mgw@cs.brown.edu (Markus G. Wloka) (02/11/91)
I remember reading on the net that the "The Audio Amateur" had a feature last year on building a pretty good analog stage for an oversampling CD player. I would like to hear from persons who have read the article or build the circuit. Is the design technically sound? Is the circuit easy to build, AND DEBUG? I don't have a spectrum analyzer. Does Old Colony Sound Labs sell a kit, and is it affordable? Reasons for modifying the player: My player is a Sony C7 ES carousel player, with 4X oversampling. The ES series is the Premium grade of Sony CD players. The player does not come with schematics, naturally, so that the customers will not notice that the difference between the premium C7 and the pedestrian C700 is a digital output (20 cent parts cost) and chrome plastic feet. The analog stage consists of : DAC out direct into a unity gain buffer, 22uf coupling cap, buffered filter (~50K Hz), 22uf coupling cap, [emphasis circuit], RC network (~50K Hz). I remember reading about a paper by Lipshitz et al. that such a design is both retarded and typical of cheap consumer CD players. The whole point of having an alias filter is to remove the high frequency noise before it reaches any amplifier stage. I am also not happy with having two amp stages, and two electrolytic coupling caps in the signal path. The op amps in the analog stage are 8 pin single-in-line chips. The designers must have been on drugs, otherwise there is no explanation of why they did not use 'normal' op amps with 4 pins on each side. The pin layout is the same as any old 741, of course. I soldered a wirewrap socket into the circuit board that converts from 8 pin single to 4 pin dual, and have PMI op249 chips in the player now. The idiot who drew the schematics switched pins 6,7, as revealed by some very strange distortion and a subsequent comparision with the circuit board. Other mods are not possible since the whole analog stage very cramped for space and located under the transformer. Smart location. There is no space for bypassing the coupling caps, let alone replacing them by polycarbonate caps. This is why I am now thinking of building a simple analog stage, instead of trying to fiddle with the components on the circuit board. - Markus G. Wloka mgw@cs.brown.edu Dept. of CS, Box 1910 mgw@browncs.bitnet Brown University, 401-863-7668 Providence, RI 02912