USB-Powered Phono Preamplifier

5V PC-operated unit for moving magnet pick-ups

Shunt feedback RIAA equalization

Circuit diagram:

USB-Powered Phono Preamplifier


R1______________47K  1/4W Resistor
R2_______________1K  1/4W Resistor
R3,R4,R11______100K  1/4W Resistors
R5,R6,R8________10K  1/4W Resistors
R7_____________120K  1/4W Resistor
R9______________10R  1/4W Resistor
R10____________470R  1/4W Resistor

C1_____________470nF  63V Polyester Capacitor
C2,C3___________47µF  25V Electrolytic Capacitors
C4_____________100nF  63V Polyester Capacitor
C5____________2200µF  16 or 25V Electrolytic Capacitor
C6,C9____________2µ2  25V Electrolytic Capacitors
C7______________22nF  63V Polyester Capacitor
C8_______________8n2  63V Polyester Capacitor (See Notes)

IC1___________LM833   Low noise Dual Op-Amp (See Text)

D1______________LED small dimensions preferable 

J1______________RCA audio input socket
J2____________3.5mm Stereo Jack socket
J3____________4-pin USB connector (See Notes)

Circuit description:

A simple way to remaster vinyl recordings into digital files through a PC is to use this moving magnet cartridge phono preamplifier.
Its power supply is taken directly from one of the USB ports of the computer and its output is carried to the analogue audio input of the PC, usually equipped with a 3.5mm mini jack socket, by a suitable cable.

Despite the low voltage supply (5V), the circuit performance is quite good, featuring high input overload capability, very low distortion and accurate reproduction of the RIAA equalization curve, thanks to a two-stage op-amp circuitry.
The first stage is a linear amplifier having a gain of about 11 times. The second stage implements the RIAA equalization by means of a second op-amp wired in a shunt feedback configuration.

The power supply smoothing capacitor C5 is generously dimensioned (2200µF) in order to eliminate any source of noise coming from the computer supply rails.

Please note that the use of LM833 ICs is mandatory in this circuit: all similar devices like NE5532 or LS4558 will perform far less well at 5V supply.


Technical data (5V supply):

Sensitivity @ 1KHz: 13mV RMS input for 200mV RMS output

Max. input voltage @ 100Hz: 18.5mV RMS

Max. input voltage @ 1KHz: 85mV RMS

Max. input voltage @ 10KHz: 125mV RMS

Maximum undistorted output voltage: 1.27V RMS

Frequency response @ 200mV RMS output: -0.4dB from 40Hz to 20KHz; -1.1dB @ 20Hz

Total harmonic distortion @ 1KHz and up to 1.27V RMS output: 0.0035%

Total harmonic distortion @10KHz and up to 1.27V RMS output: 0.02%