Impedance matching
Left and right inputs performed well on a 47kohm resistor (R2 and R7) that acts as a load resistance for the turntable.

Amplification and RIAA correction
Amplification and RIAA are provided by the NE5532 dual operational amplifier, known for low noise, and related components. There are other integrated circuits perform better than NE5532. Correction frequency following the curve whose shape depends on the value of the resistors and capacitors that are used around the op amp: R3 R5 and C2-C4 for the left channel (upper assembly), R8-R10 and C6-C8 for the right channel (the bottom of the assembly). Amplification (gain) is defined by the voltage on the resistor value.

The supply
The power supply must be symmetrical + / 9V to + /-15V (max + /-18V). Local decoupling is provided by the RC network consisting of R11, C7 and C8 positive for power lines, and R12 C9, C10 to the negative line. For this coupling to be effective, four decoupling capacitors should be placed closest to the pin of the power IC NE5532 (terminals 4 and 8).

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As you can see, this scheme is very simple. The use of a dual operational amplifier type NE5532 is a large part in simplicity, although it was quite possible to simplify even more by using a single transistor.

Polarization of the microphone
The electret microphone is fed Due to the resistor R1, 4.7 K. The value of this resistor should be adjusted (increased) if the power is chosen higher than the one proposed here (9V). The capacitor C1 prevents this voltage reaches the microphone in the first op amp, the reference voltage can be shifted and a problem of distortion of the high levels (asymmetrical clipping).

Small electret microphone gain
The op amp provides a first amplification of 10 (20 dB), this value is equal to the ratio of resistors R3 / R2. The second op amp is mounted in exactly the same way as the first, and also provides a gain of 10, and the ratio R5 / R4. The overall gain is equal to 100 (40 dB). If you want to be able to vary the gain, simply replace the resistor R5 of 100K with a 220K potentiometer in series with a resistance of 10K. The overall gain in this way can vary from 10 to 200.

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