Alesis GT Electronics

The solid‑state AM51 and AM52.

With so many good capacitor microphones already on the market, can Alesis offer anything new? Paul White puts their new large‑diaphragm FET and valve mics to the test.

Alesis started life as a company specialising in low‑cost digital signal processing, but since then they've produced the hugely successful ADAT digital multitrack tape recorder as well as a creditable range of studio monitors, both of which have taken the company into other areas of engineering. Last year it was announced that Alesis had bought US microphone manufacturer Groove Tubes, a small but well respected Californian company owned by specialist valve designer Aspen Pitman (Groove Tubes' valve marketing operation remains independent). At the outset, Alesis stated that it was their aim to apply their financial backing and manufacturing expertise in conjunction with Aspen's skills in microphone design with a view to building a range of professional capacitor microphones. The result is the GT Electronics AM series of microphones, which purports to offer high‑end performance at a mid‑market price. The series comprises two valve models and two solid‑state FET models, with a fixed cardioid and multi‑pattern version of each. Both tube mics have their own external power supply, so no phantom power is required for their operation.

All the mics in the range feature a large capsule a little over one inch in diameter — the single‑pattern mics employ single‑diaphragm capsules and the multi‑pattern mics have a dual‑diaphragm topography. All four mics use the same hand‑tooled, brass housing, but to differentiate the models, the solid‑state mics are finished in non‑reflective black while the valve mics have a satin metal plated finish. The top‑of‑the‑range AM62 also features a gold band around the lower part of the body. All four mics feel reassuringly heavy, and a very high standard of mechanical engineering is evident throughout.

Rather than emulate an existing microphone shape, Alesis have taken some elements of classic microphone design and incorporated them into a distinctive bulbous shape that looks a little like a Neumann that's been force‑fed. Both solid‑state models come with simple standmounts that fix to the bottom of the mic body via a threaded retaining ring, while the tube models come complete with shockmounts and power supplies. All models feature a switchable 10dB pad and a 75Hz/12dB/octave switchable high‑pass filter.

Thinly Veiled

The valve‑based AM61 and AM62.

Even though Alesis automate manufacturing wherever possible, capacitor microphone capsules are still extremely labour‑intensive and require dozens of manual machining operations. This is one reason why good capacitor mics will always cost more than dynamic mics. The main advantage of a capacitor mic, compared to a dynamic model, is its extended frequency response and increased sensitivity, largely due to the very low mass of its diaphragm. A dynamic mic diaphragm is relatively thick and has to drag a heavy metal voice coil around with it, whereas a typical capacitor capsule is based around an incredibly thin mylar diaphragm upon which a layer of metal (usually gold) only a few molecules thick has been deposited. This coating renders the diaphragm conductive so that the capsule can be electrically charged. In most capacitor microphones, the diaphragm is typically five to 10 microns thick, but the AM‑series capsules use a diaphragm that's just three microns thick (three thousandths of a millimetre) so as to reduce the moving mass still further. There's also a gold‑plated centre element designed to further extend the HF and transient response of the capsule.

The AM51 and 52 are the FET models, the AM51 having a fixed cardioid pickup pattern whereas the AM52 is switchable between cardioid, omni and figure‑of‑eight patterns. A Class A FET head amplifier circuit is used via a low‑distortion Cinemag nickel core output transformer.

Both the AM61 and 62 incorporate Class A valve amplifiers based around the GT5840M (selected military) miniature triode valve, and these also have transformer‑coupled outputs. The valves run with an HT of 120V DC and have 6.3V heater circuits. The valve models are supplied with a aluminium case that also holds the power supply, shockmount and cable. The two solid‑state mics come in smaller, but no less rugged ABS locking cases with metal hardware.

These mics are beautifully built, with a tapering mesh basket below which a recessed ring accommodates the pad and filter switches. The pattern‑change switch is also found here on the relevant models. I felt the switch action could have been a touch more positive, but it seems to get better once the switch has been operated a few times.

Testing Testing

First up for test was the cardioid‑pattern, solid‑state AM51 model, which I tried against three of my own capacitor mics. The two solid‑state mics have nominally flat frequency responses that deviate no more than +/‑ 1.5dB from 20Hz to 18kHz, and because the amplifier is a Class A design, there's no crossover distortion, a significant factor in keeping the sound clean, especially at low levels. It's important to mount any capacitor mic on a stand that is not going to be subject to vibration — even a headphone lead rubbing against the stand can compromise the result as some low‑frequency rumbling will be picked up. If you try to test it while it is held in your hand, it'll pick up all sorts of things, including muscle movement or even your pulse!

The main advantage of a capacitor mic, compared to a dynamic model, is its extended frequency response and increased sensitivity, largely due to the very low mass of its diaphragm.

The overall sensitivity of the AM51 was similar to the other mics used in the test, and though the tonality of each microphone varies slightly, the AM51 didn't stand out as being radically different other than seeming perhaps a touch more open‑sounding at the top end. As the mics used for comparison were models I'd chosen on the basis of their good sound quality, this was a pretty positive outcome, though I did feel that the AM51 was slightly more susceptible to popping than the others if used without a pop shield. Perhaps this is related to the thin diaphragm material? Overall, the sound was solid and detailed, with just a touch of that slightly over‑focused quality characteristic of virtually all cardioid mics.

Switching to the AM52 set to cardioid mode produced a similar tonality, but with more of an open feel to it. This is something I've often noticed when comparing single‑diaphragm cardioid mics to multi‑pattern mics switched to cardioid mode, and may well be associated with the porting systems employed within single‑diaphragm cardioid capsules. Switching to omni opens up the sound even further, while the figure‑of‑eight pattern is beautifully clean with almost perfect rejection at the sides of the capsule. In all modes the off‑axis response is excellent.

Opening The Valves

Next up came the valve models which, I have to admit, don't sound as obviously tube‑powered as some earlier Groove Tubes models I've tried, but still add that distinctive warm chestiness to vocals, as well as sprinkling a little glitter on the high end. The latter characteristic is particularly noticeable when working with acoustic guitars or delicate acoustic instruments. The multi pattern AM62 adds a super‑cardioid pattern to the repertoire of the AM52. Interestingly, the valve models have a flatter frequency response than the FET models, extending from 20Hz to 20kHz with only a +/‑ 0.1dB variation. The valve models are also just slightly more sensitive at 39mV/Pa as opposed to 35mV/Pa for the FET models. There's little to choose noise‑wise between the valve and FET models — both manage an equivalent input noise of around 20dB (A‑weighted) give or take a dB or so, which is absolutely fine for this type of microphone.

Rather than emulate an existing microphone shape, Alesis have taken some elements of classic microphone design and incorporated them into a distinctive bulbous shape that looks a little like a Neumann that's been force‑fed.

With the pad switched in, any of the mics will take a maximum level of around 140dB SPL, with the solid‑state models being 2 or 3dB more tolerant than the valve models. But then you wouldn't put your best valve mic in a kick drum, would you? As you might expect, the valve mics exhibit a higher total harmonic distortion figure than the FETs, but even then it's still a very low 0.13 percent, which I actually find encouraging, as the best valve gear I've heard to date invariably has pretty low distortion. This may sound like an odd statement when we're all led to believe that valves sound good because of the way they distort, but in reality the mechanism of valve tonality is more complicated than that and I'm not convinced anyone understands it fully. To me, the 'valve' sound always sounds more like compression than distortion.

Conclusion

All of the mics in this series are capable of producing excellent results, and even though the large‑diaphragm capsule creates a slightly flattering, larger‑than‑life sound, the subjective impression is that you're hearing a fairly honest version of what's being recorded. Because there's no dramatic coloration, these mics are actually quite versatile — they should work well with a wide range of singers, and they also sound good on pretty much any acoustic instrument. Whether the super‑thin diaphragm is directly responsible or not I don't know, but the transient response and ability of these mics to reproduce high‑end detail is certainly very good. If they have a weakness, it's only their susceptibility to popping, but this disappears completely when you work with a pop shield. And as I've said before on numerous occasions, and will doubtless repeat many times in the future, using any capacitor mic to record vocals without using a pop shield is just asking for trouble.

Clearly the new Alesis/Groove Tubes alliance has culminated in a range of well engineered, good‑sounding microphones. When you consider how competition has forced down microphone prices, though, Alesis are not going to find this an easy market to win over, with the likes of Rode, AKG, Audio Technica and even Neumann marketing affordable capacitor mics aimed specifically at the project studio owner. Having said that, there's always room for quality microphones, and I think it's fair to say that the new GT Electronics AM series mics offer better engineering quality than the original Groove Tubes mics at a more attractive price, especially when you consider that the valve mic power supplies are now included as standard. In short, there's a new addition to your 'must try' list.