| Class
AB Amplifiers from 150 Watts to 1000 Watts
Pass
Labs continues to improve its power amplifiers. After five years,
these acquired enough mass to warrant a change in designation, and
the X.5 series was announced. The improvements are:
Cascode
JFET Symmetric front end – input differential MOSFETs have
been replaced by the same JFETs we use for the input devices on
our MM and MC phono stages. They have very much less noise, higher
input impedance, and greater linearity. They operate self-biased
in a symmetric quad topology, each device cascoded and level shifted
to form the full voltage gain stage of the amplifier.
More
power output devices – Greater quantities of matched complementary
power Mosfets operated as followers, giving more current and power
handling, and also improved linearity.
New
bias circuit – A newly developed bias generator has eliminated
three sources of variation in bias current in the output stage.
It also allows approximately 20% greater power output for a given
supply voltage.
Single-ended
Class A bias – The output stage not only uses a heavy bias
current for push-pull Class A operation to a large portion of the
rated power, but also has a single-ended bias source for single-ended
Class A at low wattage. |
