Complements

December 9, 2014
 by Paul McGowan

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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5 comments on “Complements”

  1. Hallo Paul.

    I always have to grin when people say they would listen to an amplifier. In fact they are listening to a pair of loudspeakers and it’s room and amp interaction.
    I wonder why there is no common “listening-standard”. However there is no standard loudspeaker and no standard listening room, I assume. 😉

  2. I’ve read more than one time that a problem with complementary devices is that they’re not completely complementary. And that why some designers sometimes use PNP devices in push’pull(push/push?) designs.

  3. As an addition to my previous comment today. Tube amps are obviously done push/pull with non-complementary devices since they don’t exist for tubes. I do know that David Berning and Murray Zeligman researched using FETs as a complementary device to tubes many decades ago. And patents were issued. But this was only for small signal tubes at the time such as 12 AX7s, not for output tubes.

  4. Understanding how transistors work is much harder than understanding how tubes work. It’s easy to visualize a mesh of wire between two electrodes controlling the flow of electrons in one direction between them. Understanding how transistors work requires knowledge of quantum physics and is much harder. It is also much harder to manufacture two opposite type transistors that have the exact mirror image characteristic curves than it is to manufacture two of the same type that are identical. Perhaps this is why most hi fi audio power amplifier output stages are quasi complimentary symmetry and not true complimentary symmetry designs. If they don’t match exactly you have the same asymmetric harmonic distortion disadvantage you have with class A amplifiers. Why are they usually both NPN and not PNP? I don’t know but I’d hazard a guess that it’s because the bias voltage polarity is the same as for tubes and it was a kind of holdover from the transition in the early 1960s. It has proven to be a good design for the last 50 years, a kind of workhorse that is reliable, well understood, and cheap to build. However while much more efficient than class A amplifiers they aren’t nearly as efficient as class D and similar amplifier designs. Those amplifiers can deliver much more power in much lighter packages for much less money. I think they will eventually replace class AB amps as the ordinary vanilla design with digital signals straight through to the driver stage. Sorry to say it but I think the age of analog audio is drawing to a close just the way analog video did. Eventually it will be an historic curiosity for engineers to marvel that analog worked as well as it did.

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