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[ how virtual class a¨ works ]
In the normal class AB amplifiers, the negative and positive output transistors are switching in
tune with the music signal, when the output current to the speakers is larger than twice the idle
current. For example at the figure on the left side, if the idle current is 100mA, transistor Q2
switches off when transistor Q1 delivers 200mA to the speaker with a positive musical signal,
and vice versa with a negative musical signal.
In the picture, you see the current through the output transistors when one sinus pulse is delivered to the speaker. It shows
that for Virtual Class
A¨ (green line) both the negative and positive output transistors remain active the entire time (Òno
switchingÓ), because the lower edges of the scale are approximately 100mA respectively over and under zero. In Class
AB(red
line) the output transistors are switched on and off in tune with the music signal.
In Virtual Class A¨ operation the output transistors are always active, therefore they are both
always ready to start fast when the music signal changes polarity. It is the same way with auto-
mobiles, which also start faster from idle than from the ignition. Because of this faster reaction
with Virtual Class
A¨, glitches and other transient currents are avoided.
In class
AB operation the output transistors are turned on and off during zero crossing. This
switching can generate glitches and transients, which blur small treble details.
With class ABoperation, there is a characteristic change in the output impedance from the amplifier when one of the output
transistors switches off. This switching introduces an unequal distortion that sounds particularly harsh and unmusical, called
Òcrossover distortionÓ. This means that several types of feedback must be used to compensate for the crossover distortion, but
the feedback itself in some cases sounds even worse than the original distortion because of increased instability (flat Japanese
sound). With Virtual Class
A¨ the output impedance sequence is significantly smoother, ensuring that the Virtual Class A¨
generates much less distortion.
sense output current -Vcc+Vcc
Q1
I idle
Q2
variable control voltage
music signal
speaker
With Virtual Class A¨, a
variable control voltage in
tune with the musical sig-
nal, is used to set the idle
current, so that the idle cur-
rent is maintained through
both output transistors
regardless of the output
current. This means that
both output transistors
stay active the entire time.
+Vcc
-VccQ1
I idle
Q2
fixed control voltage
music signal
speaker
0 400 mA
-400 mA
virtual class a¨ principle
no switching
switch off
Virtual Class A¨ Class AB
output impedance at zero crossing
Virtual Class A¨ Class AB
0.200.25 0.30impedance Re=0.22 ohm, I idle=200mA
0.15
0.10
0.05
0.00 ohms
output current 0 -350
350 mA Òcrossover distortionÓ
CONVENTIONAL CLASS AB
OUTPUT STAGE
VIRTUAL CLASS A
¨ OUTPUT STAGE
I idle changes with music signal
Virtual Class
Acontrol circuit
I idle is constant
