Cary Audio Design CAD 300 SE - Manual (Page 3)

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INTRODUCTION____________________________________________________
Congratulations!
You have purchased one of the most exotic vacuum tube audio amplifiers available. Within its power
range, the CAD 300 SE Signature offer 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
incredible reliability and enjoyment.
For the technically minded, a review of the circuit is in order. Your new CAD 300 SE Signature operates in a
Class A single-ended mode utilizing fixed negative grid voltage on the 300B triode output tube. The input
stage utilizes two 6SN7 dual triodes. The output transformer in the CAD 300 SE Signature is the most
important component in the amplifier and has been specifically designed by Cary Audio for use in the
300-SE Signatures. The output transformer is an air-gap design with a commercial continuous rating of
200% duty cycle. The CAD 300 SE Signature output transformer is an E/I laminate, silicon impregnated,
grain oriented steel design. The windings are wax vacuum impregnated and the entire transformer is
potted in high temperature wax. The power supply is a full wave center tap configuration running a CV-
378 (heavy duty 5AR4) vacuum tube rectifier. The rectified 430 VDC is fed to a pi-L filter network. The filter
capacitors consist of two (2) 1200 MFD and one (1) 100 MFD computer grade electrolytics for a total of
over 230 Joules of energy storage. Each electrolytic capacitor is by-passed with a low impedance .22
polystyrene capacitor. The power transformer is also a 200% duty cycle rating on the CAD 300 SE
Signature. To avoid AC hum, both the 6SN7 and 300B tubes 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 design of your new CAD 300 SE Signature was concentrated on the
"overload recovery" ability of an amplifier to instantly recover from clipping is much more important than
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 and amplifier can achieve after overload. With the incredible dynamic range of live
and in turn recorded music even 2000 watts of power is not enough power. 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 SE Signature extols its merits in the ability to handle transients and instantaneously
recover from brief or even extended overloads. The CAD 300 SE Signatures will overload symmetrically at
any frequency in the audio band pass. The CAD 300 SE Signatures 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 SE Signature to recover quickly when overloaded. If one were to monitor
the high voltage rail voltage (420 VDC) of a CAD 300 SE Signature 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.
Another technical feature of your new CAD 300 SE Signature is amplifier stability. The CAD 300 SE
Signature may be operated with no load (without speaker) without damage to the amplifier, output
transformer or tubes.