Cary Audio CAD-805RS

The CAD-805 Reference Series (CAD-805RS) is the latest edition in the 805 series of single-ended mono block amplifiers that has been in continuous production since 1993.

Here is what you will see with a pair of CAD-805RS moving from front to back. The front panel is a handsome thick machined aluminum that is available in either silver or black anodized finish. The CAD-805RS is equipped with a built-in meter on the front of the chassis. The adjustment pot is located at the rear of the chassis along with a bias jack for an external meter. Looking at the amplifier from above, you see a top-plate, which is finished in a handsome Anthracite Black gloss enamel, with white silkscreened lettering. To the left, the main power switch, to the right the ‘standby/operate’ switch which powers up the 211 or 845 output tubes. To the left is a rotary switch with a green jewel-like knob that chooses the appropriate bias for your choice of 211 or 845 output tubes. To the right is a similar rotary knob that dials in ‘negative feedback’ on a global basis and is adjustable from “off” in a continual rotation to a max of 10 dB of feedback. Also, on this top front portion of the amplifier are two 6SN7 input stage tubes, a 300B driver stage tube, and your choice of either an 845 or 211 output tube.

Behind the tubes, the main real-estate of the 805 is occupied by two very impressively sized transformers—one power, and one output, and four electrolytic capacitors. Also, toward the rear of the chassis are the aforementioned setscrew adjustment pot for biasing purposes and the quarter inch bias jack for reading the bias of the 300B driver tube. These are the only user adjustments you will have to worry about, other than choosing your output tube flavor.

The back panel offers a choice of 4, 8, or 16 ohm speaker outputs that are very high quality and heavy duty. There is a RCA input jack and a XLR input jack with a user selectable switch, and two visible fuses—one for the 300B and one for the output tube. There is also a built-in fuse within the AC power socket. The power cord is detachable.

To review the circuit, the single-ended audio input signal from the outside world is passed to the control input grid of the first 6SN7 tube. This first 6SN7 stage is operating in a parallel dual triode conventional resistance-coupled Class A amplifier. This first gain stage is directly coupled to the next 6SN7 dual triode gain stage. This second stage is operating in a series anode constant-current Class A amplifier. This amplified audio signal is passed through a coupling capacitor to the grid of the 300B driver stage. The 300B triode tube operates in a Class A single-ended mode utilizing fixed negative grid voltage on the 300B driver stage. This negative grid voltage is user-adjusted by the screwdriver pot at the rear of the CAD-805RS. The 300B is directly coupled to the grid of either a 211 or 845, 100-watt plate dissipation, triode output tube. The method of coupling at this stage is called an interstage transformer. This is an air-gap design with a flat frequency response from 30Hz to 23 KHz. The audio signal from the 300B interstage transformer will drive the 211/845 output tube to 27 watts Class A output. With additional volume levels, the amplifier goes into a positive grid mode with a power output of 50 watts Class A2. In the case of the 211-output tube, the Class A2 power is on the order of 70 watts at clipping. The high-power output from the 211/845 tube is coupled to the loudspeaker via an air-gapped 150 watt continuous duty output transformer. This transforms the high impedance (8000 ohm primary winding) of the output tube to 4,8, or 16 ohms to drive the loudspeaker.