Hands-On Review:GT Electronics Vipre Variable Impedance Mic Preamp



by Emile Menasché

Next to the microphone, the mic preamp is the most critical component in the recording signal path. In fact, a bad-sounding preamp can do more to disrupt an otherwise killer sound than any other piece of gear. And the worst part is you may not even know your preamp is trash until you’ve heard the same mic in use with a better preamp. Unfortunately, the world is overpopulated with bad mic preamps, low-rent devices that look cool but which can cause subtle clipping and imbalanced frequency response.

 

None of which applies to the Groove Tubes Vipre. The Vipre is about as full-featured a mic preamp as you can find, boasting switchable impedance transformer circuitry, five variable rise times, precise and repeatable gain controls, multiple inputs optimized for a variety of sources, and extras such as onboard metering and high-pass filtering.

 

While many of these features can be found in your average preamps, the Vipre’s variable impedance capability is what puts the unit ahead of the pack. Microphones and electronic instruments are built to operate at specific impedances. One reason why your guitar sounds better plugged into your amp than into your recording console is that the amp is a better impedance match. Vintage mics are particularly sensitive to impedance mismatches, and many older mics were designed to work at lower impedances than what is typically employed in modern circuits. The Vipre lets you match the unit’s impedance to your mic and instrument, thus optimizing performance.

 

As you’d expect from a company that basks in the glow of vacuum tubes, the Vipre employs all-tube Class-A tube circuitry throughout. But unlike tube mic preamps designed to add “warmth” by distorting and darkening the signal, the Vipre provides audiophile-quality sound, wide frequency response, copious headroom and high output. Judging from our tests, it can work in any recording environment.

 

Versatility is chief among the Vipre’s virtues. The unit has rear-mounted balanced XLR and 1/4-inch jacks for connecting mic and line sources, and a front panel 1/4-inch instrument input. A front panel Input switch lets you select which input is active and determines how the preamp responds to the signal it receives. This is no subtle design detail. Whereas many devices will “defeat” the mic input when the line-in jack is occupied, the Vipre’s design lets you wire up all of the Vipre’s jacks to your patchbay and switch between them as needed.

 

The Input selector is where you optimize the Vipre’s circuitry for your source. The Vipre has two mic input settings—Balanced XFMR and Balanced Bridge—and two instrument settings—Standard, for passive guitars, and -20dB, for handling active preamps and other hot signals. The most audible change occurs between the two mic input settings. Balanced XFMR activates an input transformer that can operate under variable impedances, which you can select with the Impedance switch. The Balanced Bridge setting offers an electronically balanced input design connecting the signal to the first grids of the dual tube gain stage. In Balanced XMFR mode, the Vipre offers a choice of 300, 600, 1200 and 2400 ohms. A modern condenser like the AKG 414 sounded best at the higher impedances but also worked at the lower settings, which are optimized for vintage mics like the RCA ribbon Broadcast mic.

 

There was also a noticeable sonic difference between the XFMR and bridge modes. With the 414, the XFMR mode sounded fatter and seemed to drive the circuit a little harder. In either case, the Vipre brought out the mic’s transient response and detail. The 414’s upper midrange peak can sound gnarly through a bad preamp, but the Vipre took advantage of this presence boost and brought welcome clarity to a vocal track. The Vipre was also very effective as a direct input for bass, electric and acoustic guitar.

 

The Rise Time control, which affects the circuit’s responsiveness, is one of the Vipre’s most unique features. With it, you can optimize how the unit responds to harsh transients produced by vocal sibilants, brass instruments, piezo pickups and other instruments. I opted for the fastest settings for most material, but also found that slower settings worked nicely with vocal and bass.

 

The Vipre offers precise control over audio levels, which is particularly important in the digital environment, where you need to take advantage of headroom. The unit boosts gain with two stepped controls: the first works in 5dB increments, the second in steps of 1dB. This made it easy to set the optimal levels for a vocal track and a dynamically varied acoustic guitar track, and switch between the two.

 

Other controls include a 48-volt phantom power switch and switches for Standby and Mute (useful when you’re switching mics), a phase reversal switch (to help combat phasing issues in a multi-mic setup), and a 75Hz lo cut switch (useful for reducing unwanted rumble). The unit itself is large, requiring four rackspaces, (five if you use the recommended reinforcement rails to bear its considerable weight), but its construction is solid and the switches and dials have a satisfying feel.

 

THE BOTTOM LINE
With its versatility and excellent sound, the Vipre should find its way into the arsenals of recording engineers who are looking for the sonic depth, detail and warmth of tubes and the precision of a truly modern design.