is built around a PNP Darlington common-emitter stage.

An Audio Precision analyser plot, comparing the frequency responses of the four preamp modes at maximum gain and a 0dBu output level (note the inevitable but small variations in gain between them). The effect of the band-pass filtering in the Abbey Road TG preamp (blue) is quite obvious, as is the rather odd attenuating HF shelf in the original Olympic Sound studio preamp (brown).An Audio Precision analyser plot, comparing the frequency responses of the four preamp modes at maximum gain and a 0dBu output level (note the inevitable but small variations in gain between them). The effect of the band-pass filtering in the Abbey Road TG preamp (blue) is quite obvious, as is the rather odd attenuating HF shelf in the original Olympic Sound studio preamp (brown).

Sum Of The Parts

Integrating these four separate circuits into a single preamplifier is not a trivial matter. For a start, they all require quite different power-rail voltages, and the input transformer also has to provide different ratios to accurately replicate the gain structures and source impedances of the various preamps. Some attention was also required to normalise the output levels from each preamp, so that switching between them reveals their different characters without substantial changes in level.

The nominal gain range for all four preamp modes is 10-50 dB (with the pad deselected), switched in 4dB steps. It is inevitable that in practice the actual gains vary slightly from that ideal, although never by more than 1.5dB for any mode at any gain setting. In my tests, the signal-to-noise ratio varied as expected with the different ‘head’ modes, the SW13 setting being the noisiest at 71dB (ref 0dBu) and the NW8 mode being the quietest at 75dB (ref 0dBu). The calculated EIN figures varied similarly, with the SW13 mode being the highest at -121.2dB, and the SW1 the lowest at -124.8dB. All four preamps maintained correct polarity, albeit with small differences in phase shifts across the spectrum.

Another AP plot comparing the total harmonic distortion for each preamp with increasing output level. These measurements were taken with a  nominal 50dB of gain, so the right-hand side of the chart represents an output level of about +20dBu. As the chart shows, the amount of distortion builds rapidly as the output signal level increases above the 0dBu operating level.Another AP plot comparing the total harmonic distortion for each preamp with increasing output level. These measurements were taken with a nominal 50dB of gain, so the right-hand side of the chart represents an output level of about +20dBu. As the chart shows, the amount of distortion builds rapidly as the output signal level increases above the 0dBu operating level.

Naturally, the distortion figures are higher than would be expected of a modern mic preamp, and at maximum gain with a 0dBu output level the THD figures fell between about 0.04 percent (SW1) and 0.5 percent (NW8), rising rapidly in all cases with increasing output level. The frequency responses of the different modes also proved interesting, with the NW8 having the narrowest bandwidth and the SW13 mode the widest. The SW1 variant had the strongest bass and dramatically recessed treble, and both Olympic Studio variants had slightly recessed mid-ranges. Overall, the Trident preamp has the most neutral frequency response, but also the highest distortion. The input impedance remained at 780Ω in all four modes. That’s lower than in most modern preamps but appropriate for vintage designs like these. With the input pad selected, the impedance rises to 1.9kΩ.

This composite AP chart compares the relative levels of the individual distortion harmonic components for the four different preamps.This composite AP chart compares the relative levels of the individual distortion harmonic components for the four different preamps.

Impressions

The four different preamp designs selected for inclusion in the Hydra provide a powerful range of different sound characters and, importantly, these ‘flavours’ are all quite rare amongst other vintage preamp homages. The only significant British preamp omission from the same era that I can think of is the Neve 1066/1073 (yet another three-transistor gain stage), but countless 1073-esque preamps are available, so acquiring that tonal colour is relatively easy. And to be fair, there isn’t any spare spaced inside the Hydra to squeeze anything else in!

Compared with most modern preamps, the gain range, at a maximum of 50dB, is a little restricted, but I don’t think that’s going to prove problematic in practice with close-miked vocals and instrument amplifiers. Switching between the different preamp modes allows a quick comparison of sound characters, and I found that the most appropriate in any given situation was usually glaringly obvious. Some tweaking of the drive trimmer can also help fine-tune the relationship between distortion and input level in a very useful and creative way.

Although some might rue the absence of a dedicated instrument input, I liked the simple styling of the Hydra and its uncomplicated operation. I’m not a fan of wall-wart PSUs, but the Hydra recovered some points by using a locking power connector... and promptly lost them again when I discovered that it allows phantom power to appear on the TRS input socket! (That’s never a clever idea, and almost guaranteed to catch out the unwary with loud bangs, or potentially fry the outputs of connected line equipment.)

Overall, the Hydra is a striking and very versatile microphone preamp, which delivers masses of vintage character in four different historic flavours that are very easy to compare and fine-tune. It looks unlike anything else on the market, both in its stand-alone form and when mounted in the optional rack panel, but retains an elegant and understated vintage style which I found most appealing. Given its unique combination of four accurately recreated and relatively rare preamps, all with discrete components, and its very high-quality construction, the Hydra is also priced very attractively. Highly recommended.

Published March 2016