Features
| Brand | Cary Audio |
|---|
Tech Specs
- Circuit Type - Single-Ended Pure Class A
- Power Output - 50 Watts Pure Class A
- Feedback - 0 to 10 dB, continuously variable control
- Frequency Response - 20 Hz to 20 kHz +/- 0.5 dB at Full Power Output
- Power Consumption - 230 watts – OPERATE, 76 watts – STANDBY
- Input Impedance - 150k ohms
- Output Impedance - 4, 8 and 16 ohm speaker connection terminals
- Signal to Noise - > 80 dB below rated output
- Sensitivity - 1.0 Volt @ zero feedback for full output
- Power Supply Capacitors - 4 – 1500 Fd @ 450V
- Power Transformer - 1 – EI Laminated core power transformer - 1 – Special air-gap output transformer - 200% duty cycle on all transformers
- Tube Complement - 2 – 6SN7 input tubes - 1 – 300B driver tube - 1 – 845 output tube or (Please choose 845 or 211 during install) - 1 – 211 output tube (Please choose 211 or 845 during install)
- Warm-Up Time - 5 minutes, after initial break in period
- Break-In Period - 100 hours of music playing time
- Finish - Jaguar ‘Anthracite Black’ chassis
- Dimensions - 10” H x 12-1/4” W x 24” D
- Weight - 85 lbs.
Product Description
The Cary Audio CAD-805RS Valve MonoBlok Power Amplifier is a high-quality unit with a choice of output valves. Two units are needed for stereo and they are supplied as a pair. The output valves can be either 841 or 211 types. Each has different power capabilities. Notably, The front panel features a meter that is used to set the bias of the 300B driver stage valve.
The adjustment potentiometer is situated at the rear of the unit. There are several output modes depending on the power required. These are automatically used as the output power increases. The maximum output is 70 watts, with the 211 valve. Moreover, the output transformer has three outputs for 4,8, or 16 ohm speakers.
High build quality
All Cary amplifiers, including the Cary Audio CAD-805RS Valve MonoBlok Power Amplifier, use point-to-point wiring. This is more reliable than printed circuit boards. Notably, from the 1960’s onwards most manufacturers switched to using PCBs as it was cheaper. However. They are not ideal for valve circuits.
The constant heating and cooling cycles tend to crack the soldered joints leading to unreliability. Next, the front panel of the Cary Audio CAD-805RS Valve MonoBlok Power Amplifier is made of thick silver or black anodized aluminium. Unusually most of the controls are mounted on the top of the unit.
These include the on/off switch and a knob that sets the bias for the type of output valve in use. There is also a feedback adjustment that sets the amount of feedback used by the amplifier. Negative Feedback is used to lower distortion. The top also supports the heavy power and output transformers and the high-grade smoothing capacitors.
Circuit operation
Notably, the Cary Audio CAD-805RS Valve MonoBlok Power Amplifier has a complex multistage circuit. Firstly there is a 6SN7 input stage. This takes the audio signal from the outside world. The valve is operating as a dual triode class A amplifier. Next comes a second 6SN7 also operating in Class A. The third stage employs a 300B valve as a driver stage.
This is connected via a capacitor from the previous stage. An adjustable negative voltage on the grid sets up the operating conditions of the output stage. The output stage is then transformer coupled from the driver stage. This is known as an interstage transformer and has to be carefully designed to have a flat frequency response.
There are several different operating modes used by the amplifier, depending on the output power needed at the time. For example, at relatively low power, up to 17 watts, the unit operates in pure Class A. As the power increases it switches to class A2 mode with an output of 50 watts. However, the more powerful 211 valve can go up to 70 watts.
Next comes the output transformer. This is a large unit with three speaker impedance taps for 4. 8 or 16 Ohm speakers. A transformer is needed to convert the high impedance, 8000 ohm. primary winding to a lower level suitable for a speaker.
A word about capacitors
Capacitors are important with valve amplifiers as they have to withstand high voltages for long periods of time. In the past capacitors used a paper dielectric that deteriorated over time, becoming electrically leaky. This led to a positive voltage being applied to the grid of the output valve causing it to overheat and fail. As a result, modern capacitors are made of polypropylene or polystyrene.
These can last for decades with no degradation. The same problem occurred with the electrolytic capacitors. These are used for power supplies. The heat in valve circuits caused the electrolyte to dry up leading to increased hum and eventual failure. Modern capacitors are much more robust.
