14
Class AB operation allows both devices to be on at the same
time (as in Class A), but just bar
ely
. The output bias is set so
that curr
ent fl
ows in a specifi
c output device appr
eciably mor
e
than a half cycle, but less than the entir
e cycle. That is, only a
little curr
ent is allowed to fl
ow thr
ough both devices — unlike the
complete load curr
ent of Class A designs — but enough curr
ent
to keep each device operating so they r
espond instantly to
input voltage demands. Thus the inher
ent non-linearity of Class
B designs is eliminated, without the gr
oss ineffi
ciencies of the
Class A design. It is this combination of good effi
ciency (ar
ound
50%) with excellent linearity that makes Class AB the most
popular audio amplifi
er design.
Class AB+B design involves two pairs of output devices. One
pair operates Class AB, while the other (slave) pair operates
Class B. Class AB+B designs ar
e just as effi
cient as Class AB,
but have even less distor
tion.
To
summarize, AB+B cir
cuitr
y draws less AC power and wastes
less heat than Class A, and has less distor
tion than Class B or
Class AB.
V
a
riab
le Impedance (VZ)
VZ is the name of Cr
own’
s patented bi-level power supply
technology
. It allows Cr
own to pack large amounts of power into a
compact package while achieving ultra-low distor
tion and without
generating excessive heat.
An amplifi
er power supply must be large enough
to handle both the maximum voltage and
maximum curr
ent the amplifi
er needs to drive
its rated power into a specifi
ed load. In or
der
to meet this r
equir
ement, most conventional
supplies ar
e heavy
, large, and pr
oduce lots
of heat. In contrast, the VZ supply gets mor
e
curr
ent AND voltage out of a smaller
, lighter
, and
mor
e effi
cient package by dynamically adapting
to both signal and load r
equir
ements in r
eal-
time. This pr
ovides the best power match to the
widest range of loads.
The VZ power supply is divided into two
segments. When the output stage r
equir
es high
voltage, the segments ar
e arranged in series to
deliver twice the voltage of a single segment.
When the output stage r
equir
es high curr
ent,
the segments ar
e arranged in parallel to deliver
twice the curr
ent of a single segment.
Sensing cir
cuitr
y “watches” the voltage of the
signal to deter
mine when to switch VZ modes.
The power supply continuously adapts to the
output signal: high voltage or high curr
ent.
The switching cir
cuitr
y is designed to pr
event
audible switching distor
tion to yield the highest
possible dynamic transfer function.
It allo
ws Cro
wn
to pac
k large
amounts of po
w
e
r
into a compact
pac
kage
...
”
A Circuit Diag
ram from the
VZ P
atent.
