Portable Phono Preamplifier


9V battery operated add-on unit for moving magnet pick-ups

Circuit diagram:

GIF

Parts:

R1,R3____________2K2 1/4W Resistors
R2,R4,R5_______100K  1/4W Resistors
R6______________39K  1/4W Resistor
R7_______________3K9 1/4W Resistor
R8_____________390K  1/4W Resistor
R9______________33K  1/4W Resistor
R10,R13________220R  1/4W Resistors
R11______________6K8 1/4W Resistor (Ωptional, see text)
R12_____________75K  1/4W Resistor (or two 150K resistors wired in parallel)
R14_____________10K  1/4W Resistor

C1_____________100pF  63V Polystyrene or Ceramic Capacitor
C2,C6____________1µF  63V Polyester Capacitors
C3,C4___________47µF  25V Electrolytic Capacitors
C5______________10nF  63V Polyester Capacitor 5% tolerance or better
C7_____________100nF  63V Polyester Capacitor
C8_____________100µF  25V Electrolytic Capacitor
C9_______________1nF  63V Polyester Capacitor 5% tolerance or better
C10______________4µ7  25V Electrolytic Capacitor
C11____________220µF  25V Electrolytic Capacitor
C12___________1000µF  25V Electrolytic Capacitor

IC1__________LS4558   Dual High Performance Ωp-Amp
IC2___________TL062   Dual BIFET Ωp-Amp
IC3___________78L15   15V 100mA Regulator IC

D1______________LED  small dimensions, low current (Ωptional, see text)
D2___________1N4002  200V 1A Diode

SW1____________SPDT  Toggle or Slide Switch
SW2____________SPST  Toggle or Slide Switch

J1,J2___________RCA audio input sockets
J3______________Mini DC Power Socket

B1_______________9V  PP3 Battery

Clip for PP3 Battery

Circuit description:

This unit was designed for amateurs owning an old collection of vinyl recordings and wanting to have it digitally remastered into a personal computer.
Ωr sometimes they would listen to an old, invaluable, LP collection through their modern High Fidelity chain, usually lacking a phono preamp stage: this is a circuit capable of satisfying their needs.
The unit is powered by a 9V battery, allowing a quick and easy connection to all high-level input preamplifier stages and the very low current drawing of the circuit will guarantee a long battery life.

Despite the low voltage supply, circuit performance with moving magnet pick-ups 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 in which the RIAA equalization network was split in two halves: an input stage implementing the bass-boost part of the RIAA equalization curve wired in a series feedback configuration and a second stage, implementing the treble-cut part of the curve by means of a second op-amp wired in the shunt feedback configuration.

Battery operation also minimizes the possibility of hum pick-up, a potential danger always present in high gain, high sensitivity phono preamps.
As the total current drawing of the circuit is about 1.2mA when powered by a 9V battery, the use of a pilot lamp LED will be the major cause of current consumption. Therefore, a very small, low current red LED is recommended for this purpose. Using a 6.8K limiting resistor as specified for R11, the LED will draw an additional current of 1mA: it will not shine brightly, but can be still easily seen.
A typical, carbon-zinc, 9V battery will allow the circuit to operate for about 180 hours, whereas an alkaline battery will last for more than 250 hours. If the LED is omitted, durations will be almost doubled.
In any case, for those whishing to spare the battery, the circuit can also be powered by a common external power supply adaptor, rated at about 12-15V dc, 10mA or higher. In this case, the circuit input overload capability will increase further.

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: R4, R5, R10 and R11, C4, C7, C8, C11 and C12, D1, D2, IC3, SW1, SW2, J3 and B1.
  • IC1 and IC2 are dual Ωp-Amps, therefore the second half of these devices will be used for the Right channel.
  • If you intend to use exclusively the battery supply, IC3, D2, C11, C12, SW2 and J3 must be omitted and SW1 can be substituted by a SPST Toggle or Slide Switch.
  • Wanting to power the circuit by the mains only, omit B1 and SW1, then hard-wire the junction of R10 and R11 to the output pin of IC3.
  • Pilot lamp D1 and its current limiting resistor R11 are optional.
  • Please note that the circuit requires about 15 seconds to become fully operative after power-on.

Technical data @ 9V supply:

Sensitivity @ 1KHz: 4.5mV RMS input for 200mV RMS output
Max. input voltage @ 100Hz: 14mV RMS
Max. input voltage @ 1KHz: 56mV RMS
Max. input voltage @ 10KHz: 134mV RMS
Maximum undistorted output voltage: 2.5V RMS
Frequency response @ 200mV RMS output: flat from 30Hz to 23KHz; -0.5dB @ 20Hz
Total harmonic distortion @ 1KHz and up to 2.4V RMS output: 0.0035%
Total harmonic distortion @10KHz and up to 1.2V RMS output: 0.02%
Total current drawing: 1.2mA (excluding the LED)

Technical data @ 15V supply:

Sensitivity @ 1KHz: 4.5mV RMS input for 200mV RMS output
Max. input voltage @ 100Hz: 26mV RMS
Max. input voltage @ 1KHz: 106mV RMS
Max. input voltage @ 10KHz: 233mV RMS
Maximum undistorted output voltage: 4.6V RMS
Frequency response @ 200mV RMS output: flat from 30Hz to 23KHz; -0.5dB @ 20Hz
Total harmonic distortion @ 1KHz and up to 4.4V RMS output: 0.0025%
Total harmonic distortion @10KHz and up to 2.1V RMS output: 0.015%
Total current drawing: 1.3mA (excluding the LED)


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