AKG C-451
A modular condenser system that let engineers build the microphone they needed, one screw-on capsule at a time.
Overview
The C-451 was a platform more than a single microphone. AKG built it around screw-on capsules, and by 1974 the company was advertising it as the only modular condenser microphone system in the audio world. It accepted 27 different back-up components, ran on phantom power, AC, or battery supply, and could be set up for close-miked piano or for shotgun work, as DB Magazine (May 1974, p.15) reported. That is the company's own advertising talking, but it shows how the C-451 was positioned: not as a single-purpose studio tool, but as a system an engineer could grow into.
The capsule interface is the key. The C-451 E is the amplifier body, and the CK-series capsules screw onto it. The CK 1 cardioid was the common pairing, and the complete assembly of body plus capsule was sold as the C 451 E comb. AKG's technical documentation for that combination lists an operating principle of pressure gradient receiver, an active diaphragm diameter of approximately 15 mm, and a frequency response of 20 to 20,000 Hz within ±1 dB of the standard curve. Sensitivity at 1000 Hz is rated at 9.5 V/Pa, which works out to 0.95 mV/µbar or −60.5 dBV on open circuit. Nominal impedance is 200 ohms or less, transformer balanced, and AKG recommended a load of at least 500 ohms. The mic weighs 100 grams net and measures 18 mm in diameter by 147 mm long. The housing is all-metal, and the connector is a standard 3-pin XLR.
The self-noise figures explain a lot about why this mic ended up on so many piano and drum sessions. AKG rates weighted sound pressure level at 28 dB or less according to DIN 45 405 (the CCIR 468-2 weighting), and 18 dB-A or less according to DIN 45 412. That is not quiet by modern large-diaphragm standards, but for a small-diaphragm condenser of the period it was competitive. The sound was bright and present, with a cardioid pattern that held together well off-axis, and that was what engineers reached for when they needed a piano to cut through a dense mix.
The C-451 E on its own, without a capsule, is a different animal. AKG's documentation lists its frequency response as 5 to 30,000 Hz, dimensions of 18 mm by 120 mm, and a net weight of 80 grams. That extended top end comes from the amplifier, while the capsule shapes the final response. The C 451 E comb, with the CK 1 attached, gives up that 30 kHz extension in exchange for the ±1 dB flatness across the audible band that the cardioid capsule provides.
Then there is the C 451 EB. It is identical to the C 451 E except for a built-in three-position bass-cut switch. The first position is linear, with response extending down to 5 Hz. The second engages a bass cut starting at 75 Hz with a slope of about 14 dB per octave, giving −7 dB at 50 Hz. The third starts the cut at 150 Hz, producing −20 dB at 50 Hz. That is not a subtle tone control. It is a problem-solving tool. In a room where a grand piano is bleeding into the drum kit, or where a vocal booth is picking up low-frequency rumble from the floor, the EB lets the engineer roll off the bottom end at the microphone rather than in the console. The EB is slightly longer than the E, 140 mm versus 120 mm, and slightly heavier at 85 grams, both because of the switch and its circuitry.
The modular concept went beyond capsules. AKG's system included shotgun and short shotgun condenser components that screwed onto the same amplifier body, and all of them worked with phantom powering, AC, or battery supply. That is why the C-451 shows up in so many different contexts in the period literature. Engineers who owned one body could reconfigure it for a session without buying a second microphone.
In the mid-1970s, Recording Engineer documented Ron Capone at Stax in Memphis placing an AKG C-451 in the tuning-board holes of Isaac Hayes' Yamaha grand piano, paired with a Neumann KM-84 on the low end. Miking down into the holes rather than over the strings was about isolation as much as tone. Capone told the magazine that with the piano lid up on the short peg, he could put Hayes in the studio without a baffle and still get no leakage from the drums. The two microphones separated a heavy bass sound from a bright high sound.
At Larrabee, the same magazine reported, grand pianos were generally miked with two AKG C-451-Es placed 12 to 14 inches above the strings and aimed at the hammers, with the lid slightly open. The engineers there chose the C-451 for its bright response and good cardioid pattern, and they recorded the piano in stereo so that high and low ends could be processed independently at mixdown. The C-451 was not the only condenser in the room, since Sony C-37As covered snare and KM-84s handled overheads, but it was the piano mic.
The same pattern shows up on stage. In a documented theatrical production, piano was picked up by an AKG C-451-E with a CK-2 omni capsule inside a heavily draped lid, while overhead drums used two C-451-Es with CK-1 cardioid capsules in stereo. Off-stage chorus pickup used a pair of C-451-E/CK-1s to create what the production team described as a large-choir sound. That is three different jobs in the same show, with three different capsules.
By the late 1990s, the C-451 had become a reference point. EQ Magazine, profiling the AKG C60 in its December 1999 issue, described that older tube microphone as the mic that begat the popular C451. So the C-451 was not AKG's first condenser. The C60 was a vacuum tube condenser built from 1960 through 1969 with an original price of approximately $400, which puts it about a decade ahead of the modular system.
When AKG revisited the design in the early 2000s, the company was careful not to call the new microphone a reissue. Mix Magazine, reviewing the C-451 B in April 2002, noted that AKG's R&D department gathered 12 vintage CK-1 capsules from around the world to measure and determine what made the originals special. The magazine described the new mic as an entire rethinking of the classic C-451 E/CK-1 capsule combination rather than a reproduction. AKG sampled a dozen original capsules before designing the replacement.
The reviews that followed were positive enough that AKG ran them as advertising. Mix Magazine, in May 2002 and again in September 2002, published endorsements from engineers comparing the new C-451 B to the originals. One engineer, Chris Dietze of Clear Sound, said the new mic sounded closer to his C 451 EB + CK 1 microphones than anything else he had heard, and that he would both specify and purchase it. The C 451 EB with a CK 1 capsule was the benchmark for judging a new microphone, nearly three decades after the original system was introduced.
The documentation consulted here gives no confirmed production run. The DB Magazine advertisement from May 1974 shows the system was on the market by then, and the C-451 B reviews from 2002 show the original was still remembered and referenced. But the exact years of manufacture, the original list price, and the total number of units produced are not documented in the available sources. The mic was made long enough to be advertised in 1974 and used as a benchmark in 2002, but the boundaries of that run are not on record.
The record does support the mic's continued use. It was used on sessions at Stax and Larrabee, deployed in theatrical productions with multiple capsule configurations, and AKG spent real R&D money trying to recreate it.
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