Integrated Amplifiers

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


The Analog Electronics

In the halcyon tube days of the 1950's, Harman Kardon's top of the line Citation products were among the best and continue to sell for top dollar on Ebay. The Citation subsidiary was the first to produce state-of-the-art consumer solid-state equipment, but the pricing relative to tube equipment was severe. Harman kept refining the Citation solid-state line. Those with long memories will recall the very high-end Citation XX power amp and XXp preamp of 1980. Overseeing the designs was then VP of engineering, Matti Otala, who joined in the late 1970's.

The HK 990 amplifier is a direct descendent of the circuit topologies that Harman developed under Matti Otala. The exotic Otala design rules were intended to improve the sound quality of the unit. A search of Otala's name in the Audio Engineering Society (AES) library finds 24 papers, starting with a seminal work on transient intermediation distortion in 1974 and running through input current requirements for high-quality loudspeakers (1983). Some of these papers were controversial. Otala is perhaps best known for advocating for the use of low levels of return-loop gain that are invariant across the audio band (a so-called low feedback design).

Bob Cordell's new text, Designing the Audio Power Amplifier, examines some of the issues that engineers face with Otala's concepts. The Cordell text is a great read for those interested in the design of a modern amplifier with very different return-loop gain characteristics. The Cordell text requires some background in analog electronics. A book of similar spirit is Doug Self's Audio Power Amplifier Design Handbook.

If you are attracted to the sound of an amplifier designed with Otala rules, then he may have been on to something. I will not opine, whether an Otala design is "closer to the truth" as Robert Lawrence Kuhn would put it for matters of cosmic significance.

I do not have access to schematics of the Citation XX units, which had limited production runs and prices far above Harman's norm. Some popularly-priced Harman units, circa 1981, signaled Otala's influence, but it was not until 1987 that the Citation name was resurrected for the 21 preamp and 22 power amp (200 watts/ch into 8 ohms). The 21 and 22 had the full Otala treatment, though by then he had moved to Nokia to design cell phones. The 1993 price of the pair was $1700. The Citation name was dropped in 1993, but similar products were manufactured through 1997. The trail goes cold as Harman USA went all-in for Home Theater. Harman stereo components continued to evolve in the EU. The HK 990 is the first high-end stereo unit to make it back on this side of the Pond. The HK 990 appears to be the best implementation of the Otala design rules of anything that sold in quantity in the U.S.

The only issue I identified on the analog side is a high 330pf capacitor at the input of the moving magnet phono stage. When combined with the cable capacitance of your turntable, the total capacitance may fall outside the range recommended for the cartridge, with the exception of Grado cartridges. Most who purchase a $2600 integrated amplifier will likely be using a moving coil cartridge with most high end moving magnets out of production. That MC stage on the HK 990 has no interface problems.

There is a red glow at the top of the HK 990. This is not due to the presence of vacuum tubes inside the unit. The glow is from the phono board, which uses LEDs as part of the circuits that set bias currents.

Performance with High Resolution Files

High-resolution digital downloads are becoming more common, so any equipment you purchase going forward should have the capabilities to deal with them. From measurements provided by Harman it was clear that high resolution LPCMs streaming data from a file server made it from the SPDIF inputs to the DAC. This held true even if the room correction and bass management were active. If the high resolution files were recorded with wide bandwidth the Harman measurements show the HK 990 will pass spectral information to 45kHz.

The HK 990 lacks a USB port. Converters to SPDIF are relatively cheap for 96kHz sampling rates. 192kHz conversion is more problematic. The HK 990 does not support direct streaming of MP3, WAV, or FLAC. Almost no two channel DACs provide for this. The conversion must be done by the device storing the high resolution files. At some point, it is necessary to draw the line between the long-term standard like LPCM over SPDIF and what is constantly evolving and device specific (Apple Air Play). Replacing a $2600 unit every couple of years is not economical. Instead, use a lower cost unit to deal with all the permutations for file streaming and storage of today and next year.

Dual-Domain Tape Recorder Outputs and Tape Monitor Functionality

A dual-domain tape monitor should be a must-have for those seriously rooted in recording. With a dual- domain configuration, both analog and digital inputs appear at the analog tape output jack Analog and digital inputs also appear on the unique to the HK 990 record digital output The source on the record outputs can be independent of the program playing on the speakers.

More on the tape recorder interfaces will be found later in the review. One issue I do want to mention now is that mixing analog and digital connections may result in a recorder self loop and high-level oscillation that could damage your speakers. Properly connected this cannot occur.

Overview: A Three-Part Structure

My review has three segments. The first segment is the introduction that you just read, which highlights the special aspects of the HK 990, and this is a nice precursor to the rest of the segment, which examines the performance of the EzSet/EQ II room-correction system.

Using block diagrams, the second segment considers the challenges of adding digital inputs and digital signal processing to a stereo integrated amplifier. This segment is a good read for those interested in the internal operation of an AVR at the block level. Since the HK is a stereo unit, the blocks are less complex. Once you have a handle of the operations at a block level, then the transition from two to seven, or even eleven, channels becomes more straightforward. The second segment also comments on the quality of the data converters in the HK 990 and additional performance data when the unit processes a high resolution file.

The last segment turns to the design and construction of the HK 990 at the circuit level. The analysis starts with the analog circuits in the direct mode, which is equivalent to an analog integrated amplifier. The designs of the power amp and phono stage are in the spotlight, and we also look at the tape recorder output and monitor function in more detail.