Minimalistic Phono Preamp for LP Remastering

Circuit | Details| Components | Characteristics | Remarks | Links | Index


Making digital copies from LP/Single records is a pretentious hobby. When results are intended to reach some kind of CD-quality level, a dedicated postprocessing, editing, decrackling and denoising procedure is mandatory. All these efforts may be more than worthwhile on some rare or frequently used records that are virtually no longer available on other media.
How do I actually connect the turntable to the PC's soundcard?
Almost none of the high-grade record players were equipped with a built-in preamplifier. Their MM/MC pickup normally delivers some Millivolts, while most soundcards expect a regular LINE-level of about 0.75...1.5 Volts. For serious remastering, a phono-preamplifier is needed anyway to boost-up and frequency-compensate the turntables tiny signal.
My version of a phono preamp is powered directly by the 5V-terminals from the PC's gameport, not only for convenience, but also bypassing all possible grounding problems. A smooth frequency compensation is certainly applied by circuit design.


that's it

  • HiFi-stereo;  simplified frequency compensation to RIAA/DIN45547 directives 

  • Based on inexpensive standard components

  • No external battery required
    (powered by the PC)

  • Project cost: < 20 EUR

  • Further requirements:
    Pentium PC (>100 MHz) with Soundcard; WAV-recording and remastering software;
    medium- or high-class turntable
    with MM/MC pickup


      Circuit Diagram

      preamp circuit scheme



      Details

      Heart of the project is IC1, a low-noise double operational amplifier with optimized audio characteristics even at low voltage supply (normally used in CD-player output stages). In here, it is wired as a noninverting amplifier with a boosting factor higher than 1000x times, performed in only one stage.
      The feedback network, consisting of R2/C4 and R3/C4 causes a frequency dependent rising attenuation with good convergence to the RIAA specified equalization curve.

      Those 5 Volts, coming from the PC's Gameport, are likely contaminated with lower frequency grumbling and higher frequency spikes that could not easily being filtered sufficiently by a simple capacitor or R/C combination. Only an active decoupler (T1-R1-C1) was able to provide clean and stable 4.4 Volts supply for this touchy amplifier.
      Besides, normal current of the whole device is just about 5mA.
      Needless to say, this circuit cannot be operated in a plastic case- because of its high sensitivity, it has to be protected from any EM-interference by a grounded metal enclosure, as seen on the picture (else you like that permanent bass rumbling of 50/60 Hz in the back...)

      The circuit plan only shows one of two stereo channels for briefness. Note, that most of the passive components are doubled for stereo operation. The R/C-network components C4/C5 should be at least 2% of precision - alternatively you could match nearly identical values with a R/C-meter or wheatstone for symmetry.
      After all, the amplifier's operation is not that critical, as soon as there is a proper grounding and shielding conditions. It's absolutely no problem to operate this on a piece of punched paper... Sorry, there is no PCB layout available yet.

      Components


      PC-powered Phono-Preamp (C) 2003 by JT
      (parts indicated with * are double for left/right channel)

      R1
      		2k2
      R2*
      		1M
      R3*
      		100k
      R4*, R5*
      		150k
      R6*
      		1k
      R7*
      		560
      P1*
      		50k lin
      C1
      		1000µ / 10V electrolyte
      C2
      		1500µ / 10V
      C3
      		100n (ceramic)
      C4*
      		4n7 2% (MKC)
      C5*
      		820p 2% (FKC)
      C6*
      		4µ7 / 16V
      C7*
      		220n /63V (FKC/MKC)
      C8*
      		1n ceramic
      C8*
      		1n ceramic
      C9*
      		47µF / 50V unipolar ^ 47µF / 16 V bipolar
      IC1 a/b
      		LM1458 (NE4558, SA4558, JRC4558)
      T1
      		BC337
      Other Material
      		Cable; PCB; Gameport-connector D-SUB 15 pin (male);
      3.5mm stereo jack (male);
      fuse ~63mA/T;
      Chassis: metal case recommended, with built-in connectors




      Remarks

      Turntable

      For remastering purposes, one definitely needs a turntable which is in pretty good shape. If you are a professional, check the physical properties of the player, like speed adjustment, stability, attenuation; replace exhausted parts (transmission belts, saphire) if necessary. A high grade MM/MC (moving magnet, moving coil) pickup is absolutely recommended!  
      Do not even think about that ridiculous turntable-imitation on top of your sistaz' compact system!!!!! Its plastic third class mechanics and crystal pickup may not satisfy the minimum requirements for a serious digital remastering source...

      Hardware

      You will also need a Computer featured with newer generation CPU. For pure recording, any Pentium machine with a good soundcard, running from 100 MHz up, will do as well ;-). If you intend to do much postprocessing, a 300 MHz may be necessary to speed up these computer-bound jobs.  

      Software

      There is a wide tapestry on digital recording and remastering tools for different OS available on the Net; some pretty useful programs are still available as free- or shareware.

      Preamplifier

      And, last but not least, the tiny tiny signal of the record player's pickup (Millivolts) must be amplified to a LINE-IN sufficient level (up to 1 Vpp) for the PC's soundcard.  For best results, the signal path from turntable to the soundcards analog-digital converters should be as short as possible to minimize external distortion and noise effects. Additionally, a dedicated turntable preamp has to perform a RIAA-compliant equalization on the frequency spectrum. Theoretically (only!) this equalization could be done in a pure digital way on the recorded WAV-files - but this will cause severe distortions from the compareably high levelled input spectrum of higher frequencies, causing aliasing and other unwanted effects in the soundcard's analog-digital converters.
      Better provide the soundcard-ADCs with a natural frequency spectrum. This does not only seem the best strategy from a technical point of view, but also simplifies afterprocessing of the recorded WAV files, since there is possibly no further equalization run necessarily. (Still the efforts of decrackling, denoising and edition remains time-consuming enough...)


      Characteristics

      Channel separation
      (one input disconnected)
      		> 70 dB(@1kHz)
      Channel disparity< 5 %
      (matched components for L/R channel)
      Amplificationmax. 1000 max. (~20 Hz)
      S/N-R (dynamic range)
      		>75 dB
      Frequency Response
      referred to 1 kHz as 0dB
      sine waveforms, 1mV input,
      tested with 10k output impedance      		20 Hz = +15 dB
      1000 Hz = 0 dB
      15.000 Hz = -10 dB
      20.000 Hz = -14 dB
      Distortions
      		estimated: less than 1% over the audible spectrum 


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      last revision on this document: 1/2004

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