Discrete output stage

Discrete CD output stage

In every CD-player, some form of filtering is present between the D/A-converter and the output connectors. In most cases, it consists of an active 3rd-order low-pass filter that is constructed around one or more opamps, sometimes followed by an output buffer. With Marantz, very often multiple discrete HDAM circuits are used for this. All together, this forms a complex section, so the signal must pass through a significant amount of components before is reaches the output sockets. If the D/A-converter has two symmetrical outputs per channel, then the entire analog section after the DAC can be upgraded in one go with this CD output stage. The analog filter, the opamps and possible buffer can all be replaced by this circuit.

The circuit consists of a passive 3rd-order filter, followed by a discretely built output stage*. This is constructed around just five transistors: at the input a differential amplifier with a current source that sums the two output signals of the DAC, followed by a single-ended buffer in class-A at the output. In the differential amp and output section, either transistors or JFET's can be used. The BC550C and 2SK170 that were used here are pin compatible and fit on the PCB without any alterations. Both versions were built, and sound very good. Needless to say this does depend on the quality of the other components. The current source T3/T4 has to be set to 2mA by means of R9. This can be checked by measuring the voltage across R5 and R6 (4k7): it will be around 4.7V. The current running through the output transistor T5 is 10mA, and depends on the voltage across R6. By altering R11 (1k8), this can be adjusted.

The design is primarily meant for DAC's that have two symmetrical voltage-outputs per channel, like the SM5872B that is used in the CD63/67 series. But with some adaptions, it can be made fit for DAC's with current outputs as well. In the case of a D/A-converter with current-outputs, this output current first has to be transformed into a voltage, before it can be fed to the input of the filter (I/V-conversion). With most converters, the outputs have to be terminated with a low resistance, to cause the least possible amount of voltage swing at the output. The most simple way to achieve this, is by means of a low-ohmic resistor that is connected between the output and ground. The voltage that is obtained this way is often quite low, and in this case it is best to raise the amplification of the differential amplifier by lowering the emitter resistors R7 and R8.

Printed circuit boards of this design can be obtained through this website. See Ray's Audioshop page for details.

Under construction...

Empty home-made PCB	Work in progress...
Black Gate FK's in the power-supply and the output capacitor is an N-type	Passive filter with Holco/PRP resistors and polystyrene capacitors
Shuntreglators in place	Component-side of the PCB
	Solder-side of the PCB