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AKG C4000B

Condenser Microphone
Published April 1999
By Hugh Robjohns

Multi‑pattern microphones traditionally have true capacitor capsules and are correspondingly relatively expensive — until now. Hugh Robjohns lends an ear to AKG's affordable new C4000B, the first dual‑diaphragm, multi‑pattern mic with an electret capsule.

AKG have over 50 years of experience in the design and manufacture of high‑quality microphones, the unique large‑diaphragm C4000B being their latest offering. I say "unique" because this is claimed to be the first microphone in the world to provide a multi‑pattern polar response using a dual‑diaphragm electret capsule. All previous multi‑pattern mics have used true capacitor capsules, but recent technological advances have enabled AKG to build multi‑pattern facilities into a more cost‑effective electret design. (See 'Electret Or Capacitor?' box later for more on the differences between the two types of capsule.) Apparently the capsule used in the C4000B is a more elaborate, dual‑diaphragm, variant of that used in AKG's SolidTube (reviewed in SOS January 1998), but with a solid‑state preamplifier in place of the SolidTube's valve circuitry.

AKG C4000B condenser microphone.Aimed primarily at home and project studios, the side‑firing C4000B is reasonably priced, rugged and flexible, and is intended to serve equally well in the studio or on stage. The new mic features an internal preamplifier claimed to offer high sensitivity, low self‑noise, and a very wide dynamic range. It requires phantom powering of between 9 and 52 Volts for the head‑amp. Switches on the mic's body provide three polar patterns (omni, cardioid and hypercardioid), a 10dB pad (giving the mic a very high SPL — Sound Pressure Level — capability), and a bass‑cut filter. An integral wind and pop screen is provided, and the C4000B package is completed by a 'spider suspension' shockmount and a large expanded‑foam windshield.

Technicalities

In AKG terminology, the 'C' prefix in the model number denotes a microphone with a condenser transducer, while the 'B' suffix refers to the integrated bass‑cut filter. The construction of the C4000B's electret capsule assembly is completely conventional, featuring a pair of independent gold‑flashed plastic‑foil diaphragms, one mounted either side of a common backplate. Each foil diaphragm is roughly one inch in diameter (making it a true large‑diaphragm design) and metallised on the outer surface only, to remove any possibility of short‑circuiting against the backplate electrode if the mic is exposed to extremely high sound pressure levels.

The dual‑diaphragm assembly constitutes two pressure‑gradient transducers, engineered to form a pair of back‑to‑back cardioids. When these are combined with the same sensitivity and in the same polarity, the resultant polar response is that of an omnidirectional microphone; when they're combined in opposite polarity, with differing sensitivities, a hypercardioid pattern is produced. If the output is derived from only one diaphragm, a cardioid polar pattern is generated.

The internal preamp stage is 'transistorised' and has enough gain to produce a healthy output level of 25mV per Pascal. It also incorporates a switchable 10dB pre‑attenuator, to reduce the likelihood of overload if the microphone is placed in close proximity to extremely loud sources — and they'd have to be pretty loud, because the mic's rated maximum sound pressure level (for 0.5% total harmonic distortion) is a considerable 145dB SPL without the pad, and 155dB with it switched in! The head amp has a phenomenal dynamic range, too — 135dB (A‑weighted), with an equivalent self‑noise of a respectable 10dB‑A. This latter figure is a good 10dB quieter than its SolidTube valve‑mic sibling.

It must be said, however, that these specifications are quoted for a standard 48V phantom‑power supply. With lower voltages the performance reduces significantly: a dynamic range of something like 117dB is achieved with a 12V phantom supply. You may be thinking that this reduced dynamic range won't apply to you because your console supplies 48V phantom power — but think again! Any cable has an impedance which results in a voltage drop across its length and, although it's usually insignificant with lengths of a few metres, this voltage drop can become a problem if very cheap and nasty wire is used, or when cable runs extend for many tens or hundreds of metres. Another significant question is whether the console really does provide a phantom supply of 48V when multiple microphones are drawing current from it! Usually, the first indication of reduced power‑supply voltages is a mic's inability to capture loud transient signals without distortion, essentially because the amplifier's headroom is compromised by the diminished power rails.

The C4000B features a switchable high‑pass filter (providing a 12dB/octave slope from 100Hz), which may be employed to minimise the likelihood of high‑energy, low‑frequency signals overloading the preamp and causing distortion. Normally this filter would not be required, but if the microphone is exposed to strong air currents, or to vibrations transmitted to it through a mic stand, it's far better to remove their effects at source, rather than at the console.

Construction

The mic's case is a slightly barrelled cylinder (lending it something of a 'retro' appearance) which measures 58mm in diameter at its widest point and 133mm in length. In fact, the C4000B looks very similar to the top half of the SolidTube, upon which it is apparently based, although the body length is substantially shorter in this new solid‑state version. All‑metal casework not only makes the microphone extremely rugged, but also affords a great deal of protection for the internal electronics from the plethora of radio‑frequency signals prevalent in any working environment these days.

The supplied H100 'elastic spider suspension' is an impressive‑looking beast! Based on the typical ring‑within‑a‑ring concept, the design boasts several nice features. First, the junction of the outer metal ring with the plastic standmounting adaptor is engineered to form an S‑shape, providing a pair of cable strain‑relief slots with slightly differing dimensions. This is a very practical feature, in that it reduces the strain of the cable weight on the XLR connector and also improves the isolation from cable‑borne mechanical noise.

The inner ring is supported by a pair of elastic cord loops strung around guide posts. These are arranged into Vs, providing links to the outer ring at the four cardinal points. Most manufacturers are using rubber O‑rings in place of the elasticated shock cord these days, as they are cheaper, provide a more consistent tension over their life span, and don't creak! However, the H100 seems to be well designed and uses the traditional elastic technology properly, proving highly effective in practice.

Holding the microphone in place in the suspension is an extremely elegant clutch system, which can be easily tightened to exert an extraordinary grip on the microphone, yet can be released with a simple twist of the wrist. During my time with the microphone the system worked faultlessly, never becoming loose or rattly.

Auditions

As soon as I began listening to the C4000B it was clear that it was going to be impressive. With the spoken voice (always a challenging and revealing test) the microphone sounded well balanced and more natural than some of its peers. The accuracy of the polar patterns in both cardioid and hypercardioid modes was excellent, with a very high degree of rejection from the rear null(s). Moving off‑axis from the mic caused the expected level drop (the cardioid pattern is actually quite narrow in comparison to many other mics), but the frequency response remained generally smooth and controlled. When I used the omnidirectional pattern I noted a slight loss of treble around the sides, but this is to be expected with a large multi‑diaphragm capsule arrangement.

With challenging acoustic instruments, such as a 12‑string guitar, the C4000B was unable to capture the complex harmonic relationships in quite the same way as a top‑flight microphone such as, say, the DPA (Danish Pro Audio — formely Brüel & Kjaer) 4011. However, in the context of its price it did extremely well, and I would suggest that it's easily capable of capturing more information than the mic amps in most project‑studio mixing consoles can resolve! With voices, acoustic guitars, woodwind instruments and piano, the C4000B was able to extract the sound I was searching for very well. Its accurate polar patterns were a great asset in minimising spill during multi‑miking sessions, and its smooth and even frequency response portrayed instruments very naturally, without imposing a significant character on the sound.

Although considered by many as a mere accessory, the shockmount is a vital part of the C4000B and proved to be surprisingly effective. Even deliberate tapping on the mic stand was almost completely eliminated from the mic's output. Just as impressive was the built‑in wind and pop filter. Clearly, the foam material used behind the wire grille is pretty dense, because it's hard to see the capsule even when the microphone is held up to the light, and with normal speech at a reasonably close working distance, plosives caused no problems at all. Although it was possible to pop the mic occasionally with close singing voices, the supplied foam wind‑gag put a stop to that, too!

During my tests I never felt the need to use the pre‑attenuator, such was the phenomenal headroom of the microphone's internal preamp. Likewise, I didn't need to use the bass filter, as the proximity effect was sufficiently well controlled and predictable with close miking that I could achieve a better result with a little desk EQ, and the shockmount was so good that I didn't need to use the filter to remove unwanted rumbles.

Conclusion

Overall, I was very impressed with the C4000B. It looks smart, sounds good, is versatile enough to work well in the studio or on stage, and represents excellent value for money. It isn't quite as good as a C414ULS, but it isn't really that far off for most purposes either, and it is 40 percent cheaper! The microphone is the source of all recorded acoustic sound, so money spent on a decent mic is always money well spent, and you could do a lot worse than invest in the C4000B.

Electret Or Capacitor?

It seems to be popular across the industry these days to market microphones with electret capsules as 'condenser microphones', causing a degree of confusion as to the real difference between an electret and a traditional capacitor (or condenser) microphone.

Strictly speaking, these two versions of a condenser microphone work in the same fundamental way and share the same desirable qualities in terms of their sensitivity, accuracy and ability to capture high‑frequency detail. However, the important distinction between them is that electret capsules are self‑polarised — they are statically charged during manufacture — whereas the traditional capacitor microphone is electrically charged during use (either via phantom power or a separate external supply).

The construction of electret and true capacitor capsules is very similar but, theoretically, a capacitor mic will work forever, while the static polarising charge within an electret capsule gradually ebbs away, eventually dying completely. Consequently, the capsule will eventually become insensitive and its output more noisy. Fifteen years ago, electrets typically had a usable life span of around 10 years, but radically improved capsule and amplifier technology has extended this enormously in recent years.

My experience of microphones dates back to the days when electrets were considered vastly inferior relations of true capacitor mics. Fortunately, technology has improved so much over the last couple of decades that the performance of electret capsules is now much more comparable with conventional capacitor designs, even if still not quite being equal to them.

The Making Of AKG

The Austrian Akustische und Kino‑Geräte (Acoustic and Cinematography Equipment) company was formally established in 1947, two years after its founders, Dr. Rudolf Goerike and Ing. Ernst Pless, began constructing and supplying projectors and loudspeaker systems for cinemas at the end of the second World War.

By the early '50s the company, now with five employees and sited in a basement in a suburb of Vienna, was focusing its efforts on high‑quality microphones. In 1953, Dr Goerike patented the moving‑coil transducer with a mass‑loaded diaphragm — a microphone milestone, as it extended frequency response into the bass range, giving a properly balanced frequency response for the first time. The original example of this new design was the D12, still often found in use today.

Development of the condenser microphone followed, with the C12 and C24 large‑diaphragm valve microphones being launched in 1954. These established new standards for professional studio microphones and led to the various C414 models, which have become industry standards. The company also started making headphones at this time and their first, the K10, is also still available today.

By 1965 AKG had stopped making film projectors but were still coming up with revolutionary microphone designs, such as the world's first two‑way dynamic microphones (ie. separate bass and treble transducers, as in the D202 and D222). In the early '70s AKG used their experience of moving‑coil systems to manufacture highly regarded pickup cartridges. However, with the advent of CD forcing the demise of the record player, the company reallocated their resources to the manufacture of transistorised telephone transmitter and receiver capsules instead. This side of the business has continued to grow steadily and is now a very significant part of AKG's output.

The company floated on the Viennese stock exchange in 1984 and their shares performed extremely well for many years. AKG became able to acquire many other companies along the way, including Ursa Major, Orban, dbx Professional Products, BSS Audio, Turbosound, Precision Devices, and the AMEK Technology Group.

However, the global recession in the early '90s impacted badly on AKG and in 1993, a year after Dr Goerike died, they were taken over by Harman International Inc. Today AKG continue to be a market leader in the development and manufacture of a wide range of microphones, headphones and telecommunications products, and have been granted over 1400 international patents to date.

Published April 1999