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

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.

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.

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Download c300 High Res Schematics

60 Downes Street | Calais | ME 04619 | USA