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The c300 up close
This 300 Watts/RMS amplifier is meant for those who
are not only looking for higher power, but superior performance
as well. In order to achieve this, the c300 features some
advanced techniques that are absent in its' smaller counterpart
(c200).
Additions
in 1st Gain Stage
Cascodes
Right at the very first gain stage, cascodes (Q5,6)
are adopted. They serve to improve the high frequency performance
of the c300. These cascodes are biased to approximately midpoint
between 0V and +V by zener diode D1 (33V).
Current
Mirrors
The first stage also contains current mirror Q3,4.
As the name implies, the mirror forces equal current in the
LTP (long tail pair). It is known for its' active loading
and high gain properties.
Emitter
Degeneration Resistors
Slew rate of the input differential is improved by resistors
R6,7,8,9,10. In the absence of matched transistors, preset
R10, is used for trimming DC to a minimum at the output of
the amplifier.
Buffering
the 2nd Stage
The
VAS mod
The 2nd stage is direct coupled to the differential via
a darlington Q8. This effectively buffers Q10, the main transistor
that is amplifying the voltage from loading the preceding
stage. Q10 is biased into class A by constant current source
Q12. Capacitor C9 sets the dominant pole in Miller compensation.
Thermal
Tracking
The
remaining parts of the circuit is conventional. Vbe multiplier
Q11, adjust the bias for the output transistors which is in
full complementary EF configuration. Q11 must be thermally
coupled to the main power heatsink for proper thermal tracking.
VI Limiting network consists of Q13,14, R25~30 and D3,4. This
network is optional, and can be omitted if desired.
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THD of c300
Biasing
of output transistors
All THD readings were done with outputs biased to 20mV across
0.39 ohms emitter resistor. This works out to approximately
55mA per output transistor in idling state.
Download
High Res Schematics
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