AM11 — a true capacitor mic.
Groove Tubes' latest trio of microphones offer a choice of diaphragm and circuit designs at competitive prices. Paul White examines the sibling rivalry...
Ever since Groove Tubes' microphone division joined forces with Alesis, new models have been appearing at a prodigious rate, with three new models announced at the NAMM show earlier this year. Two are solid‑state devices, while the AM40 is a tube mic based upon a miniature military Nuvistor tube.
AM11
Even the entry level AM30 offers a class A FET preamp.
Supplied in a very nice locking transit case, the AM11 is an impressively engineered true capacitor microphone with a cardioid pattern. Unlike many of the low‑cost capacitor mics currently doing the rounds, it hasn't been designed to look like a U87. Instead, it looks like an up‑to‑date version of one of the original Groove Tubes designs with a cylindrical basket and polished stainless steel support metalwork. The main body is also cylindrical with a tough black powder coating, and the supplied stand adaptor is locked to the mic body via a threaded ring. I have to say that the standard of finish and attention to detail is exemplary.
The pressure gradient capsule features a one‑inch‑diameter diaphragm of six‑micron thickness, made from gold‑evaporated mylar, and it is configured for side address — you sing into the side, not the end. The internal preamp is class A FET, and its output stage is transformer‑coupled so that no ICs are used in the design. Two tiny recessed toggle switches allow for LF cut and attenuation (a 10dB pad) to be brought in when necessary. The low‑cut filter rolls off below 75Hz and is useful to counteract the proximity effect that all cardioid mics exhibit when used very close to a sound source. With the filter switched out, the frequency response is quoted as 20Hz to 20kHz with no more than plus or minus 1.5dB variation. The equivalent noise level of the mic is a respectable 19dBA, while the sensitivity of 35mV/Pa is typical for this type of microphone. A dynamic range of 110dB is quoted, while the maximum SPL is a healthy 135dB (145dB with the pad switched in).
AM30
The AM40 hybrid tube/FET model.
Like the AM11, the AM30 is a solid‑state mic with a class A FET preamp. However, its diaphragm measures only three‑quarters of an inch across and is designed for front rather than side address. A six‑micron, gold‑evaporated mylar diaphragm delivers a good high‑end frequency response and miniature switches on the mic body bring in a 15dB pad and a low‑cut filter, the response of which is identical to that of the AM11. Three different capsules are available providing a choice of cardioid, super‑cardioid or omni polar patterns, all of which have a frequency response which is flat within ±2dB all the way up to 20kHz, though their low‑end response varies slightly — the cardioid model quotes 50Hz, the super‑cardioid 80Hz and the omni 30Hz. The equivalent noise is quoted at 20dBA with a sensitivity of 16mV/Pa at 1kHz — all very respectable figures. The maximum SPL is 130dB with a dynamic range of 115dB and, like all the mics in the range, the output is transformer balanced.
Constructionally, the microphone is both simple and elegant with a satin black cylindrical body and a stainless‑steel grille assembly. The grille cover unscrews to provide access to the capsule while a locking ring above the outlet XLR allows the body sleeve to be removed for access to the circuitry. The standard of build quality is high with neatly assembled glass‑fibre circuit boards, and the mic is presented in a sturdy aluminium camera case.
AM40
The AM40 microphone is externally identical to the AM30 except that it is finished in satin silver rather than black and has a 6‑pin XLR connector for connection to the included power supply. Unlike the previous two models, which run on standard 48V phantom power, the AM40 is a hybrid FET/tube model and so requires a dedicated PSU. The PSU itself is built into a compact steel case with a switched IEC mains socket, a red status LED and XLR connectors for the mic and for the output. No other controls are fitted to the PSU and there's no mains voltage selector. As with the AM30, the whole kit comes in a camera case and the AM40 also includes the necessary multicore lead to connect the mic to the PSU.
The same pad and roll‑off switches are fitted and the paper spec looks virtually identical with similarly low noise and distortion figures. Opening the body reveals the tiny GT5840M Nuvistor tube, hard‑wired to the circuit board. These little tubes require only 6 Volts to operate, which is claimed to produce cooler operation and hence less noise than more conventional valves. A Cinemag nickel‑core output transformer completes the signal path.
Am I Impressed?
The AM11 exhibits a fairly flat response (though no actual plot is provided), in contrast to the majority of vocal mics, most of which feature some degree of deliberate presence lift to artificially enhance the sense of clarity and articulation. In fact, the AM11 has a very transparent, natural sound that is a good deal less 'nasal' than many other cardioid models and which responds well to equalisation should that be necessary. A benefit of the AM11's flat response is that it's more likely to suit a wide range of different vocal types — the more coloured a mic is, the more important it is to match it to the vocalist who's likely to use it — as well as being suitable for a wide range of instruments. Tests with my acoustic guitar and with assorted natural sounds confirmed that what comes over the monitors is very close to what the source actually sounds like.
Of the mics that I used for comparison, the Rode NT1 came the closest in overall tonality, though the AM11 was just a hint more open‑sounding. When you consider that you get a nice case and a stand adaptor as part of the package, the AM11 is very attractively priced for such a well‑made, well‑specified microphone.
Repeating the same tests with the AM30 showed it to have a similar sensitivity with perhaps just a little more airiness of character, though balanced by a very full and well‑rounded low end. Positioning for acoustic guitar work seemed just a little more critical than with the AM11, but a little time spent finding the best spot payed off with first‑class results. Bringing in the low‑cut switch tames the proximity effect to a reasonable extent without thinning out the tone excessively.
Comparing the AM40 with the AM30 is an interesting experience because of the difference in sonic character produced by the tube stage. While the high‑end detail is still there, transients seem to be gently limited, producing an impression of smoothness, while the low mid range also seems richer, as though compressed slightly. These are traits I've come across during numerous reviews of valve mics, though they're more exaggerated in some models than others. Here I think the designer has got it about right as the valve flavour comes across without overwhelming the sound.
The mic market is a pretty ruthless place at the moment, but these new Alesis GT models offer a combination of good sound and nice build quality at a sensible price.
Class Conscious
Class B and AB circuit design, such as can be implemented with cheap integrated circuits (ICs), use different signal paths for the positive and negative parts of a signal, and therefore cannot faithfully reproduce signals that are crossing between positive and negative values. This results in what is called 'crossover distortion' and, although the levels of this distortion can be kept very low, even small amounts can be heard — the human ear is orders of magnitude more sensitive to crossover distortion than to the effects of saturation or soft clipping, for example.
The preamps within both the AM11 and the AM30 use superior class A circuitry, however, which uses more expensive Field Effect Transistors (FETs) to pass both positive and negative voltages along the same signal path, therefore avoiding crossover distortion.
Pros
- A very nicely engineered range of microphones.
- All models come with hard case and stand adaptor.
- Detailed, uncoloured sound.
Cons
- No obvious cons at the price.
Summary
These are very well‑made, well‑specified microphones with a usefully open, neutral character. They're not the cheapest capacitor mics you can buy, but they do offer extremely good value.