Integrated Amplifiers

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

ARTICLE INDEX

HK 990 Power Amplifier

The power amplifier is full complementary from input to output. There are eighteen transistors just to implement the voltage gain section excluding the class AB bias stage. This complexity ensures the open-loop distortion is low so only a small, constant amount of feedback, as dictated by Otala, is required around the complete amplifier from 20Hz to 20 kHz.

The differential pair at the front of the amplifier is biased by current sources and a buffer stage isolates the first gain stage from the second. The differential pair and second gain stages are both cascaded. Both voltage gain stages have local feedback (emitter degeneration). The four circuit techniques linearize the open-loop distortion of the amplifier and keep the return-loop gain to Otala's desired minimum level. Just for reference, some AVRs over $1000 do a complete power amp with eight transistors including the AB bias and current gain stages.

Otala and other researchers showed much more current is required to drive a speaker than a resistor. In the HK 990, the current gain block is a triple Darlington terminating in five paralleled 15 amp continuous ON Semiconductor power transistors (MJL3281A and MJL1302A complementary pair) connected from one supply rail and the speaker terminals. These devices have 260V breakdown voltages.

Counting output transistors at the output of an amplifier is a futile exercise. You need to refer to the data sheet to find the short term and steady-state current each transistor can source or sink safely. The frequency at which the current gain goes to 1 (the point at which we could replace the transistor with a wire) should be near 30MHz. Current gain in the audio band should be greater than 20 when sourcing the maximum steady state current. Open-loop distortion under full load depends on the process technology. Typical specs sheets for an output transistor run more than four pages. Paralleling output devices has the disadvantage that the load the pre-driver sees is more difficult to deal with.

The HK 990 is dual mono down to its two transformers, a feature found on many older Citation products. All the metal in the transformers and output-stage heat sink literally weigh down this forty-five pound unit.

The HK 990 power amp uses separate transformer winding for the voltage and current gain stages. +/- 80V for the voltage stages and +/-60V for the output stages, as shown in the diagram above. This approach prevents the power supplies for the voltage stage from being modulated as the current stages send significant current to the speaker. A higher supply voltage for the voltage gain stage is rarely attempted because under a fault condition (shorted speaker terminals, for example) the number of pathways through which the transistors can be damaged multiplies. In a traditional amplifier with a high return-loop gain, the amplifier is relatively insensitive to modulation of the power supply (power supply rejection ratio). With the lower return-loop gain required by Otala, the modulation results in distortion.

Since spec sheets make a big deal of the size of the capacitor on the unregulated rails, I can report each cap on the +/-60V supply is 13600uF, slightly less than reported on the HK spec sheet. The size of these capacitors is important at low frequencies (20Hz) where the power supply is required to source or sink significant current in one direction as the sign wave remains at its maximum or minimum voltage level for a long period relative to the power supply refresh rate (120Hz). This is why THD goes up at low frequencies in some amplifiers when driving low impedance loads at the maximum output voltage swing.

The +/- 80V rails have 1000uF capacitors on the unregulated rails. This capacitor can be much smaller since the current drain on the 80V supplies from the voltage gain stages is smaller than what is flowing on the +/-60V rails driving the speaker.

The HK 990 spec sheet gives no FTC power rating into 4 ohms. The back panel of the HK 990 has a 1000 Watt rating for total power consumption from the AC line. Obviously, some of the power heats the amplifier owing to the efficiency of a class AB amp, but the construction points to the unit's capability of providing almost 300 average Watts per channel (RMS power does not exist except in the mind of some marketing people. The engineers should get the blame for not proof reading the material. HK avoids this error.) into 4 ohms under FTC test conditions across the full frequency band. Why Harman does not supply such a key spec is a mystery. The mystery deepens because fans are mounted to the large heat sinks to aid in passing the FTC preconditioning test.

Harman Kardon was the first company to adopt the Audio Graph Power Cube measurement system. The unit tests for stability into inductance and capacitive loads (+/-30 degrees, +/-60 degrees) when the amplifier is driven to full power into loads as low as 1 ohm (magnitude of the load impedance). The test signal is 20 periods at 1kHz, with the power reported at a THD of 1% I do not have results for the HK990.

www.audiograph.se

The Audio Critic had access to a Power Cube measurement system. Harman products always did well while many other higher-priced units failed. That said, some amplifiers with traditional circuit topologies also did well. Class D amplifiers tend to create poor Power Cubes.

The idle current of the output stage is very high. The unit gets hot with no input signal, so it is advisable not to rest another component on the HK 990. The high idle current is an attempt to reduce crossover distortion with the low return-loop gain specified by Otala. Doug Self has shown crossover distortion can be made extremely small at low idle currents.

Not a single bypass or blocking capacitor is found in the power amp. A DC Servo circuit is designed to prevent a DC offset voltage from appearing at the speaker terminals. A DC servo provides a compensating voltage at the power amplifier's input to remove the DC offset at the output. The servo circuit is designed to only respond to subsonic frequencies and DC. The use of a DC servo to eliminate capacitors is not a universally accepted method to improve sound quality. Some argue the additional active circuitry for the servo is more audible than a well-chosen capacitor.

Be careful with the power amplifier inputs. If the input has significant DC, the amplifier will go into protection (note the DC over-current sensor in the figure. The DC over-current detector also activates if the speaker terminals are shorted or an internal component fails in the amplifier.). An amplifier with a DC blocking cap at the input would remove the DC.

New to the HK 990 is the ThermalTrak class AB bias stage. Normally the bias circuit has a diode placed on the heat sinks so temperature of the output devices can be sensed. ON Semiconductors place a diode on the same die as the output device (NJL3281D and NJL1302D) to improve thermal tracking (please refer to the figure above). The thermal time constant associated with the heat sink is eliminated. ON semiconductor has shown ThermalTrack reduces distortion.

Harman has a odd specification in their literature: 200 amp instantaneous current. At first glance, it appears to be a typo. Just considering the primary capacitor size and power supply rail, the capacitor would be discharged in 4msec by my calculations. This spec turns out to be an old measurement from the Audio Graph Power Cube test system mentioned above. The amplifier is connected to an 0.1 ohm resistive load and driven with a 10kHz (100usec) square wave. Audio Graph looks to have deleted the made in the current system. I am hard pressed to see the value of the specification now, but some readers with long memories are going to post me that The Audio Critic did report it when I was Technical Editor.