Class A amplifier

[stestexo]

Dear all,
i build this amp:

http://img19.imageshack.us/img19/7651/schemaclassea.png

I like the sound of this ampli, but i had a problem and some question for you:
- The transistor Q2 "burn" if i have a high signal at input. Can i change it with a similiar transistor more powerful to solve this problem?
- I can't understand the schematic....generally in a classA amplifier the final transistor must be the same....in this schematic there is a PNP and a NPN transistor (Q4 and Q5). Why?
- Is it possible to increase the power? for example what happened if i make a couple of Q4 and Q5 in parallel?

Thanks in advance for your suggestions
Stefano from Rome

[KatieandDad]

Did you have a heatsink on Q2 ?

[stestexo]

Hello ;-)
there is no heatsink on Q2. I think is not needed because Q2 seems not hot. Do you think is needed?

[jerluwoo]

Q2,Q4,and Q5 are all under rated for the job. Q2 has the entire 24 volts across it at all times basically, but the amount of current through it probably more than triples when driving Q5. Q4 and Q5 are both operating at their soa limits and will probably die before to long themselves.

[stestexo]

So....you want to say that the schematic is not very good :-)
Can you suggest me some changes to do? or if you want you can suggest me a new schematic of a nice class A ampli (i already have the heatsinks and transformer, it's a pleasure for me to build a new circuit).
Thank you, regards!
Stefano

[TinTopHack]

Originally Posted by stestexo

So....you want to say that the schematic is not very good :-)
Can you suggest me some changes to do? or if you want you can suggest me a new schematic of a nice class A ampli (i already have the heatsinks and transformer, it's a pleasure for me to build a new circuit).
Thank you, regards!
Stefano

You could try adding a 5W 120 ohm resistor between collector of Q2 and +V That should limit the power dissipation of Q2. Then you can lower the value of R5 to 12K. Q4 and Q3 form a constant current source with 1A of current available from Q4's collector. Worst case dissipation will approach 24W for Q4 so you will need a large heatsink for it. Same large heatsink for Q5.

Q1 runs at a few milliwatts of power so it should not burn unless there is something wrong with C4. When you say hi signal at the input, how hi is it?

[Mooly]

Q2 needs to be of reasonable current gain, and so to Q5 that it feeds. If Q5 gain is low then Q2 is supplying a lot of current to overcome that. Q2 will run hot with 24 volts across it and a few tens of milliamps flowing.

The output transistors being different, NPN and PNP is irrelevant. Q4 is just a constant current load, it could equally be a FET.

You won't increase the output power by paralleling the transistors but you really should look at using high gain (high hfe) transistor for Q5.

Have you looked at the original JLH69 Class A amplifier ?
The Class-A Amplifier Site

[TinTopHack]

Q5 (BD243) has a gain of 15 at Ic=3A. If it has to conduct 2A, 1A from Q4 and 1A from speaker (worst case), then Q2 has to supply 133mA to the base of Q5. The peak power dissipated by Q2 would be around 3W without any resistor at its collector. But probably at IC=2A, the gain of Q5 would be higher - maybe 20 - that will requires less current from Q2 - around 100mA (<3W of peak dissipation by Q2) so you could probably get on without a resistor at Q2's collector. That's peak power. When listening to music the average power dissipated by Q2 will be a lot less so this explains your statement that it is not hot.

Q2 has a min gain of 40 at 100mA so it should be OK.

It seems the remaining issue is why Q1 fails when the input signal is high. It might help if you describe what you mean by high.