This interesting preamplifier was published in the magazine Elektor 7-72 july/august 1982
Super low noise preamp
Preamplifiers for magnetic cartridges suffer from one major problem: Their own noise. This additional noise is produced mainly by the irregular current flow in the PN junction of the input transistor. The cause of this 'irregularity' is due to manufacturing tolerances. Some manufacturers, especially Japanese have designed extremely low noise transistors, but unfortunately these components are very hard to find and rather expensive.
For these reasons this circuit uses the physical law that voltages of non-correlated noise sources that are connected in parallel add geometrically, thus reducing the over-all noise of the parallel circuit. This magnetic preamp contains 8 transistors that are connected in parallel thus lowering the noise by factor 8^0.5 which is 2.82 or 9 dB.
The completely symmetrical circuit and the class A mode output transistor stage, allows low distortion factors that cannot be reached by any integrated circuit. Another notable feature is the differential amplifier circuit. Besides other advantages, this circuit is able to suppress spurious signals produced by the supply voltage
Signal to noise ratio >86dB
