Tube Power Amp Reviews

Editor: WOW! What a review! I am grateful, and, as we say in North Carolina..."Mr.. Colloms, I am beholden to you once again."

I find it extraordinarily satisfying, after I spent untold months designing and field-testing the CAD-300-SE/LX20, that Mr. Colloms is testifying to the merits of single-ended triode sound. I can remember, years back, when Martin Colloms' views of single-ended, zero-feedback triode amplifiers were quite the contrary. Martin was not alone in his negative views. Once people have the opportunity to actually listen to a single-ended, class-A triode music system, I am of the opinion it will change any preconceived negative views they may harbor.

I believe there is a well kept secret in the circles of audio designers. Most designers will not admit to the fact that "the best-sounding audio amplifiers are low-powered." Some in the industry believe that even 9W or 10W is too much. I have heard it said in many audio forums that "If the first watt sounds bad, why do I want 200 more of the same?"

During the design effort on a Cary single-ended amplifier, I concentrate a great deal of attention on the amplifier's "overload recovery" ability. The 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. In reality, the most critical aspect is how fast an amplifier can recover after overload. With the incredible dynamic range of live and, in turn, recorded music, even 2000W of power is not enough.

Most of the music being listened to in an average home listening room requires only about 3W 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 an amplifier's overload recovery ability is of critical concern. The single-ended triode class-A amplifier hasd an exemplary ability to handle transients and instantaneously recover from brief or even extended overloads. The class-A triode single-ended amplifier will overload symmetrically at any frequency in the audio bandpass. The triode in class-A will also yield faithful reproduction of extremely low frequencies at full output levels. The design of the power transformer, power-supply regulation, and careful shaping of the overall frequency-response curve all play very important parts in the ability of an amplifier to recover quickly when overloaded. If one were to monitor the high-voltage rail of a Cary Audio single-ended class-A amplifier during soft and also loud music passages, it would be found that there is no more than a volt or so change from soft to loud passages.

Another technical feature of a low-powered, class-A SET amplifier is stability. Properly designed, such an amplifier can operate with no load (without speaker) without damage to the amplifier, output transformer, or tubes.

You will note that most of my discussion has been in the realm of vacuum-tube SET class-A audio amplifiers. The SET audio amplifier is, by nature, a low-wattage device. In staying within that realm, I conclude that "High Power Corrupts!"

Once again, I would like to thank Stereophile for the opportunity to voice my thoughts on the subject of single-ended, class-A, real triode sound.

In the words of Sam Tellig: Mr. Colloms, I believe you are in "deep triode!!!"—Dennis J. Had
President, Cary Audio Design