Grzegorz "gsmok" Makarewicz, This email address is being protected from spambots. You need JavaScript enabled to view it.

In my private beauty contest of tube amplifiers that have come into my hands, for years the first place take tube amplifiers produced by Radford (especially the STA-25). Amplifiers with lower ratings are separated from Radford by greatl gap. That was until my personal encounter with Classic 60 amplifier, produced by Audio Research. Although compared to "Radford", "Quad" and other old tube amplifiers it is a real lad (construction was established in 1989), its appearance aroused in me a shiver of excitement. No doubt - second place in the ranking .

Really hard to believe that such a beautiful and classic looking design is a hybrid amplifier .

Basic technical parameters

  • Output power 60W (16Ω loading),
  • Sensitivity 550mV RMS for full output power,
  • Power frequency range (-3dB) 12Hz - 40kHz,
  • Nonlinear distortion for rated power in the range 15Hz - 20KHz below 1%,
  • Input impedance 100KΩ,
  • Signal/noise ratio 84dB,
  • Loading resistancea 4/8/16Ω,
  • Output impedance 0.56Ω (dumping factor about 30),
  • Global Negative Feedback 20dB,
  • Maximum power consumptiony 700W (370W in standby mode),
  • Electron tubes 4 x 6FQ7/6CG7, 8 x 6550,
  • Dimensions 480mm x 180mm x 400mm,
  • Weight 28.64kg,
  • Producer Audio Research Corporation.

Mixed tube-transistor amplifier design reveals its schematic diagram. Please note the unconventional way of connecting the output transformer .

Schematic of the amplifier

Undoubtedly, such an extensive audio track also requires extensive power supply. Here it is.

Schematic of the power supply

Since the amplifier is so beautiful I am forced to limit my comments to the necessary minimum. The next six photographs present the complete (dressed) amplifier before removing the covers. Its dimensions, and (as can be seen with the naked eye) holes in the front panel have been designed for rack mounting. Of course, regular user does not insert it into the rack and treats such as other components of the home audio system.

Fig. 1.

Fig. 2.

By the transparent mesh top cover you can see the interior of the amplifier. In this way the sweet mystery of the interior is partially disclosed before removing the cover . The photo shows three solid transformers located just behind the front panel. They are responsible for the weight of the amplifier and cause the "weight unbalance". Front weighs a lot more than the rest and you need to keep this in mind moving the amplifier. Fortunately, it comes with a sturdy handles.

Fig. 3.

On the back panel unusual speaker terminals are mounted. Despite their unusual design, they are very practical .

Fig. 4.

The following photo shows additional stiffening elements in the form of two angles connecting the front panel to the chassis. In my opinion they did not look too aesthetically and seem to be very delicate. Fortunately, they are not soft and (which is not without significance) also are not visible from the front of the amplifier.

Fig. 5.

Fig. 6.

Before removing the top cover you should read the information label. Certainly, the majority of users ignore the warning contained in it.

Fig. 7.

After unscrewing and deflection of the top cover an impressive set of output tubes reveals (beautiful view ) and electrolytic capacitor with red hoods (this view is not so beautiful ).

Fig. 8.

Two small fans are hidden under the cover. The power cables are connected to the motherboard. Without their separation is not possible to fully disassemble the cover.

Fig. 9.

This is the one of two fans. It is very quiet. It performs two functions: it cools the set of output tubes of one channel   and draws inside the surrounding dust . The solution could be some kind of a filter similar to those applied in the professional measuring equipment (Bruel & Kjaer, Tektronix, etc.), although this would require the user to clean it periodically.

Fig. 10.

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Fig. 40.

At the end a special photograph showing the details of one of the output transformers. That is an impressive volume of the core, ensuring adequate bandwidth could be seen from below, and the earlier photographs. Here I wanted to emphasize the core details. As you can see the side surface is smooth as a baby's skin. That's how it's done in a professional company. I dedicate the photograph below to some domestic producers of output transformers. With their  products you can rake lawns. Fortunately, this negative phenomenon is slowly disappearing and more and more transformers available with us as vividly reminds the one from the photograph.

Fot. 41.

And this beautiful view of output transformer ends the presentation.

Written by Grzegorz "gsmok" Makarewicz,