Integrated Amplifiers

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

ARTICLE INDEX

HK 990 - Problems Identified in the Measurement of EzSet/EQ II

I made measurements of the HK 990's operation over several months in two rooms. The rooms were equipped with two brands of full-range and two brands of satellite speakers. One subwoofer was deployed. Speakers were placed in different positions to test the responsiveness of the EQ. All speaker positions were plausible for pleasure listening in the home, although the listening seat position might require some adjustment. I placed the microphone at the adjusted position. The room's acoustic response was captured with the Acoustisoft RPlusD measurement system (www.acoustisoft.com). Having used Acoustisoft products for more than ten years, I have confidence in their accuracy, especially when averaging large data sets. My microphone is from IBF-Acoustic and it is recalibrated to a reference microphone every few years.

Three problems were identified with the HK 990 room-correction system:

(1) The level of boost below the port frequency of the speaker might exceed 15dB in the case of a subwoofer and 6dB in the case of full-range speakers. While the frequency response of the speaker appeared to be extended in acoustic response curves by this boost, the signal was distorted when listening with single tone SPL at 85 dB. This was especially apparent for the subwoofer.

The bass boost was evident under most test conditions, though there were a couple of one-offs where the boost moderated or even was a cut. In several instances, there was a repeatability issue likely related to the ambient low-frequency noise level of the room. I have not observed this repeatability problem with other EQ systems. The HK 990 produces a single, very short, chirp. Competitive systems have longer chirps or MLS noise. The test signals are often repeated in competitive implementations to average-out low-frequency noise.

(2) The subwoofer level, as calculated by the HK 990, was 6 – 8 dB low and required manual correction. Also, the crossover frequency selected by the unit was far too low. Manual settings appeared to be accurate with respect to the actual crossover frequency in my measurements.

(3) The EQ2 mode, appears from the measurements to allow a max EQ of approximately 2 kHz, does not work correctly. At the approximate 2 kHz transition point, the EQ does not return to 0dB insertion loss; instead, it holds at the transition point amplitude of the uncorrected response. This behavior raises or lowers the acoustic response above approximately 2 kHz and creates an unnatural result.

I examine how the HK 990 performs as it is shipped to the customer today. The developer of the EQ system in the HK 990 reviewed my measurements, and reported the problems found are real. On a hopeful note, the developer indicated the problems can be corrected with software. It is unclear if, or when a software update may be made available.