Integrated Amplifiers

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

HK 990 Backend Analog Circuitry

The diagram above (Figure 15) shows the remaining circuits that are in place before the preamp output. We have already discussed the interface of the DSP to the DAC. Next, the reconstruction filter must remove the high-frequency folded spectra from the sampling process. Most of the filtering of the folded spectra was performed in the digital domain as was discussed above.

Below (Figure 16) is the simulation of the how second-order analog reconstruction filter in the HK 990. Only residual high frequency clock noise is removed by this filter. The filter is down -0.5db at 45 kHz (approximate maximum inband frequency for 96 kHz sampling). Since it is a low Q 2nd order filter it is down only 20dB at 600 kHz.

The next block in the diagram is the digitally-controlled volume control. Recall the volume knob on the front panel is a component of the MMI and is not a genuine potentiometer. As it is turned, the microprocessor senses movement and sends a formatted digital signal to the digitally-controlled volume control requesting a change in the tap position.

As shown in the diagram, the digital volume control does not connect to the preamp outputs. A buffer isolates the volume control output from the load at the preamp output and power amp inputs. A relay is in the signal path to prevent loud popping sounds that occur as the unit powers up or down from being transmitting to the output. The buffer and relay are also as close to the preamp outputs as possible.