A lot of very good gear has been manufactured under the American brand Harman/Kardon. Regardless of the period and price range, the company consistently made devices of at least decent, and usually good and very good quality. Probably one of the reasons for this consistency was the fact that almost from the time it was founded (the 1950s) until quite recently (2007), with a short break in the second half of the 1970s, it was run by one man, the co-founder Sidney Harman.
One of the company’s more successful products is undoubtedly the integrated amplifier Harman/Kardon HK6550. Manufactured in the years 1991-1994, this amplifier was an only slightly changed version of its predecessor Harman/Kardon HK6500 (1989-1991), which in turn was developed based on the earlier Harman/Kardon PM650 VXI.
These three models have almost identical functions and their layout (small differences can be found on the back panel), the differences in appearance are purely cosmetic, their architecture is practically the same. The PM650 VXI differs from its successors in the transistors used in the power amplifier section (HK6500 and HK6550 have the same transistors), and video signal switching function with additional sockets on the rear panel. In turn, unlike its predecessors, the HK6550 does not have a speaker impedance switch (4/8 ohms). It still can be used with 4-ohm speakers, you just don’t have to remember to set the switch in the correct position when connecting different speakers.
When it comes to functionality, we have 5 basic line inputs, including 2 tape loops. An interesting addition is a set of in/out sockets marked “External Processor”, which is normally used to connect a graphic equalizer or sound processor. But since these sockets and the switch that activates them are located between the source selector and the rest of the preamplifier, the External Processor input sockets can double as additional line inputs. What’s more, the signal from them will theoretically be slightly better, because, unlike the signal from the remaining inputs, it does not go through the “Source” and “Tape Monitor” switches – and each switch in the signal path can and often does slightly degrade the sound.
There are two phono inputs, for use depending on the type of phono cartridge you have. The built-in, very decent phono stage supports not only the popular MM (moving magnet) cartridges, but, unusually in amplifiers in this price range, also higher-end MC (moving coil) carts.
Two pairs of speakers with an impedance of 4 ohms or higher can be connected to the amplifier; they can be used interchangeably or simultaneously, with the minimum speaker impedance of 8 ohms in the latter case. The amp also has a headphone output.
Tone and sound controls include bass and treble potentiometers (unfortunately without a tone defeat option), low filter (useful if you need to eliminate rumble from a turntable), a mono switch and a loudness switch labelled “Phase Correct Loudness”. The latter filter is quite specific. In most amplifiers, the loudness filter is used to adjust the sound at low volume, simply boost lows (always) and highs (usually). Its purpose is to compensate for the characteristic of human hearing, due to which we hear less low and high frequencies at low volume. Harman’s loudness filter is not designed to compensate for the way we hear, but instead to correct the sound of small speakers with weak bass. In addition to the standard bass boost, this filter adds phase shift in the range above 400 hertz.
Pop the hood and you’ll find surprisingly good components, especially for an amp in this price range. The 13000uF main filter capacitors are particularly impressive considering that its is a 50 WPC amplifier. The amplifier is completely discreet, not a single IC in sight. The main PCB contains the power supply and power amplifier sections. On the right side there is a fairly very extensive phono stage board, moved as far away from the transformer as possible to minimize interference. Behind the heat sink there is a board with input sockets. Preamplifier boards (tone controls and filters) are attached to the front side onf the amp. The volume potentiometer, speaker switch, power switch and headphone jack are mounted on separate, smaller boards. This solution is quite convenient from a servicing point of view, but it requires connecting individual sections with a large number of wires and might contribute to a signal loss. This is one of my two small gripes with the construction of this amplifier.
The second one concerns a weak point in the power supply section. The tone amplifier’s power supply uses resistors, which unfortunately are either less durable than other components, or more sensitive to current fluctuations, and they fail in some (read: quite many) units. The symptoms of their failure include a significant drop in sound volume, distortions even at low volume and lack of response to tone controls. Fortunately, this is a minor failure and repair is quite simple. In addition, many units require a basic inspection immediately after purchase due to intermittent switches (especially the four small ones in the upper part of the front panel), crackling potentiometers and the need to adjust the currents – but this applies to most amplifiers of this age and many younger ones.
And now the most important part: how does it sound? Well, this is the greatest advantage of this amplifier. Its sound is a rare combination of very good features. First of all, the bass: the amplifier is quite generous in this respect, it has a very good amount of bass, including the lowest frequencies, and at the same time bass has a good clarity, control and speed. The midrange is smooth and pleasant, but without sacrificing clarity. It is delivered in just the right amount: neither recessed nor too forward. The highs are similar – detailed, but at the same time quite smooth. This amplifier combines sonic features that are very difficult to combine: abundance of bass with its good control and speed, detail with smoothness of sound; it has just enough energy to make the sound engaging, but not enough to be tiring in the long run. It creates a soundstage with a decent width and good depth and can handle demanding loudspeakers. As expected, the on-board phono stage is very good and will satisfy most vinyl enthusiasts. When it comes to sound, this amplifier is a very good choice for almost everyone – except for people who are looking for a very specific sound, for example with forwards mids, very aggressive, or the opposite – very warm and relaxing sound that will put you to sleep.
We highly recommend Harman/Kardon HK6550. Despite the recent increase in prices, it remains one of the best choices in the EUR 100-200 price range. For this money you get an amplifier that might not charm you with its looks, but it will with its sound, it handles various loudspeakers and music genres well, and has all the functions you need in an integrated amp (except remote control – for that you’d have to look for a specific, rare version HK6550R). Moreover, since it is a popular model, it is quite easy to buy, and if you can’t find it, you can always look for its almost identical (and usually slightly cheaper) predecessor HK6500.
Specifications
Power output (RMS): 50 watts per channel into 8Ω, 70 watts per channel into 4 Ω (stereo)
Power output (music): 75 watts per channel into 8 Ω, 115 watts per channel into 4 Ω (stereo)
Frequency response: od 1 Hz do 150 kHz
Total harmonic distortion: below 0.09% at 8 Ω, below 0.3% at 4 Ω
Intermodulation distortion: below 0.1%
Damping factor: 69
Input sensitivity: 0.12mV (MC), 2.2mV (MM), 135mV (line)
Signal to noise ratio: 76dB (MC), 80dB (MM), 81dB (line)
Bass control: +/-10 dB at 50 Hz
Treble control: +/-10 dB at 10 kHz
Low filter: -3 dB at 15 Hz
Phase Correct Loudness: +6 dB at 50 Hz, phase shift above 400 Hz
Dimensions: 443x134x359mm
Weight: 9.2kg
