Groove Tubes Convertible

If you're sceptical about manufacturers claiming their mic is suitable for both stage and studio applications, maybe this one can change your mind...

While the concept behind this microphone may seem an obvious one, the GT Convertible is the first time I've seen it put into practice. In essence, this hypercardioid capacitor microphone is two mics in one. Out of the box, it functions as a hand-held or stand-mounted end-address vocal mic, for stage or studio. But unscrew the mesh ball windshield and you have a pencil-style instrument mic.

Design & Construction

The design owes a lot to the established Groove Tubes GT44 microphone, and combines a shockmounted 0.75-inch-diaphragm hypercardioid capsule with transformerless FET circuitry to achieve a wide frequency response, low noise and low distortion. The ultra-light diaphragm is made of gold evaporated onto Mylar, and is only six microns thick, which is why it has such a good high-end response in comparison with a dynamic microphone. An internal rubber shockmount isolates the brass capsule from the mic body.

Like most capacitor microphones, the GT Convertible requires 48 Volt phantom power to operate but, as it is designed for live use as well as studio applications, some sensitivity has been sacrificed to achieve a huge SPL handling (146dB max), and to maintain parity with other types of live microphone. On paper it isn't quite as quiet as many dedicated studio mics, but used close-up as intended it is subjectively quiet enough, with an equivalent noise of 28dB SPL, and sensitivity of 5mV/Pa.

The mic comes in a zip-up vinyl pouch, with a swivel stand mount. Weighing in at a solid but comfortable 12 ounces, the mic body is finished in satin black, and its sleek lines are disturbed only by the now-obligatory sticker warning you not to dispose of the mic by throwing it into the bin. The very idea! The pop screen/ball is of the familiar dual-mesh construction, with removable foam liner, so you can clean it in the mouthwash of your choice, and it is finished in black. The capsule is protected internally by a silver basket that becomes visible when you unscrew the ball to convert to instrument use. This inner basket also has a dual-layer mesh construction so, in combination with the ball, there's rather more in the way of pop-dispersing layers than you'd get in a typical dynamic stage vocal mic.

The output is on a balanced XLR with gold-plated pins and the entire circuitry of the mic, including the capsule and output socket, is mounted on a single PCB to eliminate costly wiring. The discrete FET circuitry is augmented by a number of surface-mount components, plus a small trim pot that sets the FET bias.

The Convertible's gentle, broad presence peak helps clarity and projection, without making the sound harsh or edgy. Unlike many vocal mics, the low end isn't brutally rolled off below 200Hz, so you may need to engage the low cut filter on your desk or preamp for vocal use, but the hypercardioid pattern is useful in minimising the potential for acoustic feedback in a live situation. Similarly, the ball-shaped pop shield offers a reasonable resistance to breath noise, which is certainly no worse than for conventional stage mics of a similar construction.

Performance

Though it is possible to make this, or indeed any other hand-held mic, pop if your mic technique is bad enough, the combination of the internal and external mesh screens works as well as can reasonably be expected in this respect given that there isn't the inherent low end roll-off you'd expect from a mic designed purely for stage vocals. I'd still recommend using it with an external pop shield in the studio, as I would any other condenser vocal mic.

I tested the mic by making a live vocal recording of an acoustic band where one of the members was also the singer, and the result was clean and clear with reasonable separation from the other instruments and no noticeable circuit noise. The sound isn't quite as gutsy as from a dynamic mic, but then that's the point of using capacitor technology — you get a more honest result, with good high-end detail. Of course, being a hypercardioid mic, working close-up brings the proximity effect into play, which ups the bass level very significantly. You can get all the warmth you want by 'working' the mic appropriately in this way, but if you get too close you can also provoke popping unless you also use a separate pop shield. As I pointed out earlier, there is no significant low-end roll-off built into the circuitry or capsule so, with this in mind, it might have been better for the manufacturers to add a low cut switch for vocal use, though most mixers or preamps should also offer this facility.

As an instrument mic, the GT Convertible performs well with the top down and, though it lacks the sensitivity for distant or very quiet studio recording tasks, it works rather well on acoustic guitars, or other acoustic sources at close range. It can also handle any level that a drum kit or percussionist is likely to throw at it. The handling noise is also sensibly low, and what's more surprising is that you get this level of performance for only slightly more than you'd expect to pay for a standard dynamic vocal mic, so if you play live and have a studio, this mic is going to come in very useful in both areas.