DEMETER VTMP-2B STEREO TUBE MICROPHONE PREAMPLIFIER REVIEW

Mix Magazine (May 1992)
By Guy DeFazio

Before the introduction of solid-state components, tubes were considered to be amazingly sensitive and accurate devices capable of extremely high efficiency and bandwidth. Then, with the birth of the transistor, designers shifted from vacuum tubes toward the use of new, more efficient, silicon-based components. Despite their technical superiority, transistors could not upstage the devotion that much of the audio industry had developed for their trusted friend, the vacuum tube. Consequently, this enduring affection for tube devices allows modern manufacturers to develop affordable new products based around what is now considered to be "vintage" technology.

The VTMP-2 vacuum tube microphone preamp from Demeter Amplification represents a perfect example of an affordable new product making use of vacuum tube design. This simple, two-rackspace package incorporates two channels of variable 30-50dB gain, 48-volt phantom power and a fully regulated internal power supply. While the entire schematic might be too involved to fit on a cocktail napkin, the design is very basic, including a Jensen JT13K6C input transformer, two 12AX7 tubes as the input gain stage, a 12AT7 tube acting as a high-current/low-impedance driver following the input stage, and a 12BH7 tube as the output driver. The tube circuits incorporate polypropylene capacitors and metal film resistors throughout.

Two very simple operational features of the VTMP-2's front panel impressed me immediately. First, in addition to the three-position 30-50dB gain switch and a rotary output level attenuator, there are individual phantom power switches for each channel. I prefer switching phantom power individually, and having the switch on the front panel provides an "at a glance" evaluation of each channel's setup during fast-paced situations. Second, the input peak indicator on each channel delivers a visual confirmation of that magical threshold when the VTMP-2's front-end tubes are overdriven. After all, the sonic properties of tube compression can be very useful in some applications, and the peak LED is a nice reminder. The front panel also contains 1/4-inch instrument input jacks, with associated mic/inst. switches for selecting these transformer-bypassed, line-level inputs.

The back panel has XLR, 1/4-inch and TT output connectors, as well as standard XLR mic inputs. No matter which connection you choose, the absence of a third pin on its AC power cord requires that the output connector contain a ground-carrying conductor (i.e., a shield carried through). This requirement may be eliminated if the VTMP-2 is securely fastened to a properly grounded equipment rack.

James Demeter describes the VTMP-2 as "the simplest practical design with the fewest possible parts." The combination of full regulation of the tube filament voltage (12.6 V), with the B+ voltage (250 V), allows the VTMP-2 to deliver all of the warmth of a tube device without all of the unwanted noise typically associated with tube gear. My bench tests confirmed Demeter's noise specs (-124.70 EIN @ 50dB gain w/Rs="1"50 ohms) as reality.

Off the bench, the VTMP-2 delivered the goods reliably. Working on a country track with artist Eddy Cunningham and writer/producer Roger Wojahn, I put Demeter's mic pre to work on a good ol', down-home country ballad. Using the VTMP-2 with a pair of B&K 4007s as drum overheads, I warmed up the slightly bright characteristics of my drum room and still got the transient response I wanted. Then, once in overdub mode, I couldn't resist trying it on fiddle. This particular song had much of the fiddle part written in the instrument's lower register. So the VTMP-2 had a chance to show off its low/mid response. The result was a sound rich in lower harmonics. Finally, when it came time for group background vocals, a tube U47 routed through the VTMP-2 (directly to tape) provided a remarkably clean combination of old and new tube technology.

At $1,549, the VTMP-2 is an excellent example of how vintage vacuum tube technology, complemented by common-sense modern circuitry, obtains the best of both worlds at a reasonable cost.