A+B makes it all sound better

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Yesterday's post covered what Class B in a power amplifier means and the day before we explained Class A. It's probably instructive to review what we've learned so far as there are new readers jumping on board daily and I don't want to leave them out.

We're learning something about the different classes of amplifier outputs so that we can have a bit of background under our belts when we go to learn about the main subject of this series: Class D - which mistakenly gets called Digital Amplification.

We know that Class A and B refers to the amount of current (power) running through the output stage of the amplifier. Class A has power always running through it and Class B never has current running through it (when there's no signal).

The good news for A is it's an excellent sounding low distortion topology, the bad news: it's extremely inefficient producing lots and lots of heat. The good news for Class B is its high efficiency and the bad news: it sounds nasty at low levels of signal (when the music gets quiet).

In the early days of solid state amplification all amplifiers were Class B and most listeners who cared about musical quality shunned them as harsh sounding (they were) and stuck with the much better sounding tubes. The primary reason these early amplification devices sounded so harsh was this area of distortion caused by the lack of current running through them.

Yesterday we learned the term that defines this distorted area is called Crossover Notch Distortion. Some early pioneers in solid state amplifier design like James Sugden showed the world that solid state amplifiers didn't have to sound harsh, by producing small Class A amplifiers of around 15 watts or so. The problem of heat always plagued these designs and so they remained small wattage devices. A new solution had to be designed that could offer the efficiency of the Class B design with the sweet sounding (not harsh) sound of Class A. The technique would be (not so cleverly) named Class A/B.

While it may seem simple now, combining the best of both A and B took a few years of work and I wish I knew more of the history of this work so I could share.

In a solid state amplifier there are typically two output devices, one handling the positive half of the output and the other the negative half. The problem with Class B is each of the two devices turns completely off when the other is doing its half of the work. If this was a clean cutoff, then there really wouldn't be an issue with sounding harsh at low levels. Unfortunately, not too much is straightforward with a transistor and before they turn on, there is a small 1/2 of a volt area where they distort the signal badly. This area is sort of no man's land when it comes to making good music.

When you combine the 1/2 volt of the upper transistor with the 1/2 volt of the lower transistor, you wind up with a 1 volt area that is really distorted, harsh and nasty sounding should any music be unfortunate enough to exist in this low area of sound. Since this area is a critical region of music where most of the low level detail, harmonics, tape hiss etc. live, having distortion here is a really bad idea. And this distortion is not restricted to low level details either. Even a larger signal suffers because it must pass through this no-man's land and a bit of harshness is added to the mix and rides on the sound in a most unpleasant manner.

So here's what the engineers did: they turned the transistor on permanently so it is always drawing current and producing heat - but unlike Class A they shut this constant power drain off after we passed the no-man's land. They did this to both the positive and negative devices so that within this area of low level detail we have Class A performance and outside this area we have Class B. The amount of Class A performance has to be enough to cover the area in question, which typically takes no more than a few watts, but most designers prefer to ramp it up a bit more. Most amps I have designed run between 20 and 50 watts of class A power at these low levels, and other designers have their ideas about what's good or bad. My decisions were all listening based and I just turned up the bias on the output stage until adding more had no audible gain.

So, for example, a 100 watt Class A/B amp runs a constant 2 to 20 watts of heat to keep things going and then ramps up another 80 watts to reach full output - but those extra watts go directly into the loudspeaker - not converted to heat as are the first 2 to 20 watts.

If you'll remember our 100 watt Class A amplifier example consumed 200 watts of power which, at idle, all went into heat.

As you can see, a Class A/B amplifier is significantly more efficient than a Class A amplifier and has many of the same characteristics as the A - being identical at lower levels. The idea in the design is when the music gets really loud you can't tell any difference between the two.

Those of you familiar with the sound of Class A vs. Class A/B can attest to the fact they both sound different and have different charms: Class A always sweet, open and delicate sounding, while Class A/B amps have many of the same superlatives in different degrees but with an added bit of muscle thrown in the mix for good measure.

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Paul McGowan

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