Cascode magic

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In Bascom H. King's video of the new BHK amplifier he talks about a cascode circuit, near the end of that film and several of you have asked me to expand on the subject. The first time I remember learning about a cascode was from the designer of another PS Audio amplifier, Bob Odell, or, as we called him back then, Dr. Bob, who crafted our first superstar amp, the 200C. In his original design there was a bipolar transistor differential input pair and in our version we wanted to sweeten the sound to more closely mimic that of a vacuum tube, and so he chose a FET instead. FETs generally do not have the ability to work at the high voltages required in a power amplifier as do their cousins, MOSFETS, and this was the case with what he chose. The lower voltage FET became the input of the 200C and to compensate for its lack of voltage, a higher voltage bipolar was placed above the FET in what is known as a cascode circuit; together they form one device capable of high voltage. The thing with a cascode is really trick. It permits the smaller input transistor to pass its full DNA to the larger higher voltage device without any penalty and the results are significant: however the smaller input transistor sounds and measures is carried forward by the pair. Thus if one uses a FET, then the cascode sounds and measures identically to a FET, or any other device you wish. I'll explain how this works tomorrow.
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Paul McGowan

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