New Gear Review: STX 600 Complimiter Mic Preamp by Spectra 1964

Spectra 1964 implements their lauded 610 Complimiter circuit into the STX 600 preamp—can this unit find a permanent home in your 500 series setup?

Spectra Sonics was founded in the city of Ogden, Utah by William G. Dilley in 1964. A combat pilot and WWII veteran, Dilley published over 300 engineering papers and technical articles during his lifetime. He was even awarded a fellowship by the Audio Engineering Society for “advancing the ‘state of the art’ in audio control console design.”

Not long after the family-run company’s inception, their equipment—most notably 101 and 110A mic pres—became an indispensable tool in many famed studios, including Ardent Studios, Muscle Shoals, and Château d’Hérouville, among others. Spectra Sonics components were frequently used in consoles by Auditronics, Flickinger, and Cadco.

In 1968, Spectra Sonics began to produce consoles themselves. Somewhere around 50 consoles were constructed at an average price of $35,000 each. More expensive consoles were also procured by legendary artists like Michael Jackson and The Carpenters, as well as studios like The Record Plant.

Dilley and co. began distributing their distinguished 610 Complimiter one year later, quickly making its way into numerous recording studios. An ultra-fast compressor/limiter, the unit became noteworthy for its peak-free transients and ability to quash compression pumping, differentiating itself from other conventional compressors.

Around 40 years later, former Spectra Sonics employees Bill Cheney and Jim Romney bought the company from the Dilley family. The pair scrounged up any and all notes they could find to reverse-engineer and master the original designs without making changes to the core circuits. They went on to rebrand the business as Spectra 1964 to avoid confusion with a virtual instruments company that trademarked a similar name.

Spectra 1964 continues to breathe new life into their vintage designs to this day, still hand-testing and soldering everything in their Utah facility. In 2017, the company released the STX 100, followed shortly by the STX 500 one year later. The former is their 110A amplifier circuit in a compact and affordable 500 series package. Similarly, the STX 500, a two-band passive equalizer, brings the old model 500 EQ into the lunchbox format. Then, last year, Spectra 1964 set their sights on the C610, a reissue of the original 610 Complimiter.

And now, hot off the shelves, comes the STX 600 Complimiter Microphone Preamplifier. Let’s see what this classic design can offer the modern studio operator.

Features

The STX 600 utilizes the company’s 601 Complimiter circuit, used in the original 610, 611, C610, and V610. Needing less than 80 milliamps, powering the single-slot mono module isn’t an issue, especially considering that most racks distribute at least 200 mA per slot.

Starting from the top, the STX 600 has two small, switchable knobs beside each other. The one on the left is a 48-volt phantom power knob with an on/off indication above it. The right knob is a phase flip switch, offering polarity reversal when REV is selected.

Moving down the module, there is a three-way switched input pad. It offers pad settings of 0 dB, -10 dB, and -20 dB. Below the pad resides a large skirted input knob, used for both input level and threshold adjustment “for peak transient limiting and/or mild to moderate compression.” A threshold lamp is located to the right of the input knob, allowing a visual indication of peak transients and compression.

Lastly, a large skirted output knob lives below the input knob. It has an overload indicator lamp (OVL) to its right which lights up 3 dB prior to maximum output (+18 dBm).

In Use

The STX 600, with its all-metal full enclosure, fit flawlessly into my 500 series rack-mounted chassis, right at home next to my other mic pres, compressors, and EQs. After patching a mic into the unit to record acoustic guitar, it became apparent why the module is adorned with a gold faceplate—it’s royally smooth among compressors/limiters. Arguably the fastest compressor/limiter available in the lunchbox format, the limiter attack time is less than 90 nanoseconds, while the compressor attack time clocks in at 100 nanoseconds to 1.2 milliseconds.

Many companies tirelessly tweak their outboard gear until it has a particular coloration, vibe, or frequency bump. Spectra 1964 prides itself on not having a signature sound. The STX 600 accurately and effortlessly captures what is coming from the microphone and musician. Comparing this unit to preamps like the Burl B1D and API 512c, the STX 600 is clearly more transparent—you’re hearing the instrument’s true tone (as encapsulated by your mic, of course) in your room.

This is a piece of gear that is all about using your ears, although the threshold and overload indicator lamps come in handy too. Finely stepped input and output pots allow for painless recalls—a definite plus in my book. The compressor/limiter functionality of the STX 600 took a little bit of experimentation to properly tweak at first. Spectra 1964 co-owner Bill Cheney explains it best:

“It is important to set the input threshold first, thus establishing a baseline for minimum input level with regard to transient peak levels. In this mode, the occasional flashing of the threshold lamp will provide peak limiting only with no compression. Only after the input level is increased will the onset of dynamic compression occur. As the input level is increased, a combination of peak limiting and compression will occur (3 to 6 dB of dynamic compression). Further input level increase will result in a constant threshold lamp indication, which negates the peak limiting function, and provides compression (10:1) only.

“The peak limiter circuit operates independent of the compressor circuit, and provides an attack time of 90 nanoseconds, (inaudible). The compressor circuit, like the limiter circuit, also operates in the sub-microsecond range, thus providing seamless transition between the two functions. Regardless of the amount of applied compression, frequency response is not altered and “pumping” common to conventional compressor limiters will not occur.”

The STX 600 delivered unblemished results no matter what I threw at it—everything from evening out my bass playing, smashing a drum room mic, accentuating the “thwack” of a snare, or merely taming a dynamic vocal performance. With an overall gain of 62 dB, the STX 600 is wonderful for use as a microphone or line preamplifier. It was particularly satisfying recording percussion, specifically shaker and tambourine, with a hotter and mildly dirty delivery to give it that perfect edge in a mix.

To Be Critical

I encountered zero technical issues with the STX 600. However, aesthetically, it does have quite a simplistic, old-fashioned look and feel to it. If that’s your cup of tea, then it’s terrific.

Overall, to me, the faceplate comes off somewhat bland compared to contemporary 500 series modules. I found the numbers and lettering a little difficult to discern at times, even compared to their other 500 series modules. Different color combinations (even for the knobs) could be really pleasing—something to spruce up the general look and feel.

Summing it Up

The STX 600 is a uniquely flexible and multifaceted piece of gear. It’s no mystery how Spectra 1964 has made believers out of producers and engineers like Tchad Blake, Jack Douglas, Dave Cobb, Vance Powell, Pete Lyman, and drumming icon Matt Chamberlain.

The STX 600 is an excellent addition to the company’s line of 500 series modules. With a price tag of $985, it will be available through the Spectra Custom Shop in September 2020.

Michael Duncan is a record producer, engineer, and writer who lives in NYC.

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