The single-channel, three rack space Mercury 666 MKI is inspired by the Fairchild 666, a later cousin of the Fairchild 660/670 limiters. Unlike those units, the 666 was a hybrid solid-state and tube design. Mercury took that concept (and some characteristics of the original 666) to create something very useable and new.
The Mercury 666’s output makeup gain is from a 12BH7 tube with a custom-made Cinemag output transformer, and the gain reduction happens via Mercury's proprietary Solid State Gain Reduction Element (SSGRE). Mercury says this is a linear voltage control circuit designed to act like a variable mu tube compressor, which it does. The Fairchild 666 was an optical compressor, but I bet this Mercury 666 version is a lot faster. The Attack/Release switches between 11 positions, and I'd recommend printing out Mercury's chart explaining the different settings and keeping it around when first using the Mercury 666. On an anemic snare drum, I sparkled up the drum's attack with either position 1 or 9, whereas a far-too-dynamic kick track sounded best in position 1 or 6. Drum room mics sounded great in position 1 as well, though 6 opened up the hi-hat in the mix more.
On electric bass, using the Side Chain to alter the compressor's detector circuit to a 90 or 150 Hz high-pass kept the low frequencies passing through while stopping the limiter from clamping down too hard when deep notes were hit, which allowed for more aggressive settings without sucking the life out of the bass. Position 3 on the Attack/Release seemed to work well here. But the real benefit and fairly unique feature that came into play with the bass was the two band Low and High Baxandall EQ. What, a compressor with EQ? Is this for the Side Chain? Nope, but you can insert an outboard EQ for that if you need more than the two high-pass filters mentioned, and there's even a switch to take it out of the detector path. Instead of all that, this EQ is for the output signal, with smooth shelves that lift or cut the high and low frequencies. It added a wonderful bottom to a bright bass guitar and brightened up the earlier snare track in a pleasing way as well.
There's also a DC Adjust knob, which is part of the SSGRE circuit. Its effect can seem somewhat subtle, and I think of it as fine-tuning control for the threshold and its knee point, for where and how the compression kicks in on a track. Settings in the center or lower worked best for me in most cases. There's a "modern" mix blend knob (Wet/Dry) on the Mercury 666, and that's something I find more important as people might pipe a track out of the box and back in for mixing/processing. Having the blend allows us to print a new track while keeping it in phase with itself, as a converter’s round trip can add latency. With console mixing, I rarely use features like this, but have done so for in the box mixes. I found the Bypass switch very useful as it takes all circuitry out of the signal path. None of my tube compressors offer a true bypass, and it really helps to know if you are simply making the track louder instead of limiting properly.
On female vocals, I found the Mercury 666 added a lot of presence and clarity. Position 8 seemed to fit the first song and singer I worked with – the timing grabbed the loud bits just enough without tugging the vocal out of the mix. With just one bump up on both of the EQ knobs, the chest tones and high end popped out in the mix as well and felt ready to print.
I only had a single Mercury 666 to review (would a two-channel unit be called a “1332” or a “666-2”?), but it does feature a Link input (1/4-inch TS jack on rear) and switch. I tried sending unmastered mixes I'd done through it in mono, and while the results can be fun, it mostly acted pretty overtly when tossed into previous settings, with drum attacks sinking into the mix. When relaxing the settings quite a bit while using a light EQ boost, I was able to get it into a zone that might work well for a mix bus, but if you do this, please tread cautiously. The Mercury 666 wants to dig in and get to work on whatever we feed it!
Despite loving this limiter on mixes and tracking, I found a few things I had to question. A switch to bypass the EQ section would be useful. The front panel layout is nice and open, and feels retro-cool, yet inside of the chassis, there is a lot of open space. I can imagine there's a way to fit the Mercury 666 into a two rack space enclosure, though Mercury says it would not fit, and I know the front panel wouldn’t look as nice with the controls jammed together. Most of these knobs feel like stepped rotary switches but are actually detented potentiometers. I'm not saying this is bad – just noting it for anyone who is interested. Mercury matches the potentiometers closely, so using two linked units should not be an issue. They can even match units, if desired.
In use, I felt that the Mercury 666 tends left its mark on whatever it processed (pun intended). It's a versatile compressor that can rein in sounds while doing something quite euphonic – I was always pleasantly surprised. I'm glad Mercury decided to "update" this unit instead of just copying what came before, as the extra Attack/Release settings and other controls add a lot of flexibility to the Mercury 666. It isn't cheap, but it's similar in price to my Retro Instruments 176 limiters [Tape Op #66], and those get used on every analog mix and tracking session. I imagine the Mercury 666 MK1 could fill that role as well.
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