Leaving Class A
Article By Nelson Pass of Pass Labs
The meters on our amplifiers are
different. They reflect the current consumption of the amplifier, and when the
amplifier is operating, they don't go down to zero like the meters on other
amplifiers. This is because the electrical current consumption of our circuits
has a fairly high value at all times, a property called the bias.
The bias current runs through the amplifiers at a minimum value, determining the
class of operation – Class B, Class
AB, or Class A.
Class B has no bias current, Class AB
has a moderate bias current, and Class A has a high bias current. Class
push-pull amplifiers are hybrids
between Class B and Class A. Class
run Class A at low power levels, and become Class B amplifiers at output
currents determined by the bias.
For several years, Pass Labs has specified the nominal
wattages at which our amplifiers leave push-pull Class A operation into an
Here is a current summary of this information:
We get a lot of questions about this. A typical email reads, "I can't sleep at night – I keep worrying about where my amplifier stops
being Class A. As I listen to my system, I think I can hear the Klunk
as the special Class A part of the amplifier kicks in and out!"
For starters, there is no special Class A circuit that kicks
in and out, and for that matter, there certainly is no Klunk.
There is just a push-pull amplifier output stage, which is operated at a
constant idle current known as the bias.
In this regard, our power amplifiers are like other amplifiers on the market.
The vast majority of amplifiers are push-pull designs with a certain amount of
Push-pull amplifiers generally operate in Class A mode up to a
point where the output current is twice the value of the bias current. In the
Class A region, both halves of the circuit share the signal simultaneously.
Beyond that the signal is handled solely by the push (+) half of the amplifier
or the pull (–) half.
Let us look at this in more detail. The simplified circuit
for such an output stage looks like this:
Here we see two power transistors operated as "followers"
where the output voltage equals the input voltage. Q1 is attached to the
positive power supply and Q2 is attached to the negative power supply.
Ordinarily the common output of these two transistors would be attached to a
loudspeaker. When the input voltage is positive, so is the output voltage, and
we would look to Q1 to supply current to the loudspeaker from the positive
supply. When the input voltage is negative, we look to Q2 to supply current to
the loudspeaker from the negative supply.
Audio signal has both positive and negative voltage components
and we will see a point at zero volts where the two halves meet. Unfortunately,
all the gain devices we know have severe non-linearity (distortion)
down around zero, and this gives us a very poor transition from Q1 to Q2 and
The solution for this is to apply some idle current to the
circuit by the mechanism of the bias voltage source you see in Fig 1. This
voltage is set so that at idle, current flows through Q1 and Q2 equally from the
V+ supply to the V- supply and creates a more linear region at the crossover
If that is not clear, perhaps an analogy will help: Imagine
that the two transistors are runners in a relay race and that the signal is the
baton they carry. In a real relay race, the runner receiving the baton begins
running before the hand-off, which is made with the runners at speed. The
runners who hand over the baton at a dead stop will operate at a severe
So it is with push-pull power transistors. The higher the
bias, the smoother, more seamless is the transition.
The quantity of bias current is the key. Figure 2 shows this
from the point of view of Q1.
Fig 2a shows Q1 conducting current on the positive half of the
signal and experiencing a sharp cutoff as the signal goes negative. Fig 2b shows
the effect of a small Class AB bias current – the current shows a gentle
cutoff with less distortion. 2c shows enough bias to keep the transistor in the
Class A region, where it always conducts current and has even less distortion.
Higher bias doesn't just move the Class A transition to
higher ground – it has a profound influence on the amplifier at all power
levels. It lowers the distortion at low levels as well as high levels, as seen
in the distortion versus power curves for an amplifier with the bias set at
In Fig 3 we see the distortion of an output stage operated
without feedback driving 8 ohms from 0.10 watts up to 20 watts. The top curve
with the highest distortion has a bias of 0.016 amps. The next lower is 0.08A,
followed by 0.16A, 0.32A, 0.64A, 1.28A, and the lowest distortion curve at 2.56
amps. What we see clearly is that higher bias lowers the distortion at all power
levels, and that the distortion is inversely proportional to the bias current.
It is not simply that the distortion numbers are lower, but
the characteristic of the distortion is improved in terms of the ratio of lower
order harmonics (2nd and 3rd) to higher order harmonics (4th,
5th, 6th and so on).
Figure 4A shows the distortion at 1 watt with the output stage
biased at 0.08 amps. On the left you can see the signal waveform in blue and the
distortion waveform in red. The distortion measurement is at 0.67 percent total,
and you can see the harmonic distribution on the right, where harmonics from 2nd
through 10th are prominent.
Figure 4B shows the same test and output circuit, but at a 4
times higher bias figure, 0.32 amps. The distortion total has dropped to 0.11
percent, but of greater interest, the higher harmonics have been dramatically
reduced, leaving a dominant 2nd order harmonic.
At four times more bias in Fig 4C, the continues to drop to
0.004 percent (remember, this is without negative feedback), leaving nothing to
look at on our distortion waveform and only 2nd and 3rd
harmonic on the spectrum analysis.
The benefits of high bias current extend beyond simple
harmonic distortion measurements – you also get a reduction of intermodulation
distortion (arguably more important), and a lower, more consistent output
impedance. As a corollary benefit, the heavy hardware required to support Class
A operation will show better thermal stability and will deliver better
performance into difficult loads.
So what's the down-side of high bias operation? In two
words: Low Efficiency.
For a given amplifier circuit, the idle dissipation is
proportional to the bias current. Twice the bias current makes for twice the
heat. Usually it also means about twice the hardware and twice the weight.
Many products on the market have idle power draw at a small
fraction of rated power. A Class AB 150 watt amplifier channel with 0.1 amp bias
will idle at about 10 watts. By contrast, an X150.5 channel has that power
rating but idles at about 100 watts. The high performance of an X150.5 comes at
a price. So far we have only talked about push-pull Class A bias.
the effects of high bias also apply to single-ended
Class A bias? Yes, but in a slightly different way. Single-ended Class A bias is where a single gain stage
operates against some variety of current source, as compared to push-pull Class
A where two complementary stages operate in opposition to each other.
Single-ended Class A is often thought of as the "King of Class A" by purists
because it delivers the lowest order of harmonic distortion, 2nd
harmonic, instead of the 3rd harmonic of push-pull.
It also is the least efficient of the Class A configurations,
with an efficiency figure on the order of 20%, sometimes less. A limitation of
single-ended Class A operation is that the peak output current is limited to the
value of the bias current. By comparison, push-pull Class A can deliver twice
the bias current as peak output in Class A, and generally much more than that in
In 1991 Pass Labs developed a hybrid class topology which
paralleled a push-pull Class A output stage with a current source which biased
it into single-ended Class A. The Aleph 0 amplifier operated as a single-ended
Class A amplifier to its output rating of 75 watts into 8 ohms, and at currents
beyond that it continued to deliver current as a push-pull Class A circuit.
Subsequently in the X amplifier series we retained a small
amount of single-ended Class A bias on our output stage as a means of
controlling the amount and character of the distortion at the lowest power
levels – that all-important first watt.
By setting the value and direction of a single-ended bias
current in a push pull power stage you can reduce the distortion by canceling
the second harmonic nature of imperfectly matched Q1 and Q2 halves, or you can
arbitrarily select a desired ratio of second to third harmonic, depending on
subjective evaluation of the resulting sound.
Figure 5 shows a simple example of such a circuit.
We apply a single-ended Class A bias at about 10 percent of
the push-pull Class A bias. This is sufficient to improve the performance around
1 watt and below. Fig 6 shows the distortion plus noise (again without feedback)
of the earlier example output stage with a 2.56 amp push-pull bias and also with
an added 0.5 amp single-ended bias. You can see a distortion improvement by
roughly a factor of two in the low wattage region. Below 0.5 watts, you can also
see the noise which is the same in both circuits.
Like all bias decisions, the amount of single-ended bias and
push-pull bias is a balance between performance and efficiency. The bias is
generally set at a level suited to the heat dissipation capacity of the
hardware. At Pass Labs the bias is set to the value which raises the heat sinks
25 – 30 degrees C. above ambient temperature. The result is a heat sink which
you can put your hand on for about 10 seconds or so.
As a practical matter, this means that our X (Class
AB) amplifiers are biased to dissipate roughly half of their rated output power.
The XA (Class A) amplifiers are biased to dissipate roughly three times their
rated output power. The whole point of going to this trouble is to build an
amplifier which sounds as good as possible. We find that this is achieved by
building a simple amplifier which is intrinsically distortion-free. Getting that
depends on a high bias.
Measurements are helpful at illustrating the differences
between design approaches, but they are certainly not the last word in audio. If
they were, then numerous other approaches would sound as good or better. You can certainly imagine an amplifier, which operates with a
low bias current but has the necessary amount of negative feedback and/or
circuit complexity to insure that it measures as well. Actually, you do not have
to imagine it – such amplifiers are for sale. Do they sound better? We don't think so. Our meters don't
go to zero.
© 2008 Nelson Pass
13395 New Airport Road
Auburn, CA 95602
Voice: (530) 878-5350
Fax: (530) 367-2193