Harman Kardon PM660 Integrated Amplifier Review

Heading the company’s 600 Series audio-component line is the PM660 integrated amplifier. It is rated to deliver 80 watts per channel to 8-ohm loads from 20 to 20,000 Hz with no more than 0.02 per cent total harmonic distortion. The front-panel configuration and operating controls of the PM660 are, for the most part, conventional. The tone, balance, and volume controls are knob-operated, as are the mode and tape out selectors (mode selects normal or re-versed-channel stereo and mono operation). The PM660 offers independent program selection for listening and tape recording, and its tape out knob has positions for off (which isolates the tape outputs), source (the usual arrangement, in which the program being heard is also fed to the tape decks), and tuner (for recording from a tuner while listening to phono or another source). It also has two dubbing positions for copying tapes from either of two tape decks to the other.

The program is selected by means of five pushbuttons at the upper right of the panel. Three mechanically interlocking buttons control aux, tuner, and phono sources. Two others, which can be engaged individually or together, are the tape monitor switches for the two tape decks. Across the upper part of the panel is a narrow “dial window” behind whose tinted glass are a number of illuminated identifiers for the input selections as well as the power-on indication. When the amplifier is first turned on, wait appears in yellow, and after a few seconds it is replaced by a red ON when the internal delay circuit connects the speaker outputs to the amplifiers. A large power-switch button is at the left of the window.

Most of the other operating controls of the H/K PM660 are “push-on, push-off” buttons. They include speaker selectors for two pairs of speaker outputs, a tone-defeat control to bypass the tone-control circuits, infrasonic and high-cut filters, loudness compensation, audio mute (a 20-dB gain reduction), and a main direct button that bypasses all signal-modifying controls and circuits except for the volume control and reduces the gain by 7 dB. This is a suggested manner of using the PM660 as a basic power amplifier.

The Harman Kardon PM660 has two phono inputs, selectable with a second pushbutton switch. The capacitance of the phi input can be adjusted by a small knob on the panel to add 50, 100, or 200 picofarads to the amplifier’s “normal” (but unspecified) input capacitance. The PH2 input has a fixed termination, but it can be set for either a moving-magnet (MM) or moving-coil (MC) cartridge by another small button beneath the input selector. In the MC mode the signal passes through a head amplifier for low-output MC cartridges.

The PM660 is perhaps unique among modern amplifiers in its use of discrete transistor circuits rather than integrated circuits throughout. The advantage claimed for this is a reduction in what Harman Kardon calls “thermal distortion,” presumably caused by heat transfer between portions of an integrated-circuit chip. The rear of the PM660 presents a conventional appearance, including insulated binding posts that grip the ends of the stripped speaker wires, and three a.c. outlets, two of which are switched.

Laboratory Measurements

Since the PM660 is relatively large and massive for its power rating, it became only moderately warm above the output-transistor heat sinks during the preconditioning period and the high-power testing that followed. With both channels driving 8-ohm loads at 1,000 Hz, the outputs clipped at 100 watts per channel for a clipping-headroom rating of 1 dB. A high current-output capability in an amplifier implies an ability to deliver an increased power output as the load impedance is reduced, and the PM660 proved to be superb in that respect. Driving 4-ohm loads, the clipping output was 156 watts per channel, and into 2 ohms the output reached a remarkable 200 watts (this was not actually the clipping power, since the amplifier’s internal line fuse blew at that point). When we used the tone-burst test signal of the dynamic-power measurement, the power outputs were predictably higher. Into 8 ohms, the short-term output was 111 watts (for a dynamic-headroom rating of 1.42 dB), and into 4 and 2 ohms the respective maximum power outputs were 200 and 336 watts per channel. The PM660’s qualifications as a “fast” amplifier were confirmed by its rise time of 1.5 microseconds (rated 1.8) and slew factor of greater than 25. Its performance was unaffected when we drove a standard IHF reactive load, and with a complex reactive simulated speaker load a 10-kHz square wave showed only a moderate amount of ringing at about 50 kHz.

The 1,000-Hz harmonic distortion, driving 8-ohm loads with both channels, was in the range of 0.007 per cent (at 80 watts) to a maximum of 0.016 per cent at 10 watts. With 4- and 2-ohm loads, the distortion curve was similar, with slightly higher readings. The highest distortion was in the range of 0.022 to 0.125 per cent when driving 2-ohm loads at power levels up to 180 watts.

The distortion of the PM660 varied only slightly over the audio-frequency range. Driving 8-ohm loads, the distortion ranged from about 0.007 per cent to 0.02 per cent for outputs from 8 to 80 watts and frequencies from 20 to 20,000 Hz. We also measured the high-frequency intermodulation (IM) distortion with input signals (at 18 and 19 kHz) whose combined peak amplitude was equal to that of an 80-watt sine wave. The measured second-order (difference-tone) product at 1,000 Hz was at our test-equipment residual of —96 dB, and the third-order products at 17 and 20 kHz were at a — 87-dB level. To check the contribution, if any, of the low-level stages to the total distortion, we measured distortion with and without the main direct switch engaged. We found no difference.

The amplifier sensitivity for a reference output of 1 watt was 14 millivolts (mV) at the high-level inputs and 0.25 mV at the phono (MM) input. Through the MC input, the sensitivity was 8 microvolts. The A-weighted signal-to-noise ratio was 77 to 78 dB for both high-level and phono (MM) inputs. It could not be measured for the MC input because of external hum pickup.

The MM phono input overloaded at a very high 265 mV at 1,000 Hz and at levels equivalent to 245 and 241 mV at 20 and 20,000 Hz, respectively. The MC input overload was 6.8 mV at 1,000 Hz. The MM input impedance was 44,000 ohms in parallel with a “normal” capacitance of 140 picofarads (pF) for PH2 and 180 pF for PH1. The front-panel switch added the indicated amount of capacitance. The RIAA equalization was accurate within ±0.5 dB from 20 to 20,000 Hz. It was affected only slightly by the inductance of a typical moving-magnet cartridge, which increased the output at 20,000 Hz by about 1 dB compared to the midrange level.

The tone controls had conventional properties, with the bass-turnover frequency shifting from about 50 Hz to about 400 Hz as the control was varied through its range, and the family of treble curves hinged at about 2,000 Hz. The treble control had very little effect through the first half of its rotation from the center. The loudness compensation boosted both low and high frequencies at reduced volume settings, but the amount of boost was quite moderate and the sound was never muddied by the circuit. The filters had desirable 12-dB-per-octave slopes with cutoff frequencies of about 25 and 3,500 Hz.

Comment

There is little we can say about the sound of the PM660 other than to confirm that it is as clean and quiet as any amplifier we know of. Whether its all-discrete circuitry or wide-band, anti-TIM design contributed in any audible way to the final sound we cannot say. However, we are not at all ambivalent about the “high-current capability” of the PM660 and what it does for the sound. Although the amplifier must be rated at 80 watts or so according to current FTC regulations, its dynamic-power output (or even medium-duration output) into loads of less than 8 ohms can match or surpass that of many amplifiers with much higher ratings. Our speaker tests have shown that it is the rule, rather than the exception, for a speaker’s impedance to drop well below its nominal rated value at certain frequencies. If the amplifier cannot maintain its undistorted output into such a low impedance, it is not going to do justice to the sound, regardless of its “official” power rating. This can be heard quite clearly when one “pushes” the PM660 to very high listening levels, especially when driving more than one pair of speakers. It is without doubt a very “muscular” 80-watt amplifier, and we were never able to force it into audible compression or any lack of “punch” or clarity.

Aside from its sound, the Harman Kardon PM660 is an impressively smooth and unobtrusive amplifier. For example, after the turn-on time delay, instead of abruptly connecting the speakers with the “click” of a relay, the 660 introduces the sound with a smooth transition from silence that takes no more than a fraction of a second, adding a sense of ease and precision to the total effect of using the amplifier. This is a small thing, to be sure, but there is much more to good product design than mere numerical specifications, and Harman Kardon has done itself proud with this amplifier by paying as much attention to the details as to the basics.

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