The Sigma 500 provides a unique twist on the implementation of an analog summing mixer. The unit contains two 500-series slots to insert preamps of your choice to serve as the final gain stage for the stereo mix. Moreover, when not being used to sum, the Sigma 500 has an Alt mode that transforms the unit into a standard two-channel 500-series rack. I really liked this dual-natured feature, because I occasionally record voiceover artists, and having a rack on hand for a couple high-quality preamps is nice. Out of the shipping carton, the Sigma 500 slots accept single-width modules, but double-width devices can be installed by removing the included blank plates. With the function toggle switched from Alt to Sum, the Sigma 500 assumes its main identity, and the installed preamps become the final gain stage. A third configuration is available when the unit is powered down. This is called passive mode, which isolates the summing circuitry, permitting use of external preamps rather than those installed in the Sigma 500. Very neat. There are no pan pots, faders, or switches. Input signal is via two DB25 connectors per channel (with standard TASCAM pinout).
Recently, I've done considerable testing with summing mixers. Employing real-world projects and listening with trusted speakers reveals what a summing mixer can and cannot do. Most mastering houses would have little use for these devices. However, some of my clients record and mix themselves. My preference is to work with a mixing engineer, but this is not always possible or economically feasible. In these cases, I can master from stem mixes as an option [see "What Is Stem Mixing?" in Tape Op #87], using a summing mixer to feed my mastering chain.
My tests revealed that a summing mixer can sound better to me than mixing in the box, but results depend on several factors. First, high-quality D/A converters must feed the summing mixer. Each A/D or D/A comes at a price, both financially and in terms of sound quality. Using entry-level converters tends to offset any advantage of external summing. Second, not all DAWs calculate sums with the same result. Some are better than others, and there can be differences even between versions of the same DAW. (E.g., I think the summing in Pro Tools 10 sounds more spacious than in PT 7.) Finally, you may achieve additional improvements in sound quality and in workflow by adding external gear. In short, summing mixers are only one part of the equation in which they live. Without all of the pieces in place, you may experience few noticeable benefits, or even reduced sound quality. So don't expect to buy a summing mixer and magically have it do all the work.
From a coloration standpoint, summing boxes tend to fall into two camps: transparent or vintage-inspired. Colored boxes impart the sound of transformers or op-amps consistent with their heritage, while transparent models can be more flexible. With the Sigma 500, you can choose the flavor through the preamps. From the high-headroom clarity of a Grace Design M501, to the girth of a LaChapell Audio 583s, the choices are vast. For my testing, I tried several preamps, ultimately resting on two finalists. For high-headroom, transient-rich sounds, I liked Hairball Audio Lola preamps [Tape Op #93] loaded with John Hardy 990 op-amps. For a colored approach, I used a set of custom Eisen Audio preamps that were built in the tradition of vintage Neves. For this review, GAS also lent me an A-10 line attenuator [#96]. I fed the Sigma 500 with either a Lynx Aurora 16-VT [#73] or an SSL Alpha-Link MX 16-4.
For the first project, I broke out the stems for drums, bass, lead vocal, backing vocals, keys, guitars, lead guitars, and effects. Although the Sigma 500 can accept up to 32 input channels, I limited myself to 16, because I figured most people would be happy with 8 stereo stems. Level-matching as best as I could, I printed a master using the internal summing of Pro Tools and a master using the Sigma 500 with the Lola modules. I sent the test files to my client, a praise and worship leader at a large Oklahoma church. He listened to both versions and solicited the advice of his colleagues and band members. In the blind test, they chose the Sigma 500 version by a vote of 7 to 1. For Fault Lines, a punky garage-pop four-piece, I loaded the Sigma 500 with the Eisen Audio modules, drove them pretty hard, and attenuated the output through the A-10. I also summed a mix in Sequoia 10. The band chose the GAS-powered version. This version glued Jedi Emily Seabroke's bass line better than all the multiband compressors and all the plug-in exciters could. (I know this was not a scientific comparison. I should have played the Sequoia version through the preamps as well, but in the real world, I would not patch gear that way unless I had the GAS installed.) In a third set of tests, I added more outboard gear, running each stem through a bus compressor before it hit the Sigma 500. This workflow can be facilitated by a patchbay with normals from your converters to the Sigma 500, allowing you to insert gear when desired between the DAC and the summing circuit. If you choose a summing box setup, consider adding such patching to your installation. These masters resulted in the most significant sonic differences, coming much closer to the feel of a large-console mix.
So, the broader question is: If you have the converters and the time, is analog summing better than "in the box"? Yes? Garrett said so. His clients said so — even in blind tests. But what does "better" mean? To be honest, better is not the correct term. Analog summing sounds different. Furthermore, some styles of music may sound better mixed in the box. Plus, ITB recalls are a dream. But when I want to maintain the illusion of hearing the artist perform right in front of me, I prefer properly implemented analog summing. Flipping back-and-forth does reveal a wider soundstage with the Sigma 500 — not a vast difference, but noticeable. However, the major differences were in two areas. First, elements like vocals, guitar solos, and snare drums tend to sit in the mix with an uncanny "this is exactly where I belong" vibe. You need less processing to blend elements. Once I was out of comparison testing, I found I was removing plug-ins from various channels because they were no longer needed. Second, analog summing seems to place a big sonic blanket around the mix. Things start to sound "like a record" faster and easier than when working within the world of endless possibilities in a DAW.
Within the world of summing mixers, the GAS Sigma 500 has many advantages. It has lots of inputs. It doubles as a two-channel 500-series rack. It's got passive mode. And its construction is sturdy. Of course, you must provide your own preamps for the final gain stage, but that's part of the allure of this unit. And if you have access to multiple preamps, an array of sonic signatures can be readily at your disposal. All you have to do is pull four screws. But one last piece of advice: Don't go too far trying to find the perfect preamps, because at some point, you have to make records.