Thermionic Culture Rooster

Although many Thermionic Culture circuit designs hark back to the pre-vintage days of the mid 20th century, they've done whatever they can to reduce noise and distortion to meet the needs of today's user, through the use of high-quality resistors and capacitors where appropriate, selected tubes and carefully laid-out PCBs and hand wiring.

The mic/line XLR connections are on the rear panel, while the unbalanced instrument DI is on the front. Each channel has its own phase (polarity-invert) switch and 48V phantom-power switch. The control panel is an unfussy black, with clear legending, chunky knobs and traditional toggle or rotary switches. Even the power light is analogue, although there is a small concession to modernity in the form of simple red-green-yellow LED output-level metering. The rear panel is also very straightforward, with XLRs for the mic and line inputs, XLRs for the two channel outputs and an IEC mains inlet with adjacent voltage selector. Unlike most professional studio equipment, the output was unbalanced on the review model. Since I completed the review, a balancing-transformer version has been released too, and it may even be possible to retrofit the new option (you'd need to speak to distributors Unity Audio about this). Given that cable runs from the unit are likely to be short, the lack of balancing shouldn't present a problem, though, providing you take care to avoid ground loops. Inside, a torroidal transformer in its own screened area powers the circuitry, and all the tubes are fitted with screening cans.

The input stage employs a custom 1.2kΩ-input Sowter audio transformer, feeding an ECC81/12AT7 tube — so this part of the circuit is based on a fairly traditional tube preamp topography. The EQ section that follows is designed for sweetening or fine-tuning rather than surgical reconstruction, with a broad bass-lift control peaking at 60Hz, a switchable mid/high-lift (operating at 2.5kHz band-pass, 4kHz band-pass or 7kHz shelving) and a separate mid-cut, centred at 700Hz. There's also a six-position rotary bass-cut switch, which is derived from the circuitry used in the company's Earlybird 2 preamp, and a switchable low-pass filter to trim off the excessive highs.

So far it all seems very polite but no Thermionic Culture product would be complete without a dose of quirkiness, and you'll find plenty of that in the 'Attitude' section. The Attitude circuit uses a 5725 tube to create distortion effects, which range from subtle and musical to very obvious and confrontational. This is accomplished by switching the tube between triode and pentode modes via a front-panel toggle switch, and then setting one of six negative feedback amounts via the rotary Attitude switch. In the triode position, the tube, which runs from a reassuringly high-voltage HT rail, provides controllable distortion depending on the amount of feedback applied. This arrangement results in the amount of distortion being very low when the Attitude switch is set to minimum. When pentode mode is selected, the third-harmonic distortion becomes more dominant, and with the Attitude control set to one of its higher positions the distortion becomes much more obvious. At the maximum setting there's no negative feedback at all, so the amount of distortion is quite high — especially if the input level is also turned well up. After all this mayhem, the signal passes through an ECF80 output stage, which acts as a low-impedance output buffer.

Although a great deal of thought has clearly gone into the design, the only way to evaluate a preamp such as this, where distortion is used as an effect, is to do so subjectively. On paper, most tube-based audio gear tends to look worse than its solid-state counterparts, because tubes are inherently more noisy and produce more distortion. However, as long as the noise floor is low enough that it doesn't stand in the way of making good recordings — and the distortion is musically satisfying — the results can be sheer magic.

Switching to pentode mode ups the ante on the distortion front, although the low-numbered settings can still sound nicely smooth and musical. At higher settings and with a lot of input gain, the distortion becomes rather more obvious, to the point that it can sound quite crunchy. In the main, I'd avoid going this far, as large amounts of distortion without the low-pass filtering of a typical guitar speaker cabinet can sound unpleasantly harsh, but at least the option's there if you're working on death-metal vocals! Fortunately, the six switch settings plus the input gain setting give you plenty of control over how much tube coloration is added, and for treating drawbar organs or tine pianos (such as the Rhodes) the pentode mode may be exactly what's needed to capture that retro overdriven sound. For more subtle warming, the triode mode produces more of a genteel tape-saturation type of effect.

I have to say that I really like the sound of the EQ section of this preamp. Forget 10-band parametrics — these few simple controls are perfect for sweetening vocals and most instruments, and make it easy to produce a sound that punches through a mix without being thin or harsh. As with most well-designed analogue equalisers, you may only need to make quite small adjustments to achieve the desired result — and even when you need more extreme settings, the sound still stays musical and well focused.

The EQ section did exactly what I wanted without strangling the sound, as some less sophisticated equalisers and plug-ins tend to do, so I've no complaints at all in this department — and even the limited number of frequency options didn't seem to get in the way of great results. Adding in some low end, I was rewarded with depth and punch without everything turning into a woolly mess. The mid controls felt wide and sweet and, with the 7kHz setting, the high/mid control brought out the clarity and transparency of a sound without making it brittle or harsh. What this boils down to is that you can add a lot of this EQ if you need to, without compromising the integrity of the sound, even though in most cases you probably won't need very much to get the job done.

While the designers would clearly love to live in an all-analogue world, a preamp like the Rooster could bring some welcome analogue character to a digital studio, and if you're in the market for something a bit more exotic to use as a front end, this could be exactly what you're looking for.