Sunday, 28 August 2011

Marantz CD72

The Marantz CD72 is the top model in the CDx2 line.  This product is from the era where Marantz was owned by Philips, and hence it uses many Philips made components.  All of the players in this line were of the same configuration; they used a CDM-4/19 laser mechanism, the TDA8808 / TDA8809 servo chipset, the SAA7310 decoder IC, the SM5840 digital filter IC and the SAA7350 noise shaper and DAC IC.  Unlike previous digital filter ICs used by Marantz, the SM5840 does not output a S/P DIF signal.  A separate IC, the PCF3523, is used to format the digital output in this player.

I've never been a huge fan of the early DACs using the 'bitstream' concept.  The SAA7350 used in the CD72 was Philips' second try at making a bitstream DAC IC for audio, the first being the SAA732x family.  Because of the DAC it used I had ignored the CD72 I owned as something not worth spending any effort on.  However, I recently had that opinion reversed by a player using the same DAC IC; a Meridian 602 transport and 606 DAC pair.  This pair was the top of what Meridian offered in the mid '90s, and actually sounded very good.  This prompted me to take a second look at the Marantz CD72.

Given that it seemed that the SAA7350 isn't as deficient as I thought, I looked at where the rest of the player could be improved.  As always, it's not just about which ICs a players is using, it's how they are implemented.  I noted the following ways in which the CD72 could be improved:

  • Replacement of the signal filter capacitors.  Like any DAC, the SAA7350 analog output is filtered.  The passives used in this player were picked for economy rather than performance, with ceramic capacitors being used for the smaller values and polyester film capacitors where larger values were needed.  The ceramic capacitors are highly undesirable, they have characteristics which make them totally unsuitable for use in analog signal filtering.  Fortunately all of the smaller capacitors used in this player are through hole with a 5mm pitch, so finding mechanically compatible replacements was easy.  For replacements I used mainly Wima FKP2 series capacitors.  These are a stacked polypropylene film and foil type and give very high performance in this application.

  • Addition of  film decoupling capacitors to the opamp supplies.  Strangely the power supplies to the output stage opamps weren't decoupled with film capacitors, only electrolytic capacitors.  Decoupling an opamp's power supplies with film capacitors as well as electrolytic types is not only best practice, it's essential for stable operation if your using an opamp that is even mildly fast.  There was no existing place to mount the film capactors, so I bought some 1206 size surface mount types and mounted the between the pads of the existing electrolytic capacitors underneath the board.  I used AVX CB series capacitors here, a compact PPS (polyphenylene sulphide) film type.  These have good characteristics, but be careful when using them, they have much lower voltage ratings than most other types of film capacitor.
  • Addition of a low noise clock.  Please look at my last post for a detailed explanation of why I added a low noise clock to this player.  In addition to reasons generic to all CD players, those which use single bit DACs tend to be more sensitive to jitter.

  • I also replaced the output stage opamps and all the player's electrolytic capacitors.  Again, please see my previous posts for the reasoning behind these additions.
 Overall, these modifications pushed this from being a mediocre player relegated to the scrap heap into a pretty good performer.  The harshness that people hate these players for is now gone.  I haven't done everything I could to this player, and I might take it further in the future.  Some things that spring to mind are:

  • Better power supplies (at present there is a single rail for each voltage, share between the whole player).
  • A discrete output stage.
If it's lucky, this player may get them and be seen here again.