Tag Archives: Nelson Pass

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Last but not least

Following the past few day’s posts about audio amplifier efficiency, Class AB biasing, and Class A biasing, let’s wrap our little mini-series up with another topology most of us have not heard of, adaptive biasing.

The promise of adaptive biasing is a best of both worlds scenario: the efficiency of a Class AB circuit with the performance of a Class A amp. Sounds too good to be true, right?

The first time I ever heard about an adaptive biasing scheme was way back in the dark ages, the late 1970s. My very dear friend and one of the all-time good guys of audio, Nelson Pass, then of Threshold Corporation, had introduced the idea of what we called a sliding bias scheme, part of what later became known as the Stasis Circuit used in Nakamichi, Threshold and if memory serves correctly, even the Mark Levinson No. 33.

The core of this circuitry is covered in Nelson’s patent from 1975 titled Active bias circuit for operating push-pull amplifiers in class A mode.

Simply put, Nelson’s design raises the level of Class A bias in cadence with the rising input signal.

Recall in our discussion of Class AB design that a small amount of always-on power keeps small signals always on. In other words, we apply Class A (always-on) bias to the first 10% of the amplifiers output signal level, then switch over to the more efficient Class B for higher signals. Compare that to Class A operation which is always-on for any level of signal—always generating a shit-ton worth of heat (recall Class A amps are at their coolest when at full signal out).

What Nelson cleverly suggested was this. Take what we do in an AB amp where the first 10% of the signal is Class A and actively monitor the signal level. When any given input signal starts to exceed our 10% Class A bias, raise the limit from 10% to, say, 20% (or whatever is greater than the signal level), and continue on the path all the way up to 100%. Then back down again tracking the signal. The heat-producing bias is only enough to accommodate the signal, then goes away when it’s not needed.

Thus, we get the benefits of both worlds. Efficient, and sweet-sounding.

Why doesn’t every amp use this even today? (Nelson’s patents ran their course years ago). Well, as with any design there are problems as well as advantages and this post is long enough already. Ain’t nuthin’ perfect. (We used this for several models of amps though their model numbers and dates escape me).

In any case, a juicy piece of history I’d thought I would share.

Asheville, North Carolina ‘s Home Theater and Audio specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Class B, AB, and A

Ever wonder what the classes of amplifiers mean? There’s Class A, Class AB, Class B (and a few others). Here’s a bit of history and an explanation.

Early transistor power amplifiers had their share of problems: crossover distortion, poorly designed output stages, slew rate limitations, transient slewing, little attention paid to power supplies. These failings of solid state amps fueled the tube devotee’s negative feelings toward them; and rightfully so. Until these issues could be fixed, few if any tube people would consider moving from the problems of tube amplifiers because there really weren’t any better solutions being offered. Things would change, but not for a little while.

Perhaps the biggest failing of early solid state designs, aside from their tendency to occasionally self ignite, was crossover notch distortion; a problem easily overcome by increasing the level of class A bias. Crossover notch distortion was fairly common amongst early solid state amplifiers that took advantage of the fact transistors had two types: positive and negative. Tube designers had only one type available to them: positive going tubes. The fact that solid state amplifier designers could use a positive transistor for the top half of the waveform and a negative transistor for the bottom half removed the need for a special inverting circuit some tube amps and push pull solid state amps used, plus benefited from an increase in efficiency. Less circuitry meant better sound. Only, if the two output devices were not used properly, this new type of bright and harsh sounding distortion would pollute our music.
Here is a picture of severe crossover notch distortion.

This happens when the positive transistor shuts off before the negative transistor turns on. This type of output would be called a Class B output and was almost never used in any quality audio products. However, the next scope photo was more indicative of what actual crossover notch distortion looked like.

Note the breaks in the middle of the waveform. This area is heard as a harsh and bright coloration to the music, can be rather jarring and was fairly prevalent in early power amplifiers. To fix this problem we would turn each of the two transistors on just a little bit. This is called Class A biasing and the amount you turn it always on determines how we classify the amplifier: no bias and it is a Class B amplifier. A few watts of constant bias and we refer to it as Standard Class AB bias. Both transistor always on, even if there is no signal, full Class A. Early amplifiers, as well as many today, run at a few watts of class A bias; just enough to eliminate the crossover notch and no more than that.

The net result is three types of amplifiers: Standard or High Class AB, Full Class A. To tell, it’s reasonably easy. Just put your hand on the amplifier’s heat sink after it’s been idling with no music playing. If it’s just warm, probably standard Class AB bias. Good and hot, yet still able to keep your hand on the sink, High Bias Class AB. Hotter’n a firecracker, Class A. Here’s a picture of a true off-the-chart fully class A amplifier from our friend Nelson Pass.

So now, when you see a solid state amplifier manufacturer say they have a ‘Class AB amplifier’ (as most do) you can ask the obvious question. “How much class A bias?”

In the case of our new BHK Signature power amplifier, slated for release this spring, the answer is 40 watts per channel (High, Class AB bias). Which means that for the first 40 watts of power delivered to the loudspeaker, the amp is a true class A device. Since most loudspeakers play quite loudly at 40 watts and the majority of your listening is under 40 watts, the amplifier is essentially class A for all quiet to medium loudness music. Peaks and large dynamics are then not Class A, but since most of the benefits of class A bias are heard in inner detail and quiet passages, there’s no need. Simple, eh?

Now you know.

Asheville, North Carolina ‘s Home Theater and Audio specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.


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:

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.