Cary Audio Design CAD 300 - Manual (Page 3)

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INTRODUCTION____________________________________________________
Congratulations!
You have purchased one of the most exotic vacuum tube audio integrated amplifiers available. Within its
power range, the CAD 300 SEI integrated amplifier offers the most realistic sound reproduction one could
desire for a “high-end” home audio system. Careful design, parts selection and proper circuit topologies
contribute to the incredible reliability and enjoyment.
For the technically minded, a review of the circuit is in order. Your new CAD 300 SEI operates in a class A
single-ended mode utilizing an auto bias (cathode) system for the 300B triode output tubes. The input
preamplifier section utilizes a 6SN7 dual triode tube. One triode section of this dual tube is used for the
left channel and the other triode section is used for the right channel. This circuit is a Class A voltage gain
circuit. This is the same basic first stage input circuit utilized in the Cary Audio model SLP 98 preamplifier.
The next gain stage in the CAD 300 SEI utilizes a 6SN7 dual triode wired in a series constant-current plate-
loaded configuration. This stage is duplicated for each channel. The series constant-current configuration
then drives the 300B triode output stage in single-ended, class A. The output transformers in the CAD 300
SEI are the most important components in the amplifier and have been specifically designed by Cary
Audio for use in the CAD 300 SEI. The output transformer is an air-gap design with a commercial
continuous rating of 200% duty cycle. The CAD 300 SEI output transformer is an E/I laminate, silicon
impregnated, grain oriented steel design. The windings are wax impregnated and the entire transformer is
potted in high temperature wax. Utilizing the linear, directly heated filament-cathode 300B triode output
tube, there is no global feedback used in the CAD 300 SEI. The power supply is a full wave center tap
configuration running high voltage, high current fast switching diode rectifiers. The rectified 450 VDC is
fed to a PI-L filter network. The filter capacitor consists of two (2) 1200 MFD and 230 Joules of energy
storage. Each electrolytic capacitor is by-passed with a low impedance .22 polystyrene capacitor. The
power transformer is a 200% duty cycle rating on the CAD 300 SEI. To avoid AC hum, both the 6SN7 and
300B tube have DC voltage on the filaments. This will prevent AC ripple voltage from capacitively being
coupled to the elements in the tubes.
Additional Design Thoughts
A great deal of attention during the design of your new CAD 300 SEI was concentrated on the “overload
recovery” ability of the amplifier to instantly recover from clipping and is a much more important issue tan
is commonly believed. In the power war of amplifier manufacturers the mentality is focused on high and
then even higher power output to solve the clipping problem. When in reality, the most critical aspect is
how fast a recovery an amplifier can achieve after overload. Most of the music being listened to in an
average listening room is only requiring about 3 watts of power. It is on the transients of loud low
frequency program material that tremendous signal voltages will appear at the input of the amplifier. It is
in this situation that the overload recovery ability of an amplifier is of critical concern. The single-ended
CAD 300 SEI extols its merits in the ability to handle transients and instantaneously recover from brief or
even extended overloads. The CAD 300 SEI will overload symmetrically at any frequency in the audio band
pass. The CAD 300 SEI will also yield faithful reproduction of extremely low frequencies at full output
levels. Power transformer, power supply regulation and output transformer design and careful shaping of
the overall frequency response curve all play a very important part in the ability of the CAD 300 SEI to
recover quickly when overloaded. If one were to monitor the high voltage rail voltage (385 VDC at the
anode of the 300B tube) of a CAD 300 SEI during soft and loud music passages it will be found there is no
more than a volt or so change from soft to loud passages.