Field Test: Mercury M72s Studio Microphone Amplifier
February 1, 2006
By Barry Rudolph
MIX Magazine

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DUAL-CHANNEL TUBE, RETRO TAKE ON A CLASSIC DESIGN

Mercury Recording Equipment's M72s is a faithful re-creation of the Telefunken/Siemens Kassettenverstarker V72 tube mic/line amplifier. All of the amplifiers in the V Series were fixed-gain units, and the V72s was a rare, modified version specified for the EMI REDD 37 recording desk used by The Beatles and others at Abbey Road Studios during the 1950s and '60s. The original V72, V72s, V76 and other variants continue to be sought after for conversion into wonderful stand-alone studio mic and line amplifiers.

INSIDE THE LEGEND

The M72s, a 2-channel, two-rackspace mic preamp with an internal AC power supply, comes in an all-steel cabinet in the original Tele beige color, with a thick, militaristic-looking front panel. It uses custom-made transformers for the input and output paths, and another as a choke in the amplifier. Two EF86s — without hold-downs, but in ceramic, gold-pin sockets — are used for each channel. The handmade and roadworthy construction comprises first-class components firmly mounted throughout and welded chassis corners for extra rigidity.

FRONT PANEL CONTROLS

The front panel is adorned with 3/16-inch-thick, nickel-plated strike plates that mimic the original V72s' pull handles that were used to insert modules or remove them from the audio console's frame.

A large, old-fashioned toggle switch and a 1950s-style red-jeweled light are used for power on/off. Two large Bakelite control knobs labeled Gain (one for each channel) change gain in 11 3dB steps from 28 to 58 dB; gain switching in 1.5dB steps is an option that can be special-ordered. An input pad switch offers either -16 or -28 dB of attenuation.

Input impedance to the rear panel XLR is 2k-ohms or 4k-ohms when the -28dB pad is used. The unit's output impedance is 30 to 50 ohms, depending on gain, and the recommended source impedance is 200 ohms or 500 ohms minimum. Maximum output level is +22 dBu, with a frequency response of 20 to 20k Hz, ±0.5 dB.

There are phantom power on/off and phase-flip (polarity) switches, along with a mic/DI switch that selects either the rear panel XLR mic input or a 1/4-inch front panel DI jack. The JFET DI circuit has a 2-megohm input impedance. It is said to mimic Class-A tube circuit topology and provide high RF immunity. This impedance converter drives the input transformer so that you get the full benefit of the preamp's sound. Most DIs on preamps bypass the first amplifier stage or input transformer — or both. Finally, orange-colored recessed indicators show when the channels are ready for operation and change to red when phantom power is engaged.

IN THE STUDIO

My first test was with a female vocalist singing up close to an AKG C 414 TL II with no pad, no roll-off, in cardioid pattern and 37 dB of gain on the M72s. This is by far the thickest, warmest and fattest mic sound I've heard — even compared to other tube mic preamps. My singer has a husky, dark voice, so perhaps this wasn't the perfect mic/preamp combo for her, but the sound was still amazing: present, quiet and extremely clear.

I swapped out the 414 for an AKG C 24 tube stereo mic (top capsule in cardioid and 40dB gain) and backed my singer off about a foot. I found that I could plug the M72s' output directly into a dbx 160SL compressor without an equalizer and get a close and present sound, as well as the "air" around her voice and some of the room's tone.

I next recorded a flute (34dB gain) with a Soundelux E47C condenser mic placed 1.5 feet above the player and got a great sound without any equalization. I also did not add any EQ later on during the mix. A tube mic and tube mic preamp are just the ticket for a fat and natural flute sound!

I tried the DI input for bass guitar and noticed much more level and a percussive edge as compared to a Whirlwind IMP 2 transformer direct box. The DI input set to 28 dB was louder than the Whirlwind plugged into the XLR mic input with 37 dB of gain. Using the DI and advancing the gain upward (and backing the following compressor down) started to overload the M72s in the smoothest way. I liked that I could set the exact amount of breakup to taste — turn it way up and get the fuzz sound, just like on The Beatles' "Think for Yourself" from Rubber Soul.

A WORTHY GAIN PARTNER

With a huge sound, an excellent built-in DI and line-level capability, the Mercury M72s is a remarkably versatile recording tool. I love the vintage tube sound in a modern unit that's free of vintage problems such as noise, scratchy pots and variable performance. Getting great sounds on all of the sources I tried was never easier!

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