Harman Kardon PM655Vxi
An integrated amplifier that delivers 120 watts per channel into 4 ohms - and sounds effortless doing it.
Overview
The Harman Kardon PM655Vxi is an integrated amplifier built around high current capability. Many amplifiers of the era were rated into a single impedance and sagged when asked to drive difficult loads. The PM655Vxi is rated at 100 watts per channel into 8 ohms (20 Hz-20 kHz, both channels driven, at less than 0.09% THD) and delivers 120 watts per channel into 4 ohms under the same conditions. The 4-ohm figure is not a footnote.
The bandwidth is wide too. Frequency response runs from 0.5 Hz to 150 kHz (+0/-3 dB), well beyond the audible spectrum. Nobody is supposed to hear 150 kHz. The point is to preserve phase and transient integrity in the frequencies you do hear, so phase shift and group delay within the audio band stay negligible. This was typical of Harman Kardon's engineering approach in this period.
HiFI-Stereo (November 1988, p.80) noted the PM655 Vxi's high current capability enables it to deliver up to 260 watts per channel under certain conditions. That figure reflects the power supply and output stage rather than any continuous rating. The same coverage placed the PM655 Vxi among current-capable integrated amplifiers, alongside contemporary separates and tuners.
Key Features
High Current Capability Design
High current capability is the PM655Vxi's defining trait. The amplifier is built to deliver substantial current into low-impedance loads without strain. The 120-watt rating into 4 ohms is a 20-watt increase over the 8-ohm figure, so output does not collapse under load. Many amplifiers of this class were rated into 8 ohms only, and their output into 4 ohms was often a disappointment. HiFI-Stereo (November 1988, p.80) corroborated this, reporting the amplifier can deliver up to 260 watts per channel thanks to its high current design. That is transient headroom for dynamic peaks, not continuous power.
Twin Power Supply
The PM655Vxi uses a twin power supply configuration, which separates the power delivery for critical sections of the amplifier. This reduces interaction between stages and helps maintain stable voltage under dynamic load. Combined with the dual mono construction, the twin supply keeps crosstalk and interference between channels low.
Dual Mono Construction
In true dual mono construction, each channel has its own power supply, its own ground path, and its own amplification stages. The PM655Vxi's dual mono layout is a serious commitment to channel separation. The signal-to-noise ratio of 95 dB on line inputs reflects it. Crosstalk between channels is minimized, and the amplifier holds up during complex passages with demanding stereo imaging.
Low Negative Feedback Design
The PM655Vxi follows Harman Kardon's low negative feedback approach. Keeping negative feedback to a minimum avoids the transient intermodulation distortion and "harshness" that can plague high-feedback designs. The trade-off is higher distortion figures on paper, which the circuit design manages instead of brute-force feedback correction. The rated THD of less than 0.09% is respectable. The sound is natural and unforced, without the clinical edge some solid-state amplifiers show.
Balanced Differential Input Stage and Darling Transistor
The input stage uses balanced differential amplification, which rejects common-mode noise and improves signal integrity from the first stage. At the maximum signal stage, the PM655Vxi uses a Darling transistor, a configuration that pairs transistors to maximize gain and linearity. These details contribute to the amplifier's ability to resolve fine detail and maintain clarity at both low and high volume levels.
DC Power Amplifier with BTL Configuration
The DC power amplifier design means the PM655Vxi amplifies the full signal without coupling capacitors in the signal path. That removes a source of phase shift and low-frequency roll-off. The BTL (bridged transformerless) configuration lets the amplifier deliver more power by bridging its output stages. The horizontal power amplifier design has output transistors arranged for heat dissipation, which supports the high current capability and keeps thermal behavior stable during extended listening sessions.
Phono Stage Performance
The PM655Vxi includes a phono stage with 2.5 mV sensitivity and a maximum input of 150 mV, which accommodates most moving magnet cartridges. The input impedance of 47 kOhm is standard for MM cartridges, and the signal-to-noise ratio of 80 dB on the phono input gives a quiet background for record playback. Line inputs offer 150 mV sensitivity with an 8 V maximum. The damping factor of greater than 80 indicates strong control over speaker drivers, contributing to tight, articulate bass.
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