BGW 250 - Page 5

The output stage of your 250 uses the most powerful type of
transistors available. The large geometry single diffused
devices have the largest safe operating area of any type of
transistor available. All the semiconductors in the output
stage are in intimate contact with the heat sink. The
dynamic bias circuit is also mounted on this isotherm providing
rock steady bias stability with temperature.
The voltage gain circuits are also mounted on the same
circuit card. A true operational amplifier
integrated circuit, hermetically sealed in a metal can, acts
as the front end. The op amp (as they are called) is a
special unit featuring high speed (15MHz) and high slew rate
(50 volts/microsecond) yet still having very low noise due
to its darlington input stage and careful design. The op amp
stage is followed by a discrete complementary pair acting
as an active current source/sink and providing additional
voltage gain. The current source is the ideal way to drive
the output stage which is basically a voltage follower.
This sophisticated circuit design makes for an extremely
accurate amplifier. The open loop gain is higher than found
in the competitors' products. The accuracy of an amplifier
is a function of the difference between the open loop gain
and the closed loop gain. In this case the closed loop gain
is 26 db (a voltage gain of only 20) while the open loop
gain is about 1,000,000. The accuracy of the 250 is so great
that conventional audio test equipment is unable to provide
meaningful measurements. Harmonic distortion Measurements
are simply the residual level of the distortion analyzer.
This extremely accurate signal processing enables the 250
to drive speakers at very high levels while adding
absolutely no coloration of its own. EVen at milliwatt
levels the output waveform exhibits no sign of crossover
distortion. The dynamic range capability of a typical 250
is almost 115 db.
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