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Universal Audio Twin-finity 710

Hybrid Microphone Preamplifier
Published April 2009
By Paul White

Which is best, tube or solid state? When it comes to investing in a preamp, that's a question you may no longer need to ask...

Universal Audio Twin-finity 710

There are those engineers who swear by tube preamps for their warmth and musicality, whereas others prefer the accuracy of a good solid‑state circuit. It's also true that some types of music work best with one or the other, so the same engineer may use tube or solid‑state preamps for different jobs. With the Twin‑finity 710, Universal Audio have attempted to satisfy both camps by creating a preamplifier that has both tube and solid‑state signal paths, and a means to blend them as required.

Twin‑finity & Beyond

This 2U, half‑rack, all‑metal preamp is a single‑channel device that can accept mic, line or instrument signals, but although the name suggests that there might be elements of the Solo 610 in the tube side of the circuit, and perhaps a Solo 110 in the solid‑state side, there are aspects of this design that make it quite different from either, resulting in a unique tonal character. I know there are users out there who would want it to blend the Solo 610 and 110 sounds, but that's really not what the 710 is about. In fact it couldn't be, because additional design techniques had to be incorporated to ensure the two signal‑paths summed in‑phase, which is not a trivial task when combining two very different circuit topologies.

The term 'Twin‑finity' describes the device's tone‑blending abilities, which can be applied to both the mic/line preamp and the 'Hi‑Z' instrument DI. Unlike some low‑voltage tube preamps, the tube here runs at the full 310V in the expected class‑A configuration, and the input stage uses UA's trans-impedance input circuity, a current‑feedback arrangement that combines good phase coherence with a wide audio bandwidth. As the instrument input needs to offer a high input impedance, a discrete FET circuit is used, yielding an input impedance of 2.2MΩ. The input is split to drive separate, phase‑aligned tube and solid‑state gain stages. These are then summed back together via the centrally located Trans/Tube blend knob, which goes from all‑tube to all solid‑state, and covers all points in between. Given these details, you might expect the output stage to be transformer‑coupled, but in fact the Twin‑finity employs a monolithic (solid‑state) balanced output stage.

As with my own Solo 110, the Twin‑finity has a dual gain‑stage arrangement, with separate gain and level controls. 'Gain', of course, allows you to drive the circuitry harder to introduce deliberate coloration. A single moving‑coil, back‑lit VU meter monitors the output level by default, but can also be switched to show how much drive is being applied. In Drive mode, the meter gives a good idea of how much distortion is being added, although subjective listening tests suggest that the tube distortion becomes evident at lower drive settings than does the solid‑state contribution.

Design & Construction

Despite the complex internal circuitry, the rear panel offers the simplicity you'd expect from a single‑channel preamp, with sockets only for power, mic and line inputs, and line out.Despite the complex internal circuitry, the rear panel offers the simplicity you'd expect from a single‑channel preamp, with sockets only for power, mic and line inputs, and line out.

Built at UA's own facility at Scotts Valley in California, the Twin‑finity 710's half‑rack format (8.45W x 3.5H x 10.25D inches) allows two units to be mounted in a conventional 19‑inch rack, using an optional adaptor. A desktop handle kit is available as a further option, and the desktop format should make the Twin-finity well suited to the private DAW studio owner who needs a single, good‑quality, versatile preamp.

In appearance, the Twin‑finity has the UA look, with a pleasingly retro front panel — although the knobs are not quite as endearing as the Bakelite ones of the Solo 610.

All connections other than the instrument input jack are on the rear panel, and all the switched functions are handled by small — but not quite 'miniature' — toggles. To the left of the meter are switches for the 48V phantom power, a 15dB pad and mic/line selection. To the right there's the meter, Ouput/Drive selection, a 75Hz low‑cut filter and an Out/In switch for phase inversion. A horizontal toggle activates the mains power, which comes in the usual IEC socket. Rear‑panel connections are simply line input, mic input and output, all on balanced XLRs. When the line input is selected, plugging a jack into the front-panel instrument input automatically takes precedence, so you can leave the rear‑panel connectors hooked up at all times.

The internal construction of the unit is based around a couple of double‑sided glass‑fibre printed circuit boards, with a further board for the universal voltage (100-240V at 50/60Hz), switch‑mode power supply module. A multi‑pin connector joins the main board to the front‑panel board, with a further connector hooking up the PSU to the main board. Other wiring is minimal, and is restricted to the meter hook‑up leads and the mains inlet feed to the power switch. All the input and output sockets are mounted directly to the circuit board, as is the ceramic base for the single ECC83 (equivalent to 12AX7) dual‑triode tube. The non‑electrolytic audio‑path capacitors are chunky WIMA types, the preferred choice for a lot of high‑end audio designers, and the resistors are generously rated metal‑oxide film types. UA have clearly gone to a lot of trouble to make this product easy to assemble, while at the same time retaining the engineering integrity for which they're known.

Overall, the Twin‑finity offers over 70dB of gain, with a frequency range of 20Hz to 100kHz, +/‑ 0.2dB, for all inputs. The maximum output level into 600Ω is +18dBu or +28dBu into 100kΩ. A typical line input to which this unit might be connected is around 47kΩ, so there's plenty of level to feed converters with a DFS (Digital Full Scale) level of 16dB or thereabouts. THD+Noise at +4dBu is specified at 0.1 percent for the tube path and 0.005 percent for the solid‑state path.

Trial Time

Because tubes take a while to reach their optimum operating temperature, it's a good idea to switch on a unit like this one half an hour or so before you intend to use it. Tested on speech with range of studio mics, the solid‑state side of the Twin-finity sounds pretty flat and transparent, as long as you keep the drive‑meter reading below midway. As you increase the drive above this, the vocal tonality becomes increasingly 'forward' sounding and eventually becomes quite gritty, although having the ability to push things this far is within the design brief of the device, as you may need heavier amounts of distortion when processing some instruments.

Turning the blending control fully clockwise to fully utilise the tube section, still with the drive registering at around half scale on the meter, the tone changes in a subtle way to give a more obvious tube‑like character, where the high end seems smoother and the lower mid‑range a hint fatter. This is exactly the character I've heard from a number of good-quality tube mics and tube preamps, so the lack of an output transformer clearly hasn't been too detrimental to the creation of a true tube sound. Driving the tube harder creates the familiar 'soft and fluffy' distortion of an overdriven class-A tube, which provides a useful counterpoint to the more overt grittiness of the solid‑state part of the circuit when pushed.

Blending the two sounds provides the opportunity to mix the fatness of the tube with the more pushy high end of the solid‑state circuitry at moderate drive levels, and there are some nice hybrid tones to be had. However, I did sometimes find myself wishing that there had been two drive controls, so that I could mix a more driven tube sound with a lightly driven solid‑state sound, or vice‑versa. As it is, the gain control knob increases the drive to both stages, so you have to arrive at a compromise setting where both the solid‑state and tube paths sound close to what you want to hear before you start blending them.

After further experimentation I came to the conclusion that both signal paths sounded best on vocals with only moderate amounts of drive, usually less than half scale on the meter when switched to Drive, and I was also surprised at how warm the 100 percent solid‑state setting could be when used clean. The tube side of the preamp works nicely when you have a vocalist who might benefit from a tube mic to smooth off the high end, but as there are many singers who fall somewhere in the middle, it makes perfect sense that you can blend the two characters.

On electric guitar, I could discern very little difference between the solid‑state and tube options at lower drive levels, but the differences become very obvious as the drive level is increased. Eventually the tube path breaks up almost like a guitar amp, but without the smoothing effect of the speaker, while the solid‑state path gets brighter and more edgy — but doesn't move so quickly into really obvious distortion until you apply quite a lot of drive. Once it does, it exhibits the expected grit of a solid‑state circuit pushed into clipping. The more overdriven settings can work really well on some drum sounds and there's enough angst in the tube path to add some real overdrive to a plug‑in organ or keyboard sound, but for vocals I much prefer less obvious levels of drive. Still, there are death‑metal singers out there who I'm sure would relish the higher settings!

A Fine Blend?

The idea of being able to combine two different characters in one preamp is a novel one and will appeal to those who like to record through something that adds some definite character of its own. Personally, I prefer the safe, clean sound of my Solo 110 for most things, but then, by preference, I always try to record clean and then add 'filth' later if it proves necessary. If I need a tube mic sound I'll use a tube mic or one of my dedicated tube preamps.

That being the case, it will come as no surprise that I most like the way the 710 sounds at lower drive settings, where it provides the best of both worlds, adding character but without any obvious distortion. Still, nothing succeeds like excess, as they say, and UA have given the user plenty of scope for adding creative coloration when the need arises, which is something to be applauded.

What this all adds up to is a very versatile front end that can deliver good results with a range of microphones and vocal styles, as well as doubling as an extremely classy instrument DI. It isn't a Solo 110 or a Solo 610: the Twin‑finity 710 is very much its own thing, and more power to it for that.  


I can think of a few preamps that let you switch out the tube stage, but none that allow you to blend tube and solid-state signal paths in this way. SPL's Channel One allows you to control the amount of tube distortion being added, and SSL's VHD pre allows you to blend between different solid‑state circuits — but neither of these are quite the same thing.

Published April 2009

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