About the article

ECC83 (12AX7) Microphone Preamplifier

studio quality with valves

ECC83 (12AX7) Microphone Preamplifier
In this semiconductor age, we find valves being used increasingly often for hi-fi and guitar amplifiers, top-end condenser microphones and studio equipment. This article presents an excellent microphone amplifier with a uniquely attractive sound.Microphone amplifiers must amplify extremely small signals to much higher levels while introducing the least possible amount of additional noise. In principle, it does not matter whether a transistor, operational amplifier or valve us used as the gain element.A signal can be amplified by any desired amount, but the limit is set by the signal-to-noise ratio. If the magnitude of the noise signal is equal to or greater than that of the desired signal, any amplification is pointless. Consequently, microphone amplifiers must be designed to have the lowest possible levels of hum, noise and distortion, since every corruption of the signal originating in the microphone amplifier will be magnified by the following amplifier. Particular attention must therefore be given to the design of the input stage.
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Component list
(metal film, 1% tolerance, 0.7 watts, unless otherwise noted)
R1 = 1M?
R2 = 10k?
R3 = 180k?
R4 =1k?5
R5 =10k
R6 =100?
R7 =1M?
R8 = 1k?5
R9 =1M?
R10 = 1k?5
R11 = 470k?, metal oxide, 2% tolerance, 2W
R12,R13 = 220k?, metal oxide, 2% tolerance, 2W
R14 =4k?7
R15 = see text and Table 2
C1 = 680pF ceramic
C2,C3 = only fitted when oscillation or RF noise is noted (approx. 10-47pF)
C4,C5 = 0.22µF 630V, MKS4, lead pitch 22.5mm
C6,C7 = 10µF 450V, lead pitch 5mm
C8 = 100µF 40V, lead pitch 5mm
C9 = 100pF ceramic
D1,D2,D3 = 110V zener diode, 1.3W
R1 (Ü1) = E-11620
V1 (Rö1) = ECC83S, E83CC, 12AX7, ECC808 (see text)
1 valve socket, ceramic, PCB mount
Kits, special parts and PCBs available from
Experience Electronics, Weststrasse 1, D-89542 Herbrechtingen, Germany
Internet: www.experience-electronics.de, E-Mail: experience.electronics@t-online.de
Tel.: +49 7324 5318, Fax: +49 7324 2553
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