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Tree Audio The Branch

Valve Channel Strip
Published September 2014
By Hannes Bieger

Photo: Hannes Bieger

Have Tree Audio gone out on a limb with their vintage-voiced valve channel strip?

A number of small companies continue to develop and release high‑quality valve preamps and processors that are based very closely on designs from the golden age of recording in the '50s and early '60s. Among them are Tree Audio, a joint venture comprising Steve Firlotte of Inward Connections and Ian Gardiner of Boutique Audio & Design. Firlotte and Gardiner first teamed up over 15 years ago to create the Dymaxion recording console but more recently, under the Tree Audio moniker, they released a hybrid console called The Roots. A successor to the discontinued Inward Connections 820 sidecar desk, The Roots boasts classic valve‑based signal processing, but with a few useful modern twists. This console has met with acclaim but, unfortunately, it's priced well out of reach for most of us. So when I learned that a new 3U 19‑inch rackmount channel strip, The Branch, would offer the complete input section of a Roots channel at a price that's a little less aspirational, I was very keen to try it out.

Overview

The Branch's big gain knob, the large VU meter and the enclosure's blue‑grey hammered-paint finish combine to convey the look and feel of 1950s recording technology. The VU meter, manufactured by Hoyt, can display input or output level as well as gain reduction, and the front plate features signal mute and power switches.

The preamp stage accepts mic, line and instrument sources, the last bypassing the input transformer. The input selector switch offers a polarity‑invert position for the mic input, and hot input signals can be attenuated by ‑15 or ‑20 dB before they even hit the input transformer. There's phantom power, too, of course, but there's no associated status light, which I would like to have seen.

After the input transformer and the first valve stage (which is based on a 12AX7 dual triode), the signal is fed into the optical compressor section, which is based on dual VTL5C2 Vactrol opto cells. In fact, it's exactly the same circuit that can be found in the Inward Connections Vac Rac TSL4, a gain‑control device that has received the highest praise from some of the most celebrated engineers in our industry. As with most classic opto compressors, little is offered by way of control: there's a detented pot to dial in the desired amount of gain reduction, and then the program‑dependent nature of the gain cell does the rest. The only additional feature is a gentle 6dB/octave side‑chain high‑pass filter at 250Hz (according to the makers, this feature was added at the request of Chris Lord‑Alge).

The Branch boasts a simple stepped two‑band EQ with Baxandall curves and the optical compressor section of the Inward Connections Vac Rac limiter.The compressor section is followed by the big, detented level knob, which acts both as the make‑up gain for the compressor section and as the overall output level control, helping to keep the number of controls to a minimum: most channel strips need three pots/switches for input and output gain, as well as the compression threshold; The Branch gets away with only two gain‑related controls.

There's plenty of ventilation for the valves, both on the top and on the rear of the rackmount case.The signal is then passed to the second valve stage, which is based around a 12AY7 dual triode, and the EQ section is integrated in the feedback path between the valve's two triode elements. As a basic two‑channel filter in the vein of a typical '50s console EQ, with its wide and gentle Baxandall curves, this is no tool for drastic sound‑sculpting. Instead, the EQ can enhance sources with such gentle and unobtrusive sweetening that sometimes you hardly notice it's engaged at all — and I mean this in a very positive way! Both bands offer 2dB gain steps between ‑4 and +6 dB, the low band at 50, 80, 100 and 400 Hz, and the high band at 7, 10, 12 and 16 kHz.

The all‑valve signal path is accompanied by custom‑wound Cinemag audio transformers.After the second valve stage, there's only the output transformer, so this really is a very short signal path: all the processing is achieved with just two dual‑triode valves, and thus only four active stages. This all allows for a maximum gain of +63dB on mic signals, which may not be a stellar figure in absolute terms, but is fairly typical for an all‑valve preamp of this kind, and is plenty of gain for most applications.

Classic Modernity

The Branch is made of quality components. The second valve stage boasts a NOS ('New Old Stock') 6072A made by General Electric — according to Steve Firlotte, all NOS types of this valve sound better than what's being made today. The input and output transformers are custom‑wound devices manufactured by Cinemag. Even the power supply highlights the design effort that's been expended on this unit, with nothing but sound quality in mind: it's based on a very large mains transformer and, unusually for a unit in current production, it employs a 6BM8 valve as a voltage regulator. According to the designers, this has a tremendous effect on the sound. Ultimately, the design of the power supply determines how the amplifier stages in the signal path react to transients, and my guess is that this design trick influences the slew rate of the valve stages in a way that helps to shape their big, open, yet warm, sound character.

The Branch, then, certainly has its roots very much in 1950s technology and designs, but modernity is embraced in the circuitry too. Despite the all-valve signal path, there are a few semiconductor circuits in the amp that drives the VU meter, for example, and more operate in the side‑chain of the Vactrol opto cells. This way, manufacturing costs, and thus the asking price, are lowered without any compromise in the sonic quality of the channel strip. Indeed, although by no means 'cheap', the price really is surprisingly low for such a device.

In Use

The 3U face‑plate offers ample space for the neatly arranged controls. The structure of the controls, with just a single gain knob and another pot for the compressor threshold, allows for simple and easy operation. In such a layout the threshold control has to cover a lot of ground, simply because the channel strip doesn't offer any facilities to bring the signal level to a certain sweet spot before it hits the compressor. Yet, I never had the feeling that this control's range was too narrow, neither when recording mic signals nor when working with line‑level material.

Although I'd describe The Branch as a fairly straightforward device, it does more than the obvious bare‑bones facilities. There's the mute switch, for instance, and the versatile VU meter that I mentioned above.

In stark contrast to the kind of entry‑level devices that scream 'tube sound' from the off, The Branch offers the sort of effortless elegance and openness that can only be achieved with classic tube circuit designs. In the 1950s, equipment designers weren't concerned with adding a certain amount of distortion to infuse character. Rather, they tried to keep the coloration of the technology at bay, aiming for a signal path that was as clear and as true to the source as possible. The Branch, like The Roots from which it is derived, is very much built in this tradition. It sounds as big and open as one could wish, but with a silky sheen that seems to suit all kinds of signals very well. The preamp remains very true to input signal, conveying the character of the source and the mic in a beautiful fashion, but without sounding at all 'over coloured', it sounds as huge and warm as you'd hope, striking a very satisfying balance between, on the one hand, solidity and density, with ever so slightly rounded edges, and on the other, the clarity that's needed if the unit is to work well for a wide range of recording tasks.

The Vac Rac‑derived compressor section partners the preamp beautifully. With a reasonably fast attack, it keeps transients at bay, but does so without making the signal sound dull. The release seems about right for all typical‑use cases in which you'd choose an optical compressor. The compression sounds fairly invisible at low gain-reduction settings (up to around 5dB), and beyond this point the character becomes increasingly noticeable.

Most of the sound shaping takes place on a little daughterboard behind the front panel: the compressor section on the left employs dual Vactrol cells, while the EQ is based on RC filters placed in the feedback path of the second valve stage.I've provided some audio examples (http://sosm.ag/TreeAudioBranch) so you can hear something of what I'm describing but, in essence, I'd say the compressor feels a little more 'grabby' and less 'buttery' than a Teletronix LA2A, largely because of the faster, more modern‑sounding aspects of the Tree Audio device. But still, the compressor section acts with all the smoothness I'd expect when using a high-quality optical device.

The compressor does emphasise sibilance a bit — especially when applying significant amounts of gain reduction — but this is a very typical behaviour for a compressor that doesn't have a lightning‑fast attack time. However, the side‑chain filter helps to restore a good amount of the warm bottom end, and acts as a very powerful cure for this.

Finally, the EQ section further adds to the smooth, unobtrusive, sweet‑sounding nature of the channel strip. Being derived from a 1950s console design, it isn't the right tool for surgical work. Instead, it excels at enhancing the source, making it a little brighter and more airy, or adding a little more weight and body. With their extremely smooth Baxandall curves, the filters may be used to tilt the source material around the solid, blooming mid-range of the preamp. And while it isn't suitable at all for correcting (room) resonances, it may help to push a signal a little further in the desired direction, again in a very beautiful, open‑sounding fashion.

Rooting For It?

With all these controls and options under your fingertips, The Branch offers ample facilities to shape a signal. It proves an invaluable tool for tracking duties, of course, but its feature set also makes for a fantastic vocal‑processing strip when mixing. The only thing I found I was really missing was a high-pass filter in the signal path to help get rid of unwanted rumbles at the very bottom of the frequency range: the unit boasts precisely such a filter inside the compressor side-chain, but not in the main signal path, which I find surprising. But this minor criticism excepted, The Branch appears to me to be just perfect for a very wide range of applications — which is just as a true console input channel should be.

Those qualities make The Branch an ideal centrepiece for any recording chain, and in my view it can hold its own against units that cost twice its price. In fact, the Tree Audio channel strip is not only a superb recording front‑end by absolute standards — at this price, if you're working professionally, I'd also consider it a steal!  

Alternatives

The closest contender in terms of features is the Universal Audio LA 610, another all‑valve channel with an optical compressor and a basic two‑band EQ section. The LA 610 retails for a little less, yet the preamp section of The Branch seems to be more versatile. The Thermionic Culture Phoenix HG15 is based on a very similar concept, except that it is conceived around a variable‑mu valve compressor. Other valve‑based channel strips with both compressor and EQ sections include the Manley Voxbox, their new offering the Core, which is similar in price to the Branch, and the more expensive — and more feature‑laden — Pendulum Audio Quartet.

Published September 2014