Mercury 666 Studio Limiting Amplifier

This high‑end design began life as a resurrection of the rare‑as‑hen’s‑teeth Fairchild 666. But it soon became its own thing...

After many years spent repairing and racking vintage equipment, David Marquette wanted to develop his own designs using the best parts of the technology he’d worked on over the years. To that end, back in 1999, he founded California‑based Mercury Recording Equipment. Mercury are celebrating their 25th anniversary this year, and have released many products over the years. From their first three designs, the Fairchild 660‑inspired Mercury 66 Limiter and the EQ‑H1 and EQ‑P1 equalisers, to their highly regarded Mercury M72s and M76m mic preamps based on the Telefunken/Siemens V72s and V76m, all Mercury’s products are hand‑built by a very small team — they’re the very definition of a high‑end boutique pro audio company . While this means they don’t come cheap, it’s fair to say that they’re all much less expensive than the rare vintage designs that inspired them or that they’re intended to complement.

Overview

The latest addition to Mercury’s line‑up is the 666 Limiting Amplifier. Inspired by the Fairchild 666, this is a mono, hybrid (solid‑state and valve) compressor but it isn’t an outright clone. The original Fairchild 666 held plenty of sonic appeal but, putting it politely, a very limited set of user controls, with many adjustments requiring a screwdriver. Mercury felt this would make a straight reproduction a tough sell in today’s recording market so, early in the planning stage, they took the decision to create a new device that, while still ‘leaning on’ the sonic heritage of the original, would feature plenty of enhancements to make it more relevant to modern studios.

The solid‑state part of the equation lies in the dynamics processing, in the form of the custom‑designed Mercury SSGRE gain‑reduction element. Then comes the valve make‑up gain stage, featuring a 12BH7 valve and a custom Cinemag output transformer. Finally, among the new features is a Baxandall shelving EQ, an unusual feature to include on a compressor and one that really caught my eye. Apparently, the designers felt that the simple tone controls of the classic Baxandall filter would be a worthy addition to a compressor that users would probably already be looking to for it’s tone‑shaping qualities as much as for controlling dynamics. It’s a good call — I’ve always been a fan of the wide, forgiving curves of this style of EQ, whose controls to add or cut bass and treble may be simple but can be very effective.

Controls & I/O

As on the Fairchild units, the attack and release aren’t separately adjustable. Instead, time constants are defined by choosing one of 11 ‘program settings’ — presets, if you like — and Mercury have opted to include pairings taken from different classic compressors. Numbers 1‑6 are modelled on the first six settings of the Fairchild 660. Positions 7‑11 are designed to offer more flexibility, and include two alternative fast attack settings as well as a couple of options that mimic the behaviour of another famous vintage valve compressor, the Gates Sta‑Level.

If you’re used to the almost unlimited control options offered by digital plug‑ins, being confined to a menu of attack/release settings like this can feel a little limiting at first, but once you get to know broadly what they offer you’ll often find they give you a surprising amount of flexibility.

Gain reduction is introduced by turning down a more familiar threshold knob, but there’s also the unusual option, which is found on some other Mercury designs, to adjust the DC threshold. This impacts on the character of the compression as well as the amount, and in use is similar to the knee control found on some more recent compressor designs.

The 666 features both stereo linking and a side‑chain insert.In the lower half of the front panel you’ll find controls for all the ‘enhancements’. These include a high‑pass filter (90 or 150 Hz) for the side‑chain signal, which is something I found I nearly always wanted engaged in one or other setting. There are also link and side‑chain insert switches: a TS jack socket on the rear allows two units to be linked for stereo use, while an XLR send and combi XLR/jack return allow you to insert external devices into the side‑chain. These are joined on the rear by XLR sockets for the main audio I/O.

Back to the front panel, and we have simple controls for that Baxandall EQ: the ±10dB low gain knob controls a 20Hz shelving filter, whose gentle curves give you as much as ±5dB at around 350Hz; the ±6dB 20kHz high shelf translates to ±3dB at 3kHz. Other welcome additions are a hard‑bypass switch and a wet/dry mix control for parallel compression. It’s worth mentioning that whilst the bypass does what it says on the tin (bypasses all the 666 circuitry), you can also set the blend control to fully dry, whereby there’s no compression but the audio still pass through the circuitry.

A Steady Start

The 666 arrived at my studio during a busy period of tracking sessions, and I tended to use it quite conservatively in the first few sessions while I got to know it. The Mercury 666 ships with a handy card listing the various attack and release options — they’re just numbered 1‑11 on the faceplate — and I found it helpful to leave this on top of the 666. This is very much one of those devices you need to get to know if you’re to appreciate what it can really offer. In tracking sessions, you often don’t get the time or space to fully explore your gear, and on many sources there can be a large amount of nuance to how the 666 behaves.

During these early encounters, my ears seemed drawn to the medium attack/medium release setting, in which surprisingly heavy‑handed amounts of compression can be added to introduce a wonderful ‘roundness’ to vocals of different styles. In fact, this setting seemed to work great as a ‘set and forget’ introduction to the 666, and it remained my go‑to starting point as I got to know it better.

Its first job was on a vocal captured with a dynamic mic (a Shure SM7) while recording a full band in the live room. Despite my best efforts, there was bags of drum spill in this mic, so I had to be careful with the compression. But straight away I noticed that the 666 introduced a subtle coloration that helped the sound from this mic sound a little ‘richer’, even with no gain reduction. I leaned into this by adding a touch of low‑end boost with the onboard EQ and, with just a tickle of compression on louder sections, it all worked great.

More Than Meets The Ear

Looking back at my initial uses for the 666, it was a little like driving a high‑end car slowly down a country road, toddling along with a queue of traffic behind me! But once I had a little downtime I could experiment properly, and began to get a sense of just how flexible and impressive‑sounding this device can be. I mentioned what a great job it can do as a smooth, transparent vocal compressor, but when used on sources with more defined transients it can be nothing short of transformative.

As a drum room ‘smasher’ you can get huge, crunchy effects that can be easily shaped with the EQ section and the blend control.

As a drum room ‘smasher’ you can get huge, crunchy effects that can be easily shaped to taste using the EQ section and the blend control, and I loved how the 666 could add snap and presence to the kick or snare close mics — again, I could often then enhance the result with a quick turn of the low and high EQ controls. It’s really handy to have those here.

I can imagine that many users who are not so used to working with vintage units might perhaps be frustrated by the lack of variable attack and release controls, but I have to say that the program options presented here worked really nicely for me. They present you with plenty of different ways to approach compressing a source, and encourage you to work swiftly. Put crudely, it’s a little like browsing presets, in that you can often be very pleasantly surprised!

On sources like bass and vocals, I could achieve very similar‑sounding results to those from various high‑end vari‑mu compressors I own or have used, but you can also steer it towards more aggressive FET or VCA territory on drums or guitars. The modern control options play a big part in this flexibility, as does the Baxandall EQ; whether used as a tone‑shaping device or to pin a guitar or bass into place in a mix, it seemed to add value to anything I tried it on.

Summing Up

As is frustratingly often the case with the high‑end gear that I review for SOS, just as I found myself really falling for the 666 it was time to give it back! With my small‑studio‑owner business head on, and plenty of quality compression options in the studio, it would have been tough for me to justify the outlay required to keep it here. But putting aside the asking price, I genuinely loved the time I spent getting to know this wonderful‑sounding compressor. I’ve been very impressed by my first encounter with Mercury’s gear and approve of their approach too — not just resurrecting vintage gear for the sake of it, but combining vintage flavour with modern features. If you’re in the position to shop at the top table of the pro‑audio market and are looking for a classy but versatile valve‑style compressor, the Mercury 666 has to be worth a demo.