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Blue Notes

AKG Blue Line Microphones

A modular range of microphones covering almost every audio eventuality from broadcast to recording and AV.

Paul White discovers a truly versatile, modular microphone system designed to meet a variety of demands across a whole range of audio disciplines.

That AKG produce some very good microphones can be taken as read, but they also exhibit a flair for visual design and innovation that ensures their products are always noticed. The new Blue Line range combines all these attributes, yet it flies in the face of convention in that it attempts to cover just about every audio engineering application, from live and recorded sound to TV and theatre. Most microphone ranges are based around a single microphone type or application, but the Blue Line range is based on the concept of modularity. Furthermore, according to AKG, the advances pioneered in the Blue Line range will mean the replacement of the popular C451 within the next year.

The common component in the range is the SE300 B, a slim-line, universal powering module equipped with a standard balanced XLR connector at one end and AKG's proprietary bayonet connector at the other. Any one of eight different back-electret capacitor microphone capsules or extension accessories can be plugged into this bayonet fitting, and the connector has been so well thought out that an engineer working in the dark with gloves on would have no trouble assembling or dismantling the system. AKG see this as offering a very real advantage over screw-thread systems, which can often become cross threaded and are difficult to use in adverse conditions. If this seems a minor point, consider the outside broadcast engineer working in failing light with hands numbed by the cold!

Used with any of the capsules, the SE300 B draws less than 2mA from the phantom power source and works effectively with supplies between 9 and 52V. Incorporated into the SE300 B is a switchable 10dB pad and a bass cut filter offering a 12dB/octave cut with a corner frequency of 75Hz; the transformerless circuitry performs the role of impedance converter, producing a consistent 200 ohm output impedance from all the capsules. Each capsule contains its own preamplifier electronics, giving an output impedance at the bayonet connector of several kohms, and all the capsules incorporate gold-flashed, mylar diaphragms ranging in thickness from three to four microns. Because the capsules all use back-electret technology, no polarising voltage is required.

The complete Blue Line range comprises so many parts — which can be be assembled in so many permutations — that the most straightforward approach is to examine the capsules individually.

The Capsules

The CK 91, 92 and 93 are physically similar capsules, measuring just 19 by 52 mm and producing cardioid, omnidirectional and hypercardioid responses respectively. They are all tubular in form and match the diameter of the SE300 B. The front of each capsule is protected by a wire mesh screen fitted inside the slotted outer shell, while the side ports are screened with exceptionally fine metal mesh. Thoughtfully, the polar pattern of the capsule is printed on the side, and this design feature is carried over to the miniature capsules too.

An optional gauze pop shield may be slid over the capsules and is held in place by a rubber O-ring which grips the capsule body. Alternatively, a simple foam shield is provided. All three capsules have published frequency responses extending from 20Hz to 20kHz within +/-2dB, an equivalent noise level of 17dB-A (DIN 45412-A) and a sensitivity of 10mV/Pa.

Slightly longer than the previous capsules (58mm) is the CK94, which produces a classic figure-of-eight pickup pattern, while the CK98 shotgun capsule is a full 260mm long. A foam windshield is provided to fit the CK98 and is recommended for outdoor use. Again, these capsules operate over the full 20Hz to 20kHz frequency range, the CK94 having an equivalent input noise of 22dB-A and the CK98 a figure of 17dB-A.

Completing the range is a series of miniature microphones designed for use in broadcast, theatre, television, conferencing and so on. The CK97-0 has an omni response and is supplied with an integral 3-metre cable terminated in a bayonet adaptor to fit the SE300 B. A variety of mounting accessories is available, and though the small diaphragm diameter inevitably leads to an increase in self-noise of around 6dB, the frequency response is a healthy 20Hz to 18kHz, making the microphone suitable for musical as well as spoken word applications.

The CK97-C is slightly larger, and is a miniature cardioid with a range of mounting options and an integral 3-metre cable. A version of this capsule is also available as the CK97-CVR, which has an integral gooseneck in place of the fixed cable; this terminates in a bayonet adaptor and is suitable for lectern or table use. Both versions exhibit a degree of low frequency roll-off below 150Hz.

"While many of the large-diaphragm mics popular in recording studios are selected on the basis of their specific tonal characters and colourations, the Blue Line range produces a subjectively neutral sound."

The specified noise, sensitivity and bandwidth figures for all the Blue Line capsules are in keeping with other well-designed microphones of their physical size. The smaller capsules are noisier simply because fewer air molecules can interact with the surface of the diaphragm at one time — consider it as random molecular quantising noise if you will! In practice, this shouldn't be a disadvantage, as the smaller capsules are designed for close-up work; there's no point in using an unobtrusive microphone and then placing it ten feet from the action!


The range of accessories available in the Blue Line range is nothing if not extensive. Most would be described as mounting accessories, including both flexible and rigid extensions, hangers and extension cords; particularly useful is the A91 swivel adaptor, which allows the capsule to be turned up to 90 degrees from the SE300 B. There are also battery and mains phantom power supplies, stereo spacing bars, shock mounts, stands and so on.


While many of the large-diaphragm mics popular in recording studios are selected on the basis of their specific tonal characters and colourations, the Blue Line range produces a subjectively neutral sound, and though the individual capsules do exhibit tonal differences, they all create the impression of being fairly true to life. The miniature cardioid capsules have a broad presence peak, which aids intelligibility, and a progressive bass roll-off, which helps keep out unwanted rumble and boom. The larger capsules have a much more subtle high-end hump and the low end is essentially flat, as is the case with the miniature omni capsule.

As they are relatively small microphones, the geometry of the Blue Line range bodes well for a good off-axis frequency response, and this assumption was borne out in practice. While the miniature capsules are less likely to be used for serious recording work — except in an AV situation — their performance doesn't preclude this, though the larger capsules have a better noise performance and audio bandwidth. As well as trials with voice and music, all the capsules were used to pick up ambient outdoor sounds, including birdsong. In this application, the sound remained extremely natural and free from obvious colouration, even when the sound was well off-axis. The shotgun mic was impressive in its ability to gather tricky sounds without spoiling their tonal character, though with very distant, low level sounds, a more specialised, low-noise microphone would be more appropriate. However, for applications such as news gathering, outside broadcast, front of house spot mic and so on, the noise performance and sensitivity is more than adequate.


Even before you plug these mics in, it's obvious that they are beautifully engineered and that their design is the result of much thought. The very completeness of the range is impressive, the only obvious omission being a dual version of the SE300 B, which would have enabled the user to set up an M&S rig very quickly and simply. The styling manages to exude quality while being a little less austere than the majority of microphones, the blue legend and trim line working well against the grey background. I particularly liked the bayonet fittings, which are positive, easy to assemble and promise to be electrically reliable.

By way of audio performance, the different capsule combinations are impressive, given their overall cost and various physical restraints. Noise is never going to be a problem unless you're working with something like distant birdsong, and the impression of tonal neutrality is pretty consistent throughout the range. In designing a microphone range that has something to offer everyone, AKG must count the Blue Line series a success. Microphones of this type seldom achieve cult status — they are more often viewed simply as appropriate tools to do a specific job. In that respect, the Blue Line microphones and accessories provide a wonderfully complete tool kit.

Further Information
AKG Blue Line range: SE300 B £141; CK91 £141; CK92 £141; CK93 £141; CK94 £311.38; CK97-0 £176.25; CK97-C £176.25; CK97CVR £193.88; CK98 £240.88; A91 £141; MK90 Extension Cable Set £182.13; VR91 15" Extension Tube £141; VR92 4ft Extension Tube £282; W95 Wire Mesh Windscreen £88.13. Prices include VAT.

AKG Acoustics Ltd, (Contact Details).

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Revox C278

Recording Musician - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.


Recording Musician - Sep 1992

Donated & scanned by: Mike Gorman

Review by Paul White

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