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working circuit not simulating

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Advanced Member level 4
May 3, 2002
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pspice speaker

I have this schematic of a power amp which I don't really know the specifications. I have actually built the circuit many times and it works but sometimes it doesn't. I've tried simulating the stuff with protel99se several times but the simulation just wouldn't work. Can someone help me?

I've posted the circuit on this link:
**broken link removed**

working circuit

Could you post the schematic in some other format? You could cut and paste it into Word.


here is the link to the microsoft word format:

**broken link removed**

I am not surprised

I am not surprised that the circuit does not always work. There are so many poles and zeros in the feedback loop. I would suggest that you stick to a more conventional design that does not have the danger of breaking into oscilllation and burning out your speakers. As far as simulating, you might try the PSpice trick and start out with the power supply voltages low and slowly ramp them up. I am not sure if your Protel simulator can do this. Another trick is to pin the node voltages at what you know them to be with an .OPTION statement that makes a first guess for the bias calculations. SPICE simulators are famous for not being able to find the bias point of op amp circuits. They frequently veer off to full rail voltages.


i don't have pspice nor anyother stuff for simulation. i only have MC7 which i do not know how to use.

i will like someone to just try the circuit on any other stuff for me. it could simulate on something else

post net list

If you would post the net list, I will give it a try in PSpice. You should specify the output transistor model numbers and all other transistors that are not specified. I suspect that the circuit was experimentally cut and tried for a specific speaker with its specific impedance vs frequency characteristics and that is why you may be having trouble. Please replace Rpot1 with a fixed value resistor which we can tweak in the net list. Post the net list in .txt format and I will give it a try.

When the circuit does not work, what are the symptoms, (oscillating at frequecy {specify the frequency} or going to the supply limit {whiich one}?

The circuit could very well be analyzed in MC!

What transistors have you used for the undesignated ones in the schematic? The ones designated 'NPN' and 'PNP' only? Maybe you have used e.g. low power devices where medium or high power transistors shall be used? With e.g. VCE/VBE ratings too low? How about power dissipation?

Learning MC is not hard at all. I have heard about 14-15 year old teenagers who have learned the program within a few hours, by just using the guides and manuals enclosed with the program (they are also available at And by running a few examples - the 'learning by doing' way.

Give it a go!

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