CD65II_PSU_small.jpg  Marantz CD65II-SE modifications  CD65II_PSU_small.jpg

In this Marantz player the well-known TDA1541A DAC from Philips is used. This classic multibit converter has a current output, and uses an accurate resistor network for the D/A conversion. A current is driven through this, and the various bits of the digital input signal each switch the current on and off on different branches of the network. This way, each bit provides a piece of the output current. With all bits low the current is zero, with all bits high it is maximum. Hence the output signal is stair-shaped, each of the 16 bits adds approximately 61nA of current, untill the maximum of -4mA is reached. The output voltage must remain at zero volts for best results. Therefore, for the I/V conversion often an opamp is used, whose inverting input forms a virtual ground point.

The manufacturing of an IC like this is quite expensive, because the internal electronics are partly analog. The resistor network must be trimmed on the chip to reach the required accuracy. Also, multiple supply voltages are needed, leading to a more expensive power-supply. So it seems the cost-factor is the main reason these ICs are not manufactured anymore. Anyhow, the sound is unique, just consider the large group of fans of this type of DAC on the internet.

The 'A'-version is an improved type of the TDA1541, with slightly better distortion figures en less power dissipation. But despite that, the IC draws about 90mA from the various supply voltages!

During the modification of the player, the diodes and the electrolytic capacitors in the power supply have been replaced, as well as the opamps and the other parts in the outputfilter. The decoupling around the various ICs has been improved with small inductors and low-ESR capacitors. The ceramic SMD capacitors around the 1541 DAC have been replaced by PPS and MKT versions. The de-emphasis filter and muting transistors have been removed from the output circuit. Near the digital filter (the  SAA7220), an external  clockcircuit has been fitted.

Many thanks to Thorsten Loesch for his article 'Thermionic Valve Analogue Stages for Digital Audio', in which a lot of  attention is given to the technique behind this and other DACs.