Upgrading the NJM4560 op amp in the Philips CD753 (Marantz CD5000)

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Paul Hunting

Jul 13, 2020, 10:42:41 AM7/13/20
The NJM4560 is fitted as standard in the CD753. I wanted to see if there was an upgrade op amp for the Philips CD753. I listened and compared six op amps on two modified CD753 players. The op amps compared were: NJM4556, NJM4562, NJM4580, NJM2068, OP275 and LM6172.

The criteria i used is that the upgrade op amp must have a lower noise profile compared to the NJM4560. The noise figure of the NJM4560 is reported as about 10nV/√Hz (see figure in data sheet p3). All the op amps considered here had a lower noise compared to the NJM4560. The NJM2068 had the lowest noise figure of about 3nV/√Hz.

Over the comparison period it became apparent that two factors were important in realising an op amps low noise specification in the CD753. Firstly that the current should be at or below the current figure specified for the NJM4560 - that is about 4.3mA (both channels driven). Secondly the input bias current figure should be as close as possible to the figure stated for the NJM4560 (40nA). I found that the low noise specifications of the trialed op amp were not realised in the CD753, that is noise increased, if the op amps’ current figure and input bias current figure were significantly different compared to the NJM4560 figures.

I thought that the best overall op amp performance in the modified Philips CD753 was the OP275. This op amp has a current of 4mA and an input bias current figure of 100nA. The OP chip noise figure (about 5nV/√Hz) is half that of the NJM4560. The OP753, in my opinion, delivered a detailed and consistent performance in the CD753. Every CD played was musical and detailed. It bested all other chips considered.

The second best performer was the NJM4562: current 3,5mA, and input bias current figure of 100nA. And this chip also had about half (5nV/√Hz) the noise profile compared to the NJM4560. This op amp i believe is more detailed and had better instrument definition than the OP753 however its performance was not consistent that is with some CDs it sounded boring and on others there was a tendency to spotlight instruments. otherwise this op amp performed well in the modded CD753..

There are a few low current op amps out there but most of these either have a matching or higher noise figure compared to the NJM4560 or/and have a very different input bias current figure. Op amp rolling, i believe, is about the gains that can be had from fitting a low noise op amp. further these low noise gains will likely only be realised by fitting an op amp that is compatible to the surrounding circuitry in the CD player. An indication of the metrics of the surrounding circuitry is given by the datasheet of the op amp fitted as standard and that metrics like the input bias current, supply voltage, offset voltage and current etc. need to be noted and compared.

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