AKG C-451 E
A modular condenser that powered itself off two resistors and became a studio staple.
Overview
The C-451 E weighs 100 grams net (125 g per an alternate manual) and measures 18 mm across by 147 mm long. It has an all-metal housing and a 3-pin XLR connector. It's a small, dense, businesslike object with nothing decorative on it. The active diaphragm is approximately 15 mm in diameter, and the operating principle is a pressure gradient receiver with a cardioid polar pattern.
The interesting part of the C-451 E, to the engineers of its day, was how it got its power. DB Magazine (April 1970, p.15) billed the system under the heading "simplicity of simplexing," noting that the microphone could be powered at the cost of one or two precision resistors. No dedicated power supply required. That one design decision is why the C-451 E turned up in so many places where a studio condenser would otherwise have been impractical. The manual lays out the rules: phantom powering per DIN 45596, a positive voltage on the audio lines versus the screen of 12, 24, or 48 volts. At 12 V the standard resistor is 330 ohms drawing up to 10 mA. At 48 V it's 3300 ohms at 2 mA (6 mA max). Minimum current requirement for any AKG CMS microphone is 3 mA. For setups without an input transformer, AKG offered the A 52 powering module. It's a 20 mm diameter, 10 mm deep device that could be mounted almost anywhere, and its a.c. resistance was high enough to guarantee common-mode rejection on the order of 100 dB.
The capsule end is where the C-451 E earns its reputation, and where it demands respect. The frequency response runs 20 to 20,000 Hz within ±1 dB of the standard curve. Sensitivity is 9.5 V/Pa (0.95 mV/µbar, -60.5 dBV on open circuit; 9.0 mV/Pa per an alternate manual). The transformer-balanced nominal impedance is ≤200 ohms with a recommended load of ≥500 ohms (>2,000 ohms per an alternate manual). Maximum SPL for 0.5% THD is 20 Pa - 120 dB SPL into 1000 ohms. Self-noise is low: ≤28 dB weighted per DIN 45 405 (CCIR 468-2), or ≤18 dB-A per DIN 45 412. Hum sensitivity is 5 µV per 5 µT at 50 Hz.
The service documentation is blunt about the capsule. To reach the membrane and electrode, the user presses the microphone against a flat surface, compresses a spring, and removes four screws. The manual warns in bold that the cap must never be released during this job, because the spring will fling it off when the fourth screw comes free. The membrane lies loosely in the cap and its surface must never be touched. The electrode is the same. The insulation strip inside the cap has to be renewed or carefully salvaged on reassembly. This is not a microphone to open casually.
The C-451 E is the version without a bass-cut switch. The related C-451 EB is identical except for a three-position bass-cut: linear down to 5 Hz, a 75 Hz setting with roughly 14 dB/octave slope (-7 dB at 50 Hz), and a 150 Hz setting (-20 dB at 50 Hz). The C-451 EB measures 18 mm × 140 mm and weighs 85 g. AKG also sold the bare C-451 E body without the CK 1 condenser capsule. Its technical data is identical except that it was measured without the capsule fitted. That body is 18 mm × 120 mm and 80 g net. Optional accessories included the SA 18/1 all-metal stand adapter and the W17 A wire mesh windscreen.
In the field, the C-451 E was a working microphone. Recording Engineer (April 1972, p.38) described a session in which a pair of AKG C-451-E/CK-1 cardioid condensers handled off-stage chorus pickup and a vocal booth, while brass was miked with ribbon microphones for a softer character.
By 1979 the C-451 EB preamplifier was still being reviewed alongside far newer designs. Studio Sound (June 1979, p.86) included the AKG CK22/C451EB preamplifier in a seven-microphone survey that also covered the AKG C34, Electrovoice 1777 and RE20, Schoeps CMTS 501U, Sennheiser MZK 431 Profipower, and Shure SM81. Measurements were taken under identical conditions with tone bursts to avoid wall and floor reflections, and polar and frequency responses were plotted anechoically at the Polytechnic of North London.
DB Magazine (May 1984, p.26) returned to the AKG story with a lab tour update, noting the new C 460 B preamplifier designed to work with and improve the existing CMS microphones — equivalent noise level of just 15 dB SPL and a maximum SPL for 0.5% THD of 140 dB, or 150 dB with the 20 dB pre-attenuation engaged. The C-451 E sits at the head of that modular lineage, the microphone that established what the CMS system could do.
Anyone chasing a C-451 E today should expect examples with tired capsules, missing windscreens, and the occasional stripped screw from a previous owner who ignored the spring warning. The all-metal housing survives abuse better than the capsule does. The membrane and electrode are the fragile parts, and replacements are the parts AKG's service documentation explicitly lists as available and everything else as not. The operating temperature range is -20°C to +60°C (-10°C to +60°C per an alternate manual) with permissible humidity of 99% at +20°C and 95% at +60°C.
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