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Thermionic Culture Snow Petrel

Dual-channel Microphone Preamplifier By Hugh Robjohns
Published January 2020

Thermionic Culture Snow Petrel

Thermionic's latest valve preamp offers bags of gain and plenty of analogue warmth. Is this the perfect partner for your ribbon mics?

Thermionic Culture's chief designer Vic Keary has a fascinating reputation for using valves and circuit topologies that aren't conventionally associated with high-quality audio, yet which always deliver the goods, sonically and technically. His latest design, the Snow Petrel, is a two-channel valve mic preamp. It's a 'high-gain' design with very low noise and distortion, intended primarily for use with low-output ribbon mics.

Vic told me that the inspiration for the Snow Petrel lay in a previous career — he enjoyed using the classic Coles 4038s for brass and drums but found that most mic amps and mixers of the time had too much noise and distortion to enable their use on quieter instruments. So the Snow Petrel was designed specifically to enable low-output mics like the 4038 to be used on quiet sources. That sounds simple enough, but Vic has high standards, and it took him five years, two unsuccessful previous versions, and a rather unusual circuit design to develop it to his satisfaction!

Of course, the Snow Petrel is not a one-trick Antarctic-gull: it is equipped with both switchable phantom power and input attenuators, and it can work with dynamics and high-output capacitor mics in a wide range of different applications just as competently as it can with passive ribbons.

The Tour

A hefty 2U rackmounting device, the Snow Petrel weighs 6kg and extends 290mm behind the rack ears. It consumes 30W of power but the case is well ventilated at the sides, rear and top, so there's no requirement for a ventilation space above the unit. Rear-panel connections are minimal: two pairs of XLRs for the transformer-balanced mic inputs and line outputs, plus a standard IEC (C14) mains inlet with integrated fuse holder. There's also a voltage selector switch for 115 or 230 V AC mains supplies.

The visual styling is typical of Thermionic Culture, with the front panel carrying crisp white legends on a glossy black-painted background. Five chicken-head rotary controls and four toggle switches are provided for each channel, and another chunky toggle switch with a large green status lamp switches the unit on/off.

Another pair of toggle switches, each with red status LED, engage each channel's phantom power. These 'secure' switches have to be pulled out before they can be moved, preventing accidental operation — a feature intended to protect delicate ribbon mics. They won't help if phantom was left on accidentally in a previous session, of course, but I spent an entire BBC career 'hot-plugging' Coles 4038 ribbons (phantom power is present permanently on most BBC studio wall-boxes) without them suffering any damage!

The case is ventilated at the top, sides and rear, so there's no need to leave a  space above this preamp when rackmounted.The case is ventilated at the top, sides and rear, so there's no need to leave a space above this preamp when rackmounted.The rest of the controls are arrayed towards the left of the front panel, with channel one's controls directly above channel two's. The first toggle switch introduces a -20dB pad before the custom-designed Sowter input transformer in the signal path, so it avoids unwanted transformer saturation, as well as preventing overloads of the input stage when using high-output mics and high-level sources. (The user manual recommends using the high-impedance input mode whenever the input Pad is used.)

Controlling that input impedance is a second toggle switch, which alters the transformer's primary wiring configuration to provide alternate Lo/Hi input impedance options of 850Ω or 2.4kΩ. The user manual claims that this low-impedance setting is actually "high compared with some other mic amps". That might be true in comparison with some vintage preamp designs, but the days of 50, 300, or 600 Ω input impedances are long gone, and 850Ω is a low input impedance by modern (voltage-matching) standards. The high-impedance mode isn't particularly high either — standard mic inputs now typically range between 1.5 and 2.5 kΩ. So the high-impedance mode really is just normal; it would need to be above 5kΩ before it could be considered to present a higher-than-normal input impedance.

Moreover, most modern mic preamps specifically intended for use with ribbon mics offer a far higher input impedance to minimise the load on passive ribbon mics. An input impedance of 15kΩ is common in this application, and some are as high as 30kΩ. Still, the different impedance options will undoubtedly change the transient performance and overall tonality of passive moving-coil and ribbon mics.

A set of response curves measured with an Audio Precision test set to illustrate the Snow Petrel's extended frequency response, along with the effect of the high-pass filter and a  range of different Air boost settings.A set of response curves measured with an Audio Precision test set to illustrate the Snow Petrel's extended frequency response, along with the effect of the high-pass filter and a range of different Air boost settings.Returning to the other operational controls, a red chicken-head knob selects the preamp gain in 7dB increments. The markings indicate 40, 47, 54, 61, 68 dB and 'Max' — this last providing 75dB, and the measured performance using an Audio Precision test set was always within 0.5dB and often within 0.25dB of the stated gain. These nominal gains are only correct when the 'Lo' impedance setting is employed, though, because the adjustable impedance is achieved by altering the transformer's configuration, and the different turns-ratio inherently results in a different voltage gain across the transformer. As a result, the preamp gain is 6dB lower than marked at each setting in the Hi impedance mode.

I'll skip briefly over the next two tone-adjusting controls, as the fourth and fifth chicken-head knobs also affect the signal level. The first is a variable Output Trim for the valve stage driving the Sowter 1:1 output transformer, and its 0 to -10 dB range allows precise level matching when, for example, working with a stereo mic array. It can also be used to keep a check on the output levels when the front end is being deliberately overdriven to introduce gentle second-harmonic distortion. Amusingly, or confusingly (depending on your point of view) the control's markings don't coincide accurately with the decibels of attenuation being applied, except at the two extreme ends of the range.

The last rotary control, a three-position Output Attenuator switch, affects the signal after the output transformer. Nominal attenuations of 0, -7 or -15 dB are available, allowing the output transformer to be driven for saturation effects, while maintaining a sensible output level for subsequent equipment. The attenuation varies slightly depending on the connected destination's input impedance, but the figures above are accurate for a typical 10kΩ load. This attenuator is an essential facility, as the Snow Petrel is capable of delivering over +26dBu when running flat out — that's more than most converters, interfaces, or mixing consoles are comfortable receiving! The final toggle switch inverts the polarity of the output.

Returning to the two tone controls I skipped past, both are entirely passive and designed to compensate for common characteristics of vintage ribbons. The first (white) knob introduces a first-order high-pass filter with selectable corner frequencies of 60, 120, or 240 Hz. With a slope of only 6dB/octave, this filter is perfect for correcting the proximity-effect bass tip-up associated with bidirectional (fig-8) ribbon mics. (It's not much use in removing unwanted subsonic noises; that generally requires a much steeper slope.)

The second (blue) knob, Air, controls a variable peaking HF boost EQ centred at 22kHz. At maximum, the high-frequency boost reaches +5dB at 10kHz and +7dB at 22kHz, and this can be used to compensate for the HF roll-off associated with vintage 'long ribbon' microphones.

Technology

The mic signal is boosted by the input transformer's high 1:20 ratio into the first gain-stage valve — a Radio Technique RTC5654, which was used very successfully, in Thermionic Culture's Rooster 2. It's a rugged, wide-band RF pentode, featuring distinctive round holes in its anode plates, revealing the control grids within. Apparently the 5654 is a 'special quality' version of the Western Electric 6AK5W or Mullard M8100. The manual lists the latter as a suitable alternative, but Vic estimates stocks of the 5654 to exceed 30,000.

A very neat build, with lots of handwiring between four circuit boards. The gain stages built around a pair of pentodes for each preamp are on separate boards, positioned alongside the chunky 'canned' Sowter input transformers. The output drivers of both channels share a third circuit board, with the diminutive line output transformers below. A linear power supply is accommodated on the fourth board, located behind a metal screen.A very neat build, with lots of handwiring between four circuit boards. The gain stages built around a pair of pentodes for each preamp are on separate boards, positioned alongside the chunky 'canned' Sowter input transformers. The output drivers of both channels share a third circuit board, with the diminutive line output transformers below. A linear power supply is accommodated on the fourth board, located behind a metal screen.

The second half of the gain stage is built around a more familiar, high-gain pentode: the EF86. The Svetlana version is used, but the JJ EF806 is listed as a suitable equivalent. Both these front-end valves are strapped to operate as triodes, with anode voltages of around 240V DC, and the gain is controlled by altering the negative feedback around the pair. Driving the Sowter output transformer is a Tung-Sol 6189, which is a high-output version equivalent of the low-microphony variants of the ECC82 or 12AU7A double-triodes.

The combination of these three valves, along with the step-up gain of the input transformer, permits the unusually high (for a valve preamp) 75dB maximum gain, while maintaining an impressively quiet noise floor and low distortion. The review unit's QC test report cites figures of 0.23 percent THD at maximum gain, with a maximum output level of +21.9dBu, and noise floor at -83dBu. At lower gains, the noise floor falls well below -100dBu, distortion below 0.025 percent, and the maximum output level rises to +26.4dBu. These are impressive figures. Crosstalk varies with gain, as you might expect, but is typically around -70dB at 10kHz. The worst case (-41dB at 10kHz, at maximum gain) is still better than most vinyl pickup cartridges!

That Keary designed the Snow Petrel specifically for use with 4038s is evident in its creamy-smooth but detailed and naturally transparent sound.

In Use

I tried the Snow Petrel with both vintage Coles 4038 and modern AEA R92 passive ribbon mics and a selection of popular capacitor mics, and with a variety of sources and instruments. At just 0.5mV/Pa, the 4038 is about 10dB less sensitive than the R92, but the Snow Petrel was easily able to provide sufficient gain to allow the former to be used on quiet sources, including a cello, without getting lost in noise.

That Keary designed the Snow Petrel specifically for use with 4038s is evident in its creamy-smooth but detailed and naturally transparent sound. Dialling in some Air brings out a beautiful clarity and shine, without any hint of the edginess that's common with capacitor mics, and the high-pass filter cleaned up low-end muddiness nicely when close-miking. The Air control's boost range is very well-judged, bestowing the perfect working range with fine resolution. The turnover frequencies for the high-pass filter are also well-chosen and very effective in taming proximity boosts without sacrificing low-end weight. Similarly attractive sound qualities were obtained with the AEA mic, and while the impedance options did affect the tonality of both mics, I found that I selected different settings in different situations with no clear overall preference.

This preamp's noise floor is impressively low and entirely benign at all gains, but particularly so in the mid-range settings. I had no overload problems when using high-output capacitor mics, either, although I did need to engage the input pad on occasion (that's why it's there!).

In normal use, the Snow Petrel is very clean and neutral but certainly not sterile-sounding, and it produces a lovely clean, but rich and involving analogue sound. If desired, it can also be deliberately pushed into an overdriven character, which is gloriously musical and highly appealing in the right circumstances — although the output attenuator will be needed to avoid overloading downstream equipment. With so much gain available these overdriven effects are still possible when using low-output 4038s, too.

The Snow Petrel doesn't have a dedicated line input, and the input pad is not sufficient to allow use with normal line-level signals as the maximum input before clipping is just +7dBu, even with the pad engaged. This is a shame; it would be great if the preamp could be used for character when mixing, and I'd love to see a version with a dedicated line input. It would also be great to see direct instrument inputs, but Thermionic Culture do make a separate DI box which could add this functionality if required.

Conclusion

Overall, then, Thermionic Culture's Snow Petrel is a really nice, well-engineered preamp, with a clean yet involving sound character, unusually high gain, blissfully low noise floors, and well-judged tone-shaping. It's ideally matched to classic ribbon mics, and in partnership with the Coles 4038 it is capable of some truly magical and impressive results. It's also a very good choice for use with other popular dynamic mics like the Shure SM7B or EV RE20, and can also bring some warmth to the more clinical-sounding capacitor mics. I'm very impressed with the versatility and quality of the Snow Petrel, and highly recommend auditioning it.

Alternatives

There are several preamps designed for use with ribbon mics, such as AEA's TRP and RPQ and the Integer Audio RMP2. But these are solid-state designs; I know of no other all-valve preamps intended primarily for use with ribbon mics.

Pros

  • Fundamentally clean, but richly analogue and involving sound character.
  • Massive 75dB of gain available with a remarkably low noise floor.
  • Well-judged tone-shaping options to compensate for typical ribbon mic characteristics.
  • Input and output attenuators enable precise control of intended overdrive effects.
  • Excellent build quality.

Cons

  • No line or instrument input options.

Summary

A very elegant and carefully designed mic preamp specifically intended for low-output ribbon mics — but with enough versatility to be useful with any mic. It offers a range of appealingly analogue sound characters from crisp and clean, to powerfully raunchy.

Published January 2020