Copyright 1997 by Rick Chinn. All rights reserved. This article was originally published in AudioMedia magazine, American Ed., Jan/Feb 1998.

Rushes: AKG SolidTube Microphone

I got a call a couple of weeks ago from Frank Wells (editor of Audio Media) asking if I was interested in reviewing the new AKG SolidTube microphone. "Of course I am," I heard myself say. I admit, I have a soft spot for microphones. Iím the sort who looks at a vintage microphone with the same glint in my eye as an enophile does when looking at a rare Bordeaux.

Looking at some of the preliminary press releases, and spotting an ad for the microphone piqued my interest. Since this was a tube microphone, an AKG Tube microphone, I was prepared for another multi-kilobuck price tag. Nobody was more surprised than I when I discovered that the MSRP for the SolidTube was $1500, which includes the microphone, power supply, interconnect cable, anti-shock suspension, and a smart carrying case.

Of course, when the box arrived the first thing to do was to connect it to my mixer and just listen. It worked as expected, no surprises. It was surprisingly resistant to shock, even to thumping on the microphone body.

So, what is it?

The SolidTube microphone is a single-pattern (cardioid), large-diaphragm condenser microphone. A slide switch changes the sensitivity of the microphone to accommodate loud sources, and a small red window glows a dull red (from the tube filament) when the microphone is powered.

The power supply is housed in a sturdy metal box. On the front there is a 100Hz highpass filter switch, and on the rear are the AC mains connector, input and output connector, and a ground-lift switch. The power supply is a conventional solid-state design, with a DC supply for the tube filament, and a separate high-voltage supply for the tubeís plate supply. The microphone uses a 6-pin XLR connector for connection to the power supply (a smart choice).

The ground lift switch deserves a bit of discussion. For safety reasons, the outside enclosure of any electronic device needs to be connected to earth ground. The SolidTube uses an output transformer making it is practical to allow the user to separate the power supply ground from the console ground in the case of a ground loop. This is far better than disabling the supplyís safety ground connection.

The key to the microphoneís stellar price point is smart manufacturing.. It is clear that they have gone to great lengths to minimize the labor content, and thus, the cost. Since most users almost exclusively choose the cardioid pattern, AKG could eliminate the pattern switch and the more complex dual-membrane capsule. This represents another cost savings.

Insiderís View

The microphone itself is very sturdy. The case is diecast and substantial with a robust look to it that is at once familiar without looking like anything in its past. The single dual-triode vacuum tube (12AX7) serves as an amplifier and line driver (cathode follower). The output of the microphone is transformer-coupled. With the exception of a zener diode, there are no solid-state devices within the microphone body.

The newly-designed one-inch capsule has a six-micron mylar membrane and has been tuned for a rising frequency response. The same membrane material is also used in the C12VR and C414 microphones.

The circuitry for the new microphone is housed on two glass-epoxy circuit boards, and the tube (12AX7) has a rubber support to ensure that it stays in its socket. The circuit boards mount to a molded plastic support assembly without screws, and this assembly also holds the capsule. The capsule is enclosed by a live-rubber mount, suspended by four legs molded around its periphery, effectively shock isolating it from its surroundings. This entire assembly bayonets into the top half of the case without fasteners. Three screws secure the bottom half of the case to the circuit-board support.

In Use

In the studio, I set the SolidTube alongside an AKG "The Tube" microphone. I used it for male vocal, tenor saxophone, clarinet and acoustic guitar. The sound was full, and warm, with a nicely extended high-end that was pleasantly bright without sounding exaggerated. Neither I nor the other engineer present could tell the difference between the two microphones.

Later, I compared the SolidTube with a Neumann M149 and SM69. The M149 is Neumannís tube/solid-state hybrid microphone, and the SM69 is a stereo U67. On my wifeís spoken voice, the AKG sounded warm and natural, with a pleasing amount of sibilance. By comparison, the M149 had an upper-mid edge to it, almost bordering on being nasal, with less sibilance. For some voices, the M149ís sound might be an asset, but for my wifeís voice, the SolidTube was it. The SM69 sounded more like the AKG, with the warmth and fullness, but again with less sibilance. A glance at the frequency response curves confirms this.

In Closing

The SolidTube has the trademark AKG sound and a price within the reach of any project studio. Considering the price point, I was interested in learning what corners, if any, had been cut. Iím happy to report: there are none. Bravo!


Solidtube Pictures