Audio Source Enhancer

Vinyl or CD: which has the better sound? It’s  a question still hotly debated among audiophiles everywhere. We will try to shed a little light on what lies behind the question and  look at a simple circuit that can significantly  enhance the sound from a CD player.  Sometimes, on first hearing a new low- to  mid-range CD player, the sound is not altogether  convincing  when  compared  to  a  record  player.  It  is  worth  looking  at  the  recording and replay processes as a whole  for  both  CDs  and  records  to  see  why  this  might be. Assuming that we start from the  same source, or master recording, of a given  piece of music, the differences are broadly as  follows.
Circuit diagram :
Records and CDs use very different recording  technologies.  For  records  the  signal  first  undergoes  pre - emphasis  similar  to  that  used  in  FM  radio,  where  the  higher  frequency  components  of  the  signal  are  amplified. The resulting signal is cut into  the lacquer master disc that will be used  for pressing. Unlike CD manufacture, this is  an entirely analogue process, and it introduces a phase shift into the signal. To compensate for the pre-emphasis the preamplifier in a record player includes a de-emphasis (or ‘RIAA’) filter which attenuates the  higher  frequency  components.  The  purpose  of  pre - emphasis  is  to  improve  the  overall signal-to-noise ratio of the signal as  played back, reducing hiss and crackle. De-emphasis introduces further phase shifts,  and as a result the final signal is rather different from that produced by a CD player.  The processing involved in CD manufacture  and playback can be entirely digital (in the  case of ‘DDD’ recordings) and phase errors  are reduced practically to zero.
The circuit shown here uses a quad opamp  (two opamps per channel) to produce ‘record-like’ phase shifts. In the author’s experience  low- and mid-range CD players tend to have greatly attenuated output at higher frequencies, and the circuit therefore also offers the  facility to boost these components to taste.  The value of capacitors C8 and C14 may be  anywhere between 100 pF and 10 nF according to the frequency response desired. At the low-frequency end the response is  more than adequate, thanks to the large coupling capacitors used. The circuit also functions as a buffer or impedance converter,  which can help to reduce the effect of cable  capacitances. With CD players that have an output impedance of 1 kΩ or more the difference between cheap cables and more expensive low-capacitance cables can be noticeable. This circuit has an output impedance of  just 100 Ω and so cheaper cables should normally be more than adequate.
The circuit can of course also be used with  other digital audio sources such as minidisc  players, hard disk recorders, DAB tuners, dig-ital terrestrial and satellite television receivers and so on. The supply voltage can be any-where from 10 V to 30 V. It will often be possible to take power from the CD player’s own supply; if not, a separate  AC power adaptor can be used. The output signal for each channel is inverted (i.e., is subjected to a  180 degree phase shift) by the second opamp (IC1.B and IC1.D). This  does not affect the operation of the circuit. By changing the value of  feedback resistors R4 (for IC1.B) and R12 (for IC1.D) the overall gain of  the circuit can be adjusted so that the output level matches that of  other components in the audio system.
Author :Thorsten Steurich  – Copyright : Elektor

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