Class D amplifier driven by 555 output

[audioguru]

The NPN darlington gets hot because it is turned on with a 2.2k resistor that has a voltage loss and feeds a very low base current.

 

[darryl_co]

Biasing

The NPN darlington gets hot because it is turned on with a 2.2k resistor that has a voltage loss and feeds a very low base current.

Does that mean if I reduce the 2.2k resistor the heat would be equal?
To what value should I reduce the value?
Is a third diode required (1N4148) ?
Should there be a less value resistors in the emiters of the TIP ?
How did you arrive at the voltages?
Without signal (in practical) the voltage at the junction of the emiters of the TIP is around 8 volts.R2 680k value selected to get 8 volts.

 

[audioguru]

Does that mean if I reduce the 2.2k resistor the heat would be equal?

With a very low resistor value then the NPN would turn on better but still would not be the same as the PNP. Also the current would be too high for the driver transistor.

Is a third diode required (1N4148)?

For making a pure sinewave in a linear audio amplifier then 4 diodes are needed to match the 4 transistors in the darlingtons. Aren't you using squarewaves? Then no diodes are needed.

Since you are not making a linear audio amplifier then R2 can be removed. C2 can be replaced by a piece of wire.
The two diodes can be replaced by wire.

Should there be a less value resistors in the emiters of the TIP ?

Yes if it is a linear audio amplifier with 4 diodes.

How did you arrive at the voltages?

All the spec's are in the datasheets for the darlingtons and the BC547C transistors. Then I used Ohm's Law with simple arithmatic.

Without signal (in practical) the voltage at the junction of the emiters of the TIP is around 8 volts. R2 680k value selected to get 8 volts.

For a linear audio amplifier that you do not have.

For more output voltage swing, change the wiring a little so that R1 is bootstrapped by C1 like this:

 

[darryl_co]

Audioguru:
I tried your suggestion as shown in the circuit diagram.For safety reasons I added a 10E resistor in series with the plus line.I got the best results, just as I wanted in terms of heat dissipation in the transistors without shorting the diodes and C2 and not removing the R2.As everything was perfect except that the 10E resistor was heating up as the current through the resistor was 400mA.
On removing the resistor the circuit current shot to 700mA and conditions went back to the previous state.The NPN transistor got much hotter than the PNP!
Is it advisable to use two capacitors of 470uF/35V in parallel instead of single 1000uF/35V in the output?