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One of the reasons people tend to like the sound of single device amplifiers is inherent in their very design: a single 'sex' device. By 'sex' we are referring to the type of device (in the same way as we might describe male vs. female). Tubes have only one 'sex' but transistors have two: positive and negative devices. When both sex devices are used to form an amplifier we sometimes call this topology Complementary Symmetry because of its use of both sex transistors to amplify the music. A classic complementary symmetry amplifier can take many forms, but the most common is the output stage of the amplifier. This is typically the current amplification stage and a simple schematic for this looks like this: Picture 1 The vast majority of modern amplifiers and preamplifiers use a version of this classic design for their output stage. Not everyone is in agreement it is the best sounding topology. I remember reading a paper by Nelson Pass on the beauty of designing a one sex amplifier. He referred to that amplifier has being more like music than any he'd ever heard before. That it had major heat issues wasn't a concern, not when the incredible sound was taken into consideration. Nelson's adventurous (and one of the best designers we have today) and his single sex device was his foray into the exotic, but not his standard. Most of his designs incorporate both sex devices, as do mine and many others. The way this circuit works is by splitting up the duties of amplifying the music into positive and negative halves. In the drawing above the transistor labeled 'NPN' amplifies the positive half of any waveform while the 'PNP' handles the negative going half. By splitting up the duties we gain much in terms of efficiency, as well we solve the problem of single amplifying devices I brought up a few days ago: their inability to push and pull power at the same rate. This complementary pair does symmetrical amplification all day along. But there are several hitches (isn't there always?). Tomorrow we'll see what the major glitches are.
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Paul McGowan

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