Integrated Amplifiers

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

ARTICLE INDEX

HK 990 Improving Performance by Operating a Pair of DACS in a Balanced Configuration

The HK 990 (and HD990) handles the main channel DSP/DAC interface in a novel manner. Figure 14 above provides the block diagram details in the yellow box. One channel is converted by two DACs in the single Analog Devices AD1955 package (on the left side of the block diagram). Normally the AD1955 is a stereo DAC, but in the HK 990 it will convert only one channel. This is accomplished in the fully balanced configuration. In a balanced circuit, one input is the inversion of the other. Instead of sending stereo data to one of the AS1955 the DSP sends the same LPCM data both in phase and out of phase. Again a digital wire in the diagram above to represent 3 wires.

After each DAC in the AD1955 completes its analog to digital conversion, there are two balanced analog outputs. In the analog stages that follow the DACs (not shown in the block diagram), the balanced signals are subtracted from each other to form a single-ended output. In the process of converting the balanced analog signal to the single-ended output, some distortion produced by each DAC in the AD1955 package is partially canceled. Using two DACs for one channel also reduces the noise level at the DAC's output.

The bit-equivalent signal-to-noise ratio (SNR) of the AD1955 in the mono mode is 18.6 bits equivalent bits worst case (A-Weighted). The bit-equivalent distortion exceeds the chip's stereo specification of 16.7 bits worst-case at 1kHz at full-scale. The data Harman Labs sent me exceeded this at the preamplifier output with all the additional analog electronics adding noise to the measurement.

The balanced DAC configuration of the HK 990 is atypical for an AVR. If used at all, the balanced configuration would apply only to stereo channels.