Bloody Little Bastard ll solid stage power amp

[tip142]

[rcustoms]

a friend from brazil a.k.a Matec do some mod to improbe this design (not tested yet)
i translate from portuguese (google jajajaja)

Matec:
i agree that, for starters, a simpler design is very desirable.
But I also agree when it is said, there in the FSB, a SS amp design, average power, which is the case with this, it has to have a minimum of protection circuits to prevent a series of unforeseen events that may occur.
An amplifier such, any overload at the output terminals can burn the power transistors, and this can, by cascading burn all the equipment.

I did a simulation of this amplifier circuit and added a basic circuit of power limitation, (circled in green in the diagram), which should work with current peak values around 8A (maximum power amplifier with load of 4 Ohms) . With these values, the current limit for 100W into 8 Ohms is not affected. (Of course, a check values for a real circuit is required).

Another thing that is important is the temperature compensation circuit. In a bipolar transistor, the temperature rise is accompanied by increased current flow through the joints, leading to burning component. A higher power equipment that can happen in minutes, even without signal applied. In this circuit the temperature compensation system are the diodes D2 and D3. But for this to work these diodes have to have at least thermal contact with the heat sink of the output transistors. But that was not implemented in layouts, which is a serious flaw. In the scheme, the diodes that should be attached to the sink appear circled in red.

Still speaking of the diodes, the function they have in the circuit is to make the BIAS polarization of the output transistors. If you observe the way in which the transistors are placed, you will see that they are in Darlington configuration, this configuration requires approximately 2.4V for the pairs NPN / PNP Darlington work in class AB. To have this tension, it would take 4 diodes in series. In the original circuit appear only 2 diodes.
This will make the transistors 2SC5200 2SA1493 and work in class B, with crossover distortion, which greatly worsens the response in acute and makes om the amplifier makes a horrible sound at low volumes.

In the scheme that I presented was added a third diode, which reduces these effects somewhat (not eliminate), but I think a little better the answer. I tried a standard circuit adjustable BIAS control, but have come to a satisfactory result.

Fuses have also been added in the lines -B and + B.

thanks to matec for his mod

[grrrunge]

Looks better I would use a VBE multiplier to bias the output transistors into class AB, and mount it on the heat sink as close as possible to the output transistors.