Integrated Amplifiers

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

ARTICLE INDEX

HK 990 Looking Inside the Unit

Figure 1 shown above is the HK 990 with its top removed. The key subcomponents are identified with notations along the perimeter of the photo. The power amp (to be discussed in Part III of the review) and its associated power supplies monopolize most of the amplifier's footprint.

At the front of the unit is a PC board with the control buttons, the volume control-knob (a rotation-sensing device that sends a digital signal when the knob is turned), and the display panel. The electronics on the board, including a microprocessor, are the man–machine interface (MMI). Upon sensing a change on the panel, such as a depressed button, the PC board converts the action to digital data that guide the internal electronics of the HK 990. In turn, the internal electronics respond with digital data to control the display so you can tell what the unit is doing.

The preamp section has one large board at the base of the unit and four daughter boards (obscured in the photo for lack of resolution) positioned at right angles to the main board. This configuration shoehorns the maximum electronics in the minimum amount of space. Surprisingly, the phono board is as large as the mixed-signal boards. Almost all digital interconnection from the front panel run to the preamp section.

The vertical placement of the daughter boards allows external I/O jacks at the rear to be mounted directly on one of the boards. From the rear of the unit, shown in figure 2 below, the locations of three of the boards are apparent. The phono board has more jacks than anticipated because the analog tape recorder connections are also on the bottom of the phono board.

The main board interconnects the daughter boards. Almost all the interconnections among the boards are digital. A second on-board microprocessor, the digital volume control, and the preamp outs are also on the main preamp board. The second microprocessor coordinates the operation of the digital circuits on the daughter boards as well as interfacing to the front panel.

The on-board position of the preamp outputs puts them in very close proximity to the volume-control circuits. You can see the preamp outputs at the bottom of the rear panel. The preamp output jacks define the position of the horizontal main board. Imagine all the wires in an analog integrated amplifier running between the front and rear panels. This would manifest itself in measurements as degradation in crosstalk or a high-end rolloff.

Those who find wire to make a sonic difference often forget how much of it is running inside a piece of electronics. Worse, each of these wires terminates with connectors that slide over pins on the PC board. Those concerned about the quality of an RCA plug and jack would be disappointed to see these connections. The ability to remove the wires from the PC board is of necessity because they must be removed to service the interconnections and/or allow replacement of a defective PC board. Yards of wires, including long traces on PC boards, are not needed when the MMI is up front and the analog signals as proximate to the rear panel jacks.

The main board also contains the power supply regulators. Multiple regulators isolate the different circuit elements. A few sub-regulators are resident on the daughter boards and power some components. With all the power supply regulation on the preamp boards instead of a separate power supply board, there is little wiring of the regulated supplies to the analog components. In front of the main board is its power transformer, which is independent of the power amps.

The big takeaway is the brevity of the analog and regulated power supply interconnections. In addition the long interconnects and multiple RCA jack – plug connections required if you used separate DAC, phono preamp, or power amp are eliminated. In the HK 990 no analog signal leaves from the area of the main preamp board to keep analog connections short. The sole exceptions are the connections to the power amps and headphone jack.