Integrated Amplifiers

Harman Kardon HK 990 Stereo Integrated Amplifier with Digital Room Correction and Dual Subwoofer Bass Management – Part III


HK 990 Analog Circuitry Connected to the DACs

The analog circuitry in this stage is a typical design and does not reflect any influence of Otala so I will not spend much time discussing it.

The Analog Devices AD1955 has a current source output. No on chip opamps are present.

Analog Devices OP275 and TI OPA2134 are used. The characteristics of these opamps are more conventional than those of the AD825. I suspect the higher power supply rejection, lower noise and lower input capacitance of these parts may have played a role in their selection. The absence of discrete circuitry is going to disappoint purists wedded to Otala design.

A DC servo circuit senses the DC offset voltage at the output of the DAC signal chain and introduces an offset current at the DAC to correct this (recall the DAC has a current output). The topology eliminates all coupling and DC blocking capacitors from the signal chain This is the first time I have seen a DC servo integrated in the digital-to-analog conversion block.

The output of the analog electronics connected to the DAC goes directly to the volume control bypassing the buffer in the direct path. In total three active opamp stages are present before the signal enters the power amp. One DC blocking capacitor is in the signal path. From the perspective of an engineer who does not slavishly adhere to Otala design rules, the execution of the direct analog and DAC paths is extremely well done.