Audio Research Classic 60 Tube Amplifier

Grzegorz "gsmok" Makarewicz, (2018)

In the ranking of the beauties of tube amplifiers that have fallen into my hands for years, the leaders are power amplifiers manufactured long ago by Radford (in particular the STA-25 model). Amplifiers from further places were separated from Radford by a real abyss. That was until my personal third-degree encounter with the Classic 60 amplifier, manufactured by Audio Research. Although compared to "Radfords", "Quads" and other tube oldies it is a real youth (the design saw the light of day in 1989), its appearance gave me a shiver of excitement. Second place in the ranking is like nothing else .

And to think that such a beautiful and classic looking amplifier is a hybrid design. Not nice .

Basic technical parameters of the amplifier:

• Output power 60W at 16Ω load,
• Sensitivity 550mV RMS for full output power,
• Power band (-3dB) 12Hz - 40kHz,
• Nonlinear distortion at full power in the 15Hz - 20KHz band less than 1%,
• Input impedance 100KΩ,
• Signal to noise ratio 84dB,
• Load resistance 4/8/16Ω,
• Output impedance 0.56Ω (damping factor about 30),
• Global feedback 20dB,
• Maximum power consumption 700W (370W in idle state),
• Electron tubes used 4 x 6FQ7/6CG7, 8 x 6550,
• Dimensions 480mm x 180mm x 400mm,
• Weight 28.64kg,
• Manufacturer: Audio Research Corporation.

The mixed, tube-transistor construction of the amplifier is revealed in its schematic diagram. Please note the unconventional way of connecting the output transformer.

Schematic diagram of the amplifier

Undoubtedly, such an extensive audio path requires an equally extensive power supply. Here it is.

Schematic diagram of the power supply

Since the amplifier is so beautiful, all that is left for me is a photographic presentation with commentary limited to the necessary minimum. In the next six photos I have presented shots of the complete (not disassembled) amplifier before removing the covers. Its dimensions and (as can be seen with the naked eye) the mounting holes in the front panel have been adapted for mounting in a "rack" type frame. Of course, no normal user will put it in such a frame, but will treat it like other components of a home audio system.

Photo 1.

Photo 2.

Through the translucent mesh of the top cover, you can see the inside of the amplifier. Unfortunately, the sweet secret is partially revealed before the cover is removed. The photo also shows three solid transformers placed just behind the front panel. They determine the weight of the amplifier and are the cause of its "imbalance". The front weighs much more than the rest, and you have to keep this in mind when lifting the amplifier. Fortunately, it has been equipped with solid handles.

Photo 3.

The rear features unusual (that's an understatement) speaker terminals. Despite their unusual design, they are very practical.

Photo 4.

The shot below shows additional stiffening elements in the form of angle brackets connecting the front panel to the chassis. I have to admit that they don't look very aesthetically pleasing and seem extremely delicate. Fortunately, they are not delicate and, what's more, they are not very visible from the front of the amplifier.

Photo 5.

Photo 6.

Before removing the top cover, it is worth reading the information plate. Most users certainly ignore the warning contained therein.

Photo 7.

After unscrewing and lifting the cover, our eyes see an impressive set of output tubes (a truly beautiful sight ) and a battery of electrolytic capacitors with red caps-covers (this sight, however, is not so beautiful).

Photo 8.

There are two fans hidden under the cover. The power cables are connected to the motherboard. Without disconnecting them, it is not possible to fully dismantle the cover.

Photo 9.

This is what the fan looks like. It is very quiet. It has two functions: it cools the complete set of output tubes of one channel  and draws all the surrounding dust into the amplifier. The solution could be some filters, e.g. similar to those used in professional measuring equipment (Bruel & Kjaer, Tektronix, etc.), although this would require the user to clean it periodically. Besides, I am not sure whether, with such an openwork construction of the cover, filters would not be just a pointless addition without practical functionality.

Photo 10.

The cover itself is a small work of art. It is worthy of being immortalized in separate photographs. Here they are.

Photo 11.

Here's the solution to the riddle about how much work it takes to separate the fan's power cable from the motherboard. As befits a solid construction, it doesn't require a soldering iron, just a little tug and a click - the plug pops out of the socket. Great idea .

Photo 12.

Here you can see better how the fan power socket is installed.

Photo 13.

After completely removing the cover, the amplifier's interior is revealed in all its glory. As you can see, I dusted the amplifier off and in the following photos the tubes simply shine .

Photo 14.

This shot shows the capacitor connection wires. They are struggling to "climb out" from under the caps, stretching them unsightly and causing them to slide off the capacitors.

Photo 15.

And now a bit of acrobatics, but this is the best position the amplifier can take, for example, during...

Photo 16.

... control of the quiescent current values ​​of the output tubes.

Photo 17.

The photo below shows exactly what fancy speaker terminals were used. Some users of the amplifier replace these terminals with traditional ones (this can be seen, for example, in photos posted on the Internet), which in my opinion is a reprehensible act .

Photo 18.

And now a mini photo gallery with details. Look and admire.

Photo 19.

A complete set of sockets of single-channel: two noval and four octal (actually, following the current fashion, I should write "Octal" ).

Photo 20.

As many as four "Little Red Riding Hoods". Their appearance will scare many a wolf from the well-known children's fairy tale, forcing him to flee the forest.

Photo 21.

How to torture yourself, to the fullest. It sparkles in your eyes and makes you clench in your stomach .

Photo 22.

The amplifier's serviceability is almost at the highest possible level. Almost every element is perfectly visible, the terminals are accessible not only from the top but (as shown later in the gallery) also from the bottom. I emphasized the word "almost" because finding the potentiometers used in the power tube quiescent current control circuits requires hiring detective Monk. I didn't think they could be hidden as well as they were. But more on that later.

Photo 23.

Look at this little relay by the electrolytic capacitors . And how beautifully it clicks.

Photo 24.

Another view of the sockets taken from above the capacitors.

Photo 25.

Mounting of octal tubes sockets - simply "armored".

Photo 26.

The "noval" sockets are held in place solely by their own pins without any additional fasteners. They are not needed here anyway.

Photo 27.

The radiators are also in good condition. There are slight traces of corrosion on the electrolytic capacitor clamps.

Photo 28.

After removing the top cover and looking at the interior from above, it's time to inspect the lower parts. The bottom cover is just as openwork as the top. The cutouts are beautiful, so it's worth...

Photo 29.

... immortalized in a separate photo. These are not just any holes made with a drill. Here, a laser or sheet metal punch was used.

Photo 30.

After removing the bottom cover, we can see the other side of the motherboard. This is what it looks like in its entirety. This is where the measurement points are located, which can be used during...

Photo 31.

... amplifier control and regulation.

Photo 32.

Let's get acquainted with the assembly details.

Photo 33.

Large solder pads, few wires. Excellent access for service technician.

Photo 34.

And here you can clearly see the ventilation holes drilled into the motherboard. There are a lot of them.

Photo 35.

The capacitors made my eyes turn yellow. What are these capacitors? Here you go...

Photo 36.

... here are the details. You can read about it.

The capacitors are really "solid", although without any audiophile accretions. I would like to draw special attention to the complete lack of glue or other gooey inventions that are smeared on capacitors in audiophile devices. The belief in improving sound through ubiquitous "glues" on capacitors is becoming more and more common, and it is not helped by the fact that manufacturers do not glue capacitors in really well-playing top-shelf amplifiers. Perhaps they have come to the conclusion that such technology, apart from making servicing more difficult and spoiling the appearance of the device's interior, does nothing .

Photo 37.

And now a fragment showing the transformers. They have aesthetic covers on top. On the bottom they are inserted into holes cut in the chassis. And here is a small unpleasant surprise for me. The manufacturer used "flangeless" casings, which even in the 1950s were considered "poor man" casings in Soviet literature .

This is of course just my little spiteful digression, because such a construction of the carcass has no effect on the electrical parameters of the transformer and may only pose a problem with its winding in amateur conditions without the appropriate equipment (when winding "by hand", the phenomenon of the outer coil turns slipping may occur). I do not think that Audio Research wanted to save money in this way.

Photo 38.

Here's a photo of the second speaker transformer, and further on...

Photo 39.

... also the so-called "flangeless" mains transformer body.

Photo 40.

Finally, a special photo showing the details of one of the speaker transformers. The fact that it has an impressive core volume, providing the appropriate bandwidth from the bottom (low frequencies) was visible in the previous photos. Here I wanted to emphasize the details of the assembly of this core. As you can see, the side surface is as smooth as a baby's bottom. This is how it is done in a professional factory. I dedicate the photo to some domestic manufacturers of speaker transformers (and mains transformers, by the way), whose products can be used to rake lawns. Yes, yes - the surfaces of some cores resemble combs. This does not bode well for the approach to a potential customer. Fortunately, this negative phenomenon is slowly disappearing and more and more transformers available from us vividly resemble the one in the photo.

Photo 41.

And with this beautiful and inspiring view of the output transformer I end my presentation and at the same time greet all the happy owners of this amplifier .

Prepared by: Grzegorz "gsmok" Makarewicz, www.grzegorz-makarewicz.pl, www.mytriode.com