Integrated Amplifiers

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


HK 990 Volume Control

The volume control is a good starting point because it is the one spot in the HK 990 where Otala design concepts are violated. Two stages of operational amplifiers are present when an analog source, including phono, is selected.

The Analog Device AD825 opamp is used. This is not a discrete circuit, which is surprising. A discrete circuit allows the amplifier stage to be optimized to all Otala requirements. A signal coming from digital media often sees many opamps in the recording studio, but this is not true for analog signals entering the HK 990 or signals from the phono preamp. It is especially surprising given that a discrete circuit is at the output of the HD 990 CD player.

The AD825 has some unusual characteristics that are more consistent with Otala design rules. That said, the opamp designers were probably unaware of these rules and coincidentally came upon them during optimization of the opamp for applications outside audio. The AD825 in the unity-gain configuration has a return-loop gain of 2000 (66dB) from 20Hz to 10 KHz. It then declines at 6dB per octave. A typical opamp has an open-loop gain of 100,000 to 1 million and will start rolling off around 10 – 100Hz. The AD825 has a high slew rate (115V/us), wide bandwidth (34MHz as a unity gain buffer), and sizeable open-loop linear input range. Indeed, Walt Jung references an Otala Audio Engineering Society (AES) paper in his Electronic Design article of  December 1, 1994 on the AD825. Still, it is not fully complementary like other Harman discrete circuits and methods that that prevent oscillations when the feedback loop is closed are different.

The downsides of the AD825 include higher noise levels than the typical audio opamp and some discrete designs.

The digitally-controlled volume IC in the HK 990 is a JRC NJW1159 comprised of only silicon switches and a resister ladder. There is no operational amplifier on the chip. With a specified maximum supply voltage of maximum +/- 7V (limited by breakdown voltages of the switches in the IC fabrication process used), the JRC NJW1159 requires a pair of dedicated sub-voltage regulators.

The 50k ohm input impedance of the JRC NJW1159 is typical of an analog control. However, MOS transistors are sensitive to electrostatic discharge and stress when the input voltage exceeds the +/-7V power supplies. The AD825 unity gain buffer in the signal path protects the JRC part. A DC blocking capacitor is at the input of the buffer. Another is at the input of the volume control. A second AD825 with a gain of 2.4 (8dB) at the output of the volume control prevents it from being loaded by the preamp output jacks. I calculated the input impedance of the power amp at a low 10k ohm, meaning the second AD825 is required for the direct power amp connection. The power amp has an above-average gain of 32dB to compensate for the low gain in the volume control section (the line stage if it was an independent preamp).