Cary Audio SLI-80 Signature

Grzegorz 'gsmok' Makarewicz

Stereo tube amplifier producedCary Audio Design company. This is not only an US amplifier, but unlike many of such products is not assembled in China, but in the United States of America. A real shock.

Coming back to the point, this is the all tube amplifier - even rectifier diodes  are vacuum tubes. Like in the most tube amplifiers from outside of China I traditionally could not find any circuit diagram that allows a detailed analysis of the amplifier. I had to draw a schematic diagram based on the actual amplifier. The process of creating the schematic from scratch is enjoyable and interesting, but on the condition that someone has a lot of time. In my case, unfortunately, I can't always afford this luxury.

Fig. 1 shows a general view of the amplifier. The amplifier does not have a factory tube protective cover. From esthetic point of view, this is very well, because amplifier with angled output transformers looks great. A little pity that the housing of the mains transformer and two output transformers are not standardized.

Fig. 1

Fig. 2 shows the front panel of the amplifier. It is quite complex compared to the purist audiophile design which contains only the power switch and possibly gain control knob.

Fig. 2

Here we have some more elements. Their functions are clearly described, as shown in Fig. 3 and Fig. 4. We have a power switch, the inputs switch (CD, AUX1 and AUX2), volume control (large knob on the center) ...

Fig. 3

... balance control, remote gain control sensor, switch between work with headphones and speakers and a headphone jack.

Fig. 4

In Fig. 5 and Fig. 6 the details of construction of the amplifier seen from above are shown.

Fig. 5

In Fig. 6 looking shyly from behind the output transformers bulky electrolytic capacitors operating in power supply filters can be seen. I think that the manufacturer did not treat them as decorative elements, but simply failed to pack them inside the housing.

Fig. 6

And here's the set of tubes and proudly exposed card with the type of amplifier. Now we know why the manufacturer has not applied protective cover for tubes.

Fig. 7

In Fig. 8 amplifier seen from the side is shown. On the side walls of metal casing there are no elements related to the operation of the amplifier. This is important. Many amplifiers have switches mounted on the side walls and near the rear panelTheir service is both inconvenient and dangerous. The owner of the listening system must, in the case of closely spaced components reach such hidden controls by hand blindly and bear the risk of touching hot tubes. In this case, there is no such danger.

Fig. 8

The amplifier can operate in triode and pentode ultralinear mode. Switching between modes is performed independently for each channel with switches presented in Fig. 9 and Fig. 20.

Fig. 9

Yet another view of a set of tubes, power transformer and ...

Fig. 10

... let us go to the photographs taken from the back of the amplifier (Fig. 11 to Fig. 19).

Fig. 11

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In Fig. 19 a set consisting of a potentiometer and large Jack Mono socket for adjusting the quiescent current of power tubes is shown. The amplifier has no autobiasing topology and after each time the power tube or tubes are replaced it is required to set the specified amplifier quiescent current manually. According to the amplifier user guide, current meter should be connected by means of plug Jack and the current value should be adjusted using the suitable potentiometer. After adjustment position of the potentiometer axle can be locked by gently tightening the nut visible in the photograph.

Curiosity, which might surprise users of tube amplifiers, which require adjusting the quiescent current is the fact that for this purpose, an ammeter and not a voltmeter is used. Inserting the plug into the measuring socket opens the connection between output tubes' cathodes and the ground, so to make a cathode-ground connection an ammeter is required, which allows anode current to flow. It's a pretty unique approach, given the fact that inside the amplifier very good resistors 10Ω / 1% are mounted between cathodes and the ground. I think that they do not serve the user of the amplifier but rather the serviceman, because access to them is possible only after removing the bottom chassis cover.

As you can see in the photographs amplifier is equipped with two potentiometers regulators, one for each channel. This means that the correct (same) division of quiescent current between the tubes of each channel requires tubes of selected parameters (you need two pairs of power tubes).

Fig. 19

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Fig. 22

The amplifier is standing on four robust rubber feet.

Fig. 23

Here is the foot with the plug...

Fig. 24

... and without the plug. Visible inscription leaves no doubt who produced the amplifier.

Fig. 25

Time for the most interesting piece of photographic relation. Dismantled session. In the following two photos the whole inside of the amplifier after unscrewing the bottom cover is shown.

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 And now the details. Look and admire.

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In Fig. 44 the central part of the system for producing a negative control grid bias is shown. It consists of a rectifier and CRC filter. The second filter capacitor (electrolytic 47μF/350V) is further shunted by a polypropylene capacitor MKP (2μF/400V). The resistor used in the filter is DALE - the samy type as in electronic circuits in a military aircraft and the systems for audiophiles. As you can see the manufacturer has not spared on the quality elements.

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Written by Grzegorz 'gsmok' Makarewicz

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