Harman Kardon Pm655 Vxi High Voltage/high Current Integrated Amplifier Review

The sweetness spot in a three-strong series of tardily '80s amps, this high current integrated promised to handle low impedance speakers without breaking a sweat. We listen...

Almost hi-fi enthusiasts know how many watts their amplifier tin produce, simply does that figure tell the whole story? In the early on '80s, Harman Kardon's HCC (Loftier Current Capability) range of integrated amplifiers gave listeners another number to think about, which was how much current an amplifier was able to source.

The PM650 of 1982 reviewed here was ane of a three-part series of integrated amps produced in Nihon. Rated at 50W per channel, the amp offered greater ability and more facilities than the basic 35W PM640 model in the lineup, while for those wanting a little actress, the range-topping PM660 offered 80W per channel and a few more knobs and buttons to dabble with.

Carry That Load
The purpose of the loftier current output capability of the PM650 is difficult to fathom at first. Schoolboy mathematics propose that the superlative current drawn by an 8ohm load at 50W will be three.5A, then why go to the trouble of supplying more than? The answer lies in the truthful nature of loudspeakers, which do not present a load that is equivalent to a perfect 8ohm resistor. The impedance contour of a typical speaker tin vary widely over the sound frequency range, from perchance only a couple of ohms upwards to many tens. The more than exaggerated these deviations are from the loudspeaker'southward nominal impedance the more taxing it is for the amplifier to drive.

Amps similar the PM650 were designed to be as shut as possible to an ideal voltage source, eg, the output voltage is an verbal multiple of the input voltage regardless of the electric current that is fatigued by the loudspeakers. This can clearly but be the case if sufficient electric current is available to satisfy this condition, hence the generous allowances made in the PM650 excursion.

What would the effect of running out of electric current exist? In a moderate case, a not-uniformity of response would issue, the amp existence unable to provide enough drive at a particular frequency (eg, that corresponding to a depression impedance point of the loudspeaker) and elsewhere in the spectrum. In extreme cases distortion, performance of protection circuits or the failure of the amp's output devices might occur, though this is rare in practise.

Heavy Metal
The design of a high current amplifier isn't greatly different to that of a conventional ane at first sight. The showtime requirement is that the output impedance must be depression, simply that is not the complete story. The tiny amplifiers found in the servo systems of CD players too have a very low output impedance by virtue of the negative feedback loops that surround them. But in do this is a theoretical position merely as they cannot supply any real power into the sensitive loads they drive.

As well equally a low output impedance generated by negative feedback, a high current amplifier also requires a high current power supply, output devices that are sturdy and minimum circuit resistance betwixt the output devices and speaker terminals. All these factors were engineered into the PM650, which had a large mains transformer and dual rectifier/reservoir sets – i per channel. This latter arroyo was something of an HK trademark at the time and was also seen in the 'twin powered' HK505 amplifier [HFN Oct 'sixteen]. To proceed the output resistance low one finds thick PCB tracks, heavy wiring and big speaker binding posts within the PM650, along with the absence of the usual speaker protection relay. In its place are ii thermal cutouts, but these are ho-hum interim and may not be fast enough to save your speakers if something goes awry.

Feature Packed
The minor speaker switches seem to exist at odds with HK'south high current principles, but tests undertaken at the time suggested that a remarkable ±39A (peak) might be obtained from the PM650. High current circuitry aside, the PM650 was a typical middleweight amplifier of the immediately pre-CD era. Three inputs and 2 tape loops with cross-dubbing options were provided, the turntable input being switchable betwixt MM and MC cartridges. This facility was especially well engineered, with separate input sockets for the two different types of cartridge and variable capacitance loading for the MM input. No fewer than 30 transistors were employed in the RIAA preamplifier lonely, a far from penny-pinching specification for an integrated amplifier in its class.

Meanwhile, loftier and low cutting filters, a loudness excursion and the power to play in mono or with the two stereo channels reversed were also included in the design, although a well planned fascia meant that the PM650 was still an attractive product that was unproblematic to operate.

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Source: https://www.hifinews.com/content/harman-kardon-pm650

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