Free circuit schematics - wiparat.com

An ultra low noise audio preamplifier particularly suited to microphone preamplification including balanced microphones. The IC features wide bandwidth, low distortion only 0.007% at a gain of 100, and very low noise only 1.3nV/Hz for source impedance s up to 4kohm. Link
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Power Amplifier OTL 30W by IC TDA2030
2 Watt stereo amplifier with LM1877
Linear Amplifier [...]

A high performance op-amp that sets a new standard of excellence in noise performance only 0.9nV/Hz with low source resistances. Total harmonic distortion is less than 0.01%. The op-amp is suitable for use in high quality audio, low noise frequency synthesizers, infrared detectors etc. particularly where the source resistance is under 1Kohm. Link
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Microphone Preamplifier [...]

P1______________2K2 Linear Potentiometer
R1,R2,R3______100K 1/4W Resistors
R4______________8K2 1/4W Resistor
R5_____________68R 1/4W Resistor
R6______________6K8 1/4W Resistor
R7,R8___________1K 1/4W Resistors
R9____________150R 1/4W Resistor
C1______________1µF 63V Polyester Capacitor
C2,C3,C4______100µF 25V Electrolytic Capacitors
C5_____________22µF 25V Electrolytic Capacitor
Q1___________BC560C 45V 100mA Low noise High gain PNP Transistor
Q2___________BC550C 45V 100mA [...]