Integrated Amplifiers

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

ARTICLE INDEX

HK 990 Digital to Analog Conversion (DAC)

The block diagram of the DSP board is repeated as Figure 13 above with the Subwoofer DAC highlighted in the yellow box. All digital-to-analog converters are driven by the DSP. It is typical to place the DACs on the same board as the DSP since a significant amount of wiring runs between the two (a single wire is used in the diagram above to represent 3 wires).

The HK 990 can deal with two independent subwoofers. The DSP connection to the stereo subwoofer channel is typical of a DAC connection in an AVR. In the HK 990, the stereo DAC for the subwoofer is the Wolfson XMB8740. The Wolfson XWM8740 is below the DSP in the previous figure.

In an AVR, the number of DACs connected to the DSP depends on the number of main and subwoofer channels offered. The plethora of DACs complicates the block diagram, but nothing fundamentally changes. Typically, in a 7.1 channel AVR, eight channels must be converted with four stereo DACS.

The Wolfson XMB8740 DAC is above average. The bit-equivalent signal-to-noise ratio (SNR) is 18 bits worst case (A-Weighted). The bit-equivalent distortion is 15.5 bits worst case at 1kHz at full-scale. This is more than adequate for the subwoofer channels exceeding the performance of DACs for the main channels in most AVRs.

Some may find it strange that each data converter is sourced from a different vendor (Cirrus, AKM, Wolfson and Analog Devices). The DSP is from Texas Instruments, who also manufactures audio DACs. In general, large electronics enterprises (not just in the audio space) do this to ensure no single IC company commands significant pricing leverage and, at the extreme, ensures no vendor is indispensible. This makes life difficult for smaller companies who have no pricing power given their small order quantities.