Circuit diagram:
Parts:
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.
Notes:
- The circuit board should be preferably enclosed into a small metal case.
- The circuit diagram shows the Left channel only and the power supply.
- Some parts are in common to both channels and must not be doubled. These parts are: R3, R4, R9 and R10, C3, C4, and C5, D1, J2 and J3.
- A 8n2 value for the Polyester Capacitor C8 can be difficult to locate. The problem can be solved by wiring two capacitors in parallel (6n8 + 1n5).
- Even the USB connector could be difficult to locate. In the prototype the connector has been obtained by dismantling a "dead" USB flash drive, by sawing the plastic side for a length of about 10mm from the metal part and then removing the plastic scraps, in order to solder easily the two outer pins (see J3 connections in the Circuit Diagram).
- The sensitivity of the circuit can be increased by reducing the value of R2. Due to the characteristics of modern PC audio inputs, this change is usually not necessary.
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%
