Re: Audio amplifier (part 2)

John Popelish wrote:
Olivier Scalbert wrote:
Hello, it is me again ...

After "jouling" some transistors, I decide to move to LTSpice. Which is funny as I am doing electronics as hobby to change from being in front of computers all day long !


I have some questions on this schema: http://scalbert.dyndns.org/electronics/amplifier/schema1.png
Simulation is here:
http://scalbert.dyndns.org/electronics/amplifier/simul1.png

I have only 2N3055 at disposal so I use a Sziklai pair (at least I think!)...

BC546 and 556 will be replaced by bigger ones

Questions:

1/ Any big mistakes ?

I don't think so. It looks like you have a basic understanding of what you are doing. Of course, that strongly depends on what you are trying to do.

2/ To move from A to AB, I think I need to change the value of R1 so Q1 conducts more or less. By doing that I will have DC on the load, so I have to put a cap no ?

You have two separate but interconnected problems. How to change the bias point for the two output transistors, and how to produce a zero signal output of zero volts.

If you change R1 and R2 at the same time, you should be able to change the bias point and still maintain the desired output voltage. But this circuit depends strongly on component matching and power supply voltage stability. More practical versions use negative feedback to correct for variations in components and supply voltages.

3/ What can be improved before building it ?

First, you should make a functional list of "improvements" (what would define a functional improvement in the operation). Then you are in a better position to consider circuit changes and how they interact with those functions.

First, I think you should explore frequency response. Your test waveform is 100 Hz. Change that to 10,000 Hz and I think one of the weaknesses of this circuit will start to show up. Both of your output devices have lots of turn on drive, but no turn off drive, so once they are on, they will be very slow to turn off. This will produce increasing distortion and higher bias current as frequency rises.


Thanks John for your help.

Do you think it is possible to change the design in order to have only one potentiometer to change the bias of the transistors, instead of changing R1 and R2 ?

For the negative feedback, I imagine that I have to measure the DC level at the load resistor, compare it with 0, negate it and re inject it somewhere in the input bias, but I do not see how ...

I have explored the frequency response. It is still ok at 10k and even more . Pehaps the spice model of my components are too ideal. I should put some parasitic inductances and capacitors ... or find other models ...

Olivier



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