REVIEW: Unit Audio UNIT Passive Summing Mixer

Unit Audio UNIT Passive Summing Mixer
by Brandon Hickey

Unit Audio makes a small array of hand-build utility products for recording and mixing. Their primary product line offers passive analog summing mixers for DAW-based workflows. The benefits of analog summing is that combining electrical signals through an analog mixing buss sounds different from summing inside of a computer. Though DAW-based mixing offers the advantages of easy recall and automation as well as plugins which are more cost friendly than outboard hardware, there are also distinct advantages to analog mixing. Consoles usually offer greater headroom than DAW mixers, and in many cases color the sound with a unique flavor.

Unlike summing systems like the Dangerous Music 2-Bus which offers a fully active circuit path, the UNIT falls in line with devices like the ROLL Folcrom which features no active circuitry. Passive mixers combine signals by using a network of resistors to sum them together. The resulting output is very low in level, so a microphone preamplifier is necessary to restore it to line level before hitting the mix-down deck. One supposed benefit of this type of strategy is that the mic preamp adds coloration to the signal. Naturally this is a point of debate. For example, how many engineers plug a mic into a mic pre, then to a re-amp, then a DI, then to another mic pre to add flavor to the signal? Once a signal hits line level, why not keep it there, right? That is how I felt too, before I tried the UNIT. After using it, I haven’t necessarily been born again, but I will say that the results were not quite what I had expected.

The UNIT is 16 x 2 having two DB-25 connectors, each accepting eight balanced signals, and outputting to a pair of balanced Neutrik TRS connectors. UNIT Audio also produces an 8 x 2 mixer with all TRS connections, and may have XLR output connectors upon request. Several of the models have a pair of switches that can break a stereo pair of input into two mono sources, so that kick and vocal, for example, can be inputted individually without having to waste a stereo pair with dual mono signals.

The Sum Of All “Hears”
The primary material for this listening test was a jazz trio which had been tracked to tape, and transferred to Pro Tools. I mixed in the box using only plugin-based effects, then produced stems of drums, kick, snare, bass, sax, and a few effects stems. I then combined those stems together using the Pro Tools mixer, the software mixer in Harrison’s Mixbus DAW, the UNIT, and two analog consoles (SSL 4000E and an API Legacy Plus), with faders set at 0 dB VU.

It is important to note, at this point that the sound of the Xicon resistors and Neutrik connectors will play a role in the sound, but maybe not as significant as the roles played by the DA converters outputting the DAW signals to the box, the sound of the mic preamp gaining the signal back up, nor the AD converters feeding the mixdown deck. In fact, even the cabling at either end will color the sound, and perhaps even more so than the resistor network. All that said, subjectively grading the sound of the UNIT itself is nearly impossible. Despite that, my goal was to color the mix minimally and merely take advantage of the added clarity, enhanced stereo image, and tighter bottom end that the UNIT promised. I used Monster snakes at the front end, Canare Star-Quad cable with Neutrik connectors on the back end, and fed the clean, quiet, Sound Devices USBPre2 for mic pre and A to D conversion. For the sake of convenience and consistency I used the D to A converters of a Digidesign 96 I/O.

A/B Tests
Referencing all mixes through a number of monitors and headphones revealed subtle differences. Color and frequency response changes were minor, but there were also a few big things that really stood out. The most profound factor was the way the mix sounded when things got really fast and busy. For example, there were moments in the tune when the sax started playing higher notes at a point in the groove where the bass was making fast transitions and a drum fill took place. With all of that happening at the same time, the Pro Tools mix quickly turned muddy. Harrison’s Mixbus stayed clearer through these dense sections than Pro Tools, but still had a tendency to lose definition when compared to any of the analog mixes. The SSL sounded more like the Harrison software mixer though subtly clearer on quick fills. Both the API and the UNIT shared a similar character with each of them producing a sound which was like a pleasant, natural, harmonic distortion which accented the attack of each drum hit, propelling it through the mix. At the same time, this sound was persistent throughout the mix, creating an unnatural high-frequency boost.

On one hand, I would say that having heard the comparison, the way complex signals combined through the UNIT was preferable to Pro Tools. The API shined, adding all kinds of warmth and flavor through the midrange and low frequency range. That said, the sound of either of these solutions is far from transparent, and you would have to be careful to choose the right pair of mic pre’s to compliment each mix. Even then, you’ll never get a colorless sound, and may find yourself EQ’ing around the mic pre. Also worth noting was the change in stereo image across these different mixes. Relative to the Pro Tools sum, Mixbus and API both widened the mix a bit, with the SSL mix being the widest of all. The UNIT mix sounded close to Pro Tools, but if anything, it seemed like the midrange was a bit narrower.

The Verdict
When it comes down to it, summing outside of the box is undeniably popular, and engineers are looking for that sound. Some passive summers, like the Shadow Hills Equinox include the makeup gain pre-amplifier which drives up the cost significantly. Meanwhile, the UNIT is about a third of the price of the ROLL Folcrom, and does the same thing, minus some unnecessary routing switches. While this product is not going to be as clean as an active summing amp, it does provide an interesting sound, which, for many musical applications may be just the kind of thing you are looking for. Certainly it is a different character than a mathematic mixer, and if your mixes are stuck in a rut, this might be just the thing to help you think outside of the box.

PRICE: $149 – $399 plus shipping

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